Molecular Weight Calculator

Enter a chemical formula and get instant results. This molecular weight calculator parses your input, identifies each element, and returns the molar mass in grams per mole (g/mol). It also shows the elemental composition by mass percentage.

Whether you need the molecular weight of NaCl, glucose, or a complex organic compound, type the formula and let the tool do the math.

Molecular Weight Calculator Tool

The calculator on this page accepts any standard chemical formula. Type it in, and the tool returns:

• Molar mass in g/mol
• Elemental composition as a percentage of total mass
• Atomic mass contribution from each element

Results update automatically. No extra buttons to click.

This is an estimate based on standard atomic weights from the periodic table. For regulatory, pharmaceutical, or analytical work, verify results with a primary reference source.

How to Calculate Molar Mass From a Molecular Formula

Molar mass calculation follows a simple process. Identify each element in the formula, find its atomic mass, multiply by the number of atoms, and sum everything up.

Enter a Chemical Formula

Type the molecular formula exactly as you would write it in chemistry notation. A few rules:

• Use standard element symbols with correct capitalization (e.g., Ca, not CA or ca)
• Place atom counts as integers immediately after the element symbol (e.g., H2O)
• Use parentheses for groups, followed by a subscript count (e.g., Ca(OH)2)
• For hydrates or molecules of crystallization, enter the full formula (e.g., CuSO4(H2O)5 or CuSO4·5H2O, depending on what format the tool accepts)

If the calculator doesn't recognize your input, check for typos in element symbols or misplaced parentheses. Capitalization matters.

Atomic Mass, Element Composition, and the Periodic Table

Every element has a standard atomic mass listed on the periodic table. This value represents the weighted average of all naturally occurring isotopes for that element, expressed in unified atomic mass units (amu).

One amu is defined as one twