Molecular Weight Calculator

Use this free Molecular Weight Calculator to calculate molecular weight, molar mass, molecular mass, moles, molarity, and molar concentration from a chemical formula or solution values. Enter a formula such as H2O, CO2, NaCl, Ca(OH)2, or C6H12O6 and the calculator will add the atomic masses of each element to return the compound’s molar mass in grams per mole.

This page also includes a Molarity Calculator, Mole Calculator, Mass-to-Molarity Calculator, and Dilution Calculator. The guide below explains how to find molar mass, the molarity formula, molecular weight units, atomic mass, formula mass, molecular mass versus molecular weight, and common chemistry calculations using moles, grams, litres, and molar concentration.

How to Use the Molecular Weight Calculator

Choose Molecular Weight mode when you have a chemical formula and want its molar mass or molecular mass. Enter the formula using normal element symbols and subscripts as numbers, such as H2O, CO2, NaCl, Al2O3, Ca(OH)2, or C6H12O6. The calculator reads the element symbols, counts the atoms, applies the appropriate atomic weights, and adds them together.

The other modes solve common chemistry problems that are closely related to molecular weight. Molarity mode calculates molar concentration from moles and solution volume. Mass ↔ Moles converts grams to moles or moles to grams. Mass to Molarity combines molar mass with solution volume, while Dilution solves M1V1 = M2V2.

If your calculation also involves general arithmetic or scientific notation, our Scientific Calculator can help verify multiplication, powers, and exponent calculations.

What Is Molecular Weight?

Molecular weight is the sum of the atomic weights of the atoms in a molecule. For water, H2O, there are two hydrogen atoms and one oxygen atom. Using approximate atomic weights of 1.008 for hydrogen and 15.999 for oxygen gives a molecular weight of about 18.015.

The term molecular weight is widely used in chemistry even though molecular mass is often the more precise term when discussing the mass of an individual molecule. Molecular weight is technically dimensionless because it is a relative quantity, while molecular mass can be expressed in unified atomic mass units, u or Da.

In everyday chemistry calculations, the numerical molecular weight and molar mass are closely connected. A molecule with a molecular mass of approximately 18.015 u has a molar mass of approximately 18.015 g/mol.

What Is Molar Mass?

Molar mass is the mass of one mole of a substance. It is usually expressed in grams per mole, written as g/mol. One mole contains approximately 6.02214076 × 10²³ specified particles, known as Avogadro’s constant.

For example, the molar mass of water is approximately 18.015 g/mol. This means one mole of water molecules has a mass of about 18.015 grams. Two moles have a mass of about 36.03 grams.

Searches for molar mass, molar weight, molecular weight, molecular mass molecular weight, formula mass, and molecular weight calculator are closely related because they all involve adding atomic masses according to a chemical formula.

Molecular Mass vs Molecular Weight

Molecular mass refers to the mass of a molecule and is commonly expressed in atomic mass units. Molecular weight is a relative measure based on atomic-weight values. Molar mass describes the mass of one mole of the substance and is expressed in g/mol.

In many classroom and practical calculations, people use molecular weight and molar mass almost interchangeably because the numerical value is nearly the same. The calculator therefore displays both.

How to Find Molar Mass

To find molar mass, identify every element in the chemical formula, count how many atoms of each element are present, multiply each count by that element’s atomic weight, and add the results.

• Write the chemical formula clearly.
• Count each element, including atoms inside parentheses.
• Find each element’s atomic mass or atomic weight.
• Multiply atomic mass by the number of atoms.
• Add the element contributions.
• Express molar mass in g/mol.

This same process answers searches such as how to find the molar mass, how to calculate molar mass, how to compute for molar mass, how to get molar mass, and how do you calculate molecular mass.

Molar Mass Formula

The general molar mass formula can be written as:

M = Σ(nᵢ × Aᵢ)

Here, n is the number of atoms of an element and A is that element’s atomic mass. The sigma symbol means to add the contributions from all elements in the chemical formula.

Atomic Mass and Formula Mass

Atomic mass describes the mass of an atom on the atomic mass scale. The atomic weights shown on a periodic table are values used to calculate molecular mass and formula mass. For a compound, multiply each atomic weight by the number of atoms represented in the formula.

Formula mass is especially useful for ionic compounds that do not exist as separate molecules. Sodium chloride, NaCl, is commonly described using formula mass or molar mass rather than the molecular mass of an individual NaCl molecule.

How to Calculate Atomic Mass

In basic chemistry, you normally look up the atomic weight of an element rather than calculating it from scratch. Atomic weights reflect isotope abundances and isotope masses. The molecular-weight calculation then uses these atomic-weight values as building blocks.

For example, carbon has an atomic weight of about 12.011 and hydrogen about 1.008. Methane, CH4, therefore has a molar mass of approximately 12.011 + 4(1.008) = 16.043 g/mol.

Molar Mass of Common Elements

These values help answer common searches such as molecular weight of hydrogen, molecular weight of carbon, Al molar mass, Mg molecular weight, molar mass of C, molar mass of S, and molecular wt of Co or other elements.

Molar Mass of Hydrogen

The atomic weight of hydrogen is approximately 1.008. A hydrogen molecule, H2, contains two hydrogen atoms, so its molecular mass is about 2.016 u and its molar mass is about 2.016 g/mol.

Molar Mass of Aluminum

Aluminum, Al, has an atomic weight of approximately 26.982. Because elemental aluminum is represented by one Al atom in its formula, its molar mass is approximately 26.982 g/mol.

Molarity Calculator

Molarity is a concentration measurement that tells you how many moles of solute are present per litre of solution. It is one of the most common concentration units in chemistry.

The Molarity Calculator above accepts moles and solution volume. If your volume is entered in millilitres or microlitres, it converts that value to litres before calculating concentration.

Molarity Formula

The molarity formula is:

M = n / V

M is molarity in mol/L, n is the number of moles of solute, and V is total solution volume in litres.

If 0.5 mole of solute is dissolved to make 1 litre of solution, the molarity is 0.5 M. If the same 0.5 mole is in 0.25 litre, the molarity is 2 M.

This formula is the basis of searches such as molarity equation, how to calculate molarity, how to find molarity, calculate molarity, how do you calculate molarity, and how to solve for molarity.

What Does 1 Molar Mean?

A 1 molar solution, written 1 M, contains one mole of solute per litre of total solution. A 10 millimolar solution is 10 mM, which equals 0.010 M. A concentration of 1 mM equals 0.001 M.

When converting mM to molar, divide by 1,000. When converting molar to mM, multiply by 1,000.

Molar Concentration

Molar concentration is another name for molarity when concentration is expressed as moles per litre. A concentration in molar units can be written as M or mol/L.

For example, a solution with 0.25 mol of solute in 500 mL has a volume of 0.500 L. Its molarity is 0.25 ÷ 0.500 = 0.5 M.

Molar Concentration Calculator From Mass

Sometimes a chemistry problem gives grams rather than moles. In that case, first divide the solute mass by its molar mass to calculate moles. Then divide the moles by solution volume in litres.

The combined calculation is:

Molarity = mass ÷ molar mass ÷ volume in litres

The Mass to Molarity mode above performs both steps. This makes it useful as a concentration molar calculator, molar concentration calculator, or chemistry calculator when the problem starts with grams.

Mole Calculator: Mass, Moles, and Molar Mass

The relationship among mass, moles, and molar mass is:

moles = mass ÷ molar mass

Rearranging gives:

mass = moles × molar mass

For example, 36.03 g of water divided by a molar mass of 18.015 g/mol equals approximately 2 moles of water.

This same relationship is used by a mole calculator, molecular mass calculator, molar mass calculator, and many stoichiometry calculations.

Molecular Formula and Molecular Mass

A molecular formula tells you which elements are present and how many atoms of each occur in one molecule. For example, C6H12O6 contains 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

To calculate molecular mass, multiply each element count by its atomic weight and add the results. The formula itself therefore contains all the information needed to calculate molecular mass, provided the element symbols and subscripts are known.

How to Determine a Molecular Formula

Determining an unknown molecular formula is different from simply calculating molecular weight. A molecular formula problem typically starts with elemental composition, empirical formula, molar mass, combustion data, or another experimental measurement.

If you already know the empirical formula and both the empirical formula mass and measured molar mass, divide the measured molar mass by the empirical formula mass. The resulting whole-number factor tells you how many times to multiply the empirical-formula subscripts.

Molecular Weight Units

Molar mass is expressed in grams per mole, or g/mol. Molecular mass is commonly expressed in unified atomic mass units, u, or daltons, Da. Molecular weight is technically relative and therefore dimensionless, although the phrase is often used informally when people mean molar mass.

Other mass units such as milligrams, micrograms, and nanograms may appear in chemistry problems. The molar mass itself remains in mass per mole; you convert the sample mass into a compatible unit before calculating moles.

Molarity Units

The standard molarity unit is mol/L, commonly abbreviated M. Millimolar is abbreviated mM and equals 10⁻³ M. Micromolar is µM and equals 10⁻⁶ M.

Dilution Formula

When a solution is diluted without changing the amount of solute, the initial concentration and volume are related to the final concentration and volume by:

M1V1 = M2V2

The Dilution mode solves for one missing value when the other three are known. Use the same concentration unit for M1 and M2 and the same volume unit for V1 and V2.

Molar Mass Examples

Water: H2O

2 hydrogen atoms × 1.008 + 1 oxygen atom × 15.999 = approximately 18.015 g/mol.

Carbon Dioxide: CO2

1 carbon × 12.011 + 2 oxygen × 15.999 = approximately 44.009 g/mol.

Sodium Chloride: NaCl

1 sodium × 22.9898 + 1 chlorine × 35.45 = approximately 58.44 g/mol.

Glucose: C6H12O6

6 carbon + 12 hydrogen + 6 oxygen gives a molar mass of approximately 180.156 g/mol.

Calcium Hydroxide: Ca(OH)2

The parentheses mean the O and H are each counted twice. The result is approximately 74.092 g/mol.

Potassium Phosphate: K3PO4

K3PO4 contains three potassium atoms, one phosphorus atom, and four oxygen atoms. Adding their contributions gives a molar mass of approximately 212.27 g/mol.

Why Molar Mass Matters

Chemical equations describe substances in moles, but laboratory measurements are often made in grams. Molar mass provides the conversion between those two quantities. It is essential for stoichiometry, solution preparation, concentration calculations, reaction yields, analytical chemistry, and many laboratory procedures.

For percentage calculations related to composition or concentration, the Percentage Calculator can help with general percent math.

Common Molecular Weight Calculation Mistakes

• Ignoring subscripts in a chemical formula.
• Forgetting to multiply atoms inside parentheses.
• Confusing atomic number with atomic mass.
• Using millilitres directly in the molarity formula instead of litres.
• Confusing molar mass in g/mol with sample mass in grams.
• Using lowercase and uppercase element symbols incorrectly.
• Confusing molecular mass, formula mass, and molar mass terminology.

Molarity vs Molality

Molarity and molality are different concentration measures. Molarity is moles of solute per litre of solution. Molality is moles of solute per kilogram of solvent. Because solution volume can change with temperature, molarity can change slightly with temperature while molality is based on mass.

Molecular Weight and Avogadro’s Number

One mole contains 6.02214076 × 10²³ specified particles. Once you calculate moles, multiplying by Avogadro’s constant estimates the number of molecules, atoms, ions, or formula units present.

For example, 2 moles of water contain approximately 1.2044 × 10²⁴ water molecules.

Molar Mass and Significant Figures

Atomic weights are often reported with several significant digits, but the final molar-mass precision should make sense for the problem. Classroom exercises may use rounded periodic-table values, while analytical calculations may retain more digits.

The calculator uses commonly published standard atomic-weight values. Slight differences can occur when another periodic table rounds values differently or when an element has no stable isotope and a representative isotope mass is used.

Chemistry Calculator Connections

Molecular weight is connected to many other chemistry calculations, including stoichiometry, percent composition, empirical formula, molecular formula, solution concentration, dilution, density, gas calculations, reaction yield, and limiting reagent problems.

If your chemistry problem involves physical volume, container capacity, or litres and gallons rather than molar concentration, our Volume Calculator can calculate geometric volume and capacity conversions.

For additional reference on chemical elements and atomic-weight data, see the NIST atomic weights and isotopic compositions reference.

Molar Weight Calculation Step by Step

A molar weight calculation follows the same basic process as a molar mass calculation. Start with a chemical formula, identify each element, count how many atoms of that element are present, multiply each count by the element’s atomic weight, and add all of the contributions. The calculator automates those steps, but understanding the process makes it easier to check whether a result is reasonable.

For example, consider sulfuric acid, H2SO4. Hydrogen contributes 2 × 1.008 = 2.016, sulfur contributes about 32.06, and oxygen contributes 4 × 15.999 = 63.996. Adding those values gives a molar mass of approximately 98.072 g/mol.

How Parentheses Change Molecular Weight

Parentheses in a chemical formula multiply every element inside the group. This is an important source of mistakes when calculating molecular weight by hand. In calcium hydroxide, Ca(OH)2, the subscript 2 applies to both oxygen and hydrogen. The formula therefore contains one calcium atom, two oxygen atoms, and two hydrogen atoms.

Nested groups can appear in more complicated formulas. The calculator supports parentheses and brackets so grouped atoms are counted before the total molecular mass is calculated.

Hydrates and Molecular Weight

Some compounds contain water molecules in a crystal structure and are written as hydrates. Copper(II) sulfate pentahydrate, for example, is written CuSO4·5H2O. The five water molecules contribute additional hydrogen and oxygen to the total formula mass.

The Molecular Weight Calculator accepts either a centered dot or a period between hydrate groups. CuSO4·5H2O and CuSO4.5H2O are interpreted as the same composition.

Molarity Solver

The Molarity mode also works as a simple molarity solver. If moles and solution volume are known, it solves M = n/V directly. When a problem begins with grams instead of moles, use the Mass to Molarity mode so the calculator first converts mass to moles and then divides by solution volume.

For example, suppose 5.844 g of sodium chloride is dissolved to make 1.00 L of solution. Using a molar mass of about 58.44 g/mol gives 0.1000 mol of NaCl. Dividing by 1.00 L gives a molarity of approximately 0.1000 M.

How to Work Out Molarity From Millilitres

Molarity requires volume in litres. If the problem gives millilitres, divide by 1,000 first. A solution volume of 250 mL equals 0.250 L. If that solution contains 0.050 mol of solute, molarity is 0.050 ÷ 0.250 = 0.200 M.

The calculator handles this conversion automatically when mL is selected. This is useful for questions phrased as how to work out molarity, how to compute molarity, or how to find out molarity when the volume is not already in litres.

Molarity From Millimolar

Millimolar concentration is often used for dilute laboratory solutions. The prefix milli means one-thousandth, so 1 mM = 0.001 M. A 10 millimolar solution is therefore 0.010 M, and 250 mM is 0.250 M.

To convert mm to molar in the sense of millimolar to molar, divide the mM value by 1,000. To convert molar to millimolar, multiply by 1,000.

How to Calculate Concentration From Mass

When concentration is requested in molar units but the solute amount is given as a mass, two equations are required. First calculate moles from mass and molar mass. Then calculate molarity from moles and litres of solution.

moles = grams ÷ g/mol

molarity = moles ÷ litres

This combined process is common in solution preparation. The Mass to Molarity mode keeps the steps together and shows the intermediate amount of substance in moles.

Molecular Weight of Common Compounds

These values are useful as quick reasonableness checks. If a formula calculator returns a result far from the expected range, check the capitalization of element symbols and the placement of subscripts.

Molar Mass of CO

Carbon monoxide, CO, contains one carbon atom and one oxygen atom. Using atomic weights of approximately 12.011 and 15.999 gives a molar mass of approximately 28.010 g/mol.

Molecular Weight of Magnesium

Magnesium, Mg, has a standard atomic weight of about 24.305. A formula containing one Mg atom contributes approximately 24.305 to the molecular or formula mass. For elemental magnesium, the molar mass is approximately 24.305 g/mol.

Molecular Weight of Carbon

Carbon has an atomic weight of approximately 12.011. In a compound, multiply 12.011 by the number of carbon atoms in the formula. Six carbon atoms in glucose contribute approximately 72.066 g/mol to the total molar mass.

Molar Mass and Percent Composition

Once the total molar mass is known, the mass percentage of each element can be calculated by dividing that element’s mass contribution by total molar mass and multiplying by 100. The Molecular Weight mode displays an approximate elemental composition by mass automatically.

If you want to check or reuse those percentages in another calculation, our Percentage Calculator is useful for general percentage math.

Moles and Number of Molecules

After finding the number of moles, the number of molecules can be estimated using Avogadro’s constant. Multiply moles by 6.02214076 × 10²³. A 0.5 mol sample contains approximately 3.011 × 10²³ specified particles.

The correct particle description depends on the substance. Molecular substances are counted as molecules, ionic compounds are often counted as formula units, and elemental samples may be described in atoms.

Molecular Weight and Stoichiometry

Stoichiometry uses balanced chemical equations to relate amounts of reactants and products. Mole ratios come directly from equation coefficients, while molar mass converts those mole amounts into measurable masses.

For example, if a balanced equation requires 2 mol of a compound, multiply 2 by that compound’s molar mass to determine the corresponding gram amount. If you need help comparing numerical ratios before using them in a chemistry problem, the Ratio Calculator can help with ratio and proportion arithmetic.

Dilution Example

Suppose you have a 2.0 M stock solution and need 400 mL of a 0.5 M solution. Using M1V1 = M2V2:

2.0 × V1 = 0.5 × 400

V1 = 100 mL. Measure 100 mL of the stock solution and dilute to a final total volume of 400 mL.

Molarity, Volume, and Unit Conversion

Many concentration errors are really unit-conversion errors. Litres, millilitres, microlitres, grams, milligrams, and nanograms may all appear in one laboratory problem. Convert units before applying the equation and keep track of whether the requested result is mass, amount of substance, or concentration.

If a problem requires a general numeric conversion after the chemistry formula is set up, our Simple Math Calculator can be used for quick arithmetic checks.

What Molecular Weight Calculator Results Mean

The main result in Molecular Weight mode is molar mass in g/mol. The calculator also reports the numerical molecular weight, number of distinct elements, total atom count in one formula unit, element counts, approximate mass composition, and an equivalent molecular-mass value in unified atomic mass units.

For compounds containing ions or extended lattices, formula mass may be the more precise description than molecular mass. The arithmetic is still the same: add the atomic-mass contributions represented in the formula.

Molecular Weight Calculator Frequently Asked Questions

Final Thoughts

Molecular weight and molar mass connect chemical formulas to measurable quantities in the laboratory. Once the formula is known, add the atomic-mass contributions to get molar mass, then use that value to convert between grams and moles or to calculate solution concentration.

The Molecular Weight Calculator above combines the most common related calculations in one place: molecular mass, molar mass, moles, molarity, molar concentration, mass-to-molarity conversion, and dilution. Keep units consistent, pay close attention to subscripts and parentheses in chemical formulas, and convert solution volume to litres when using the molarity equation.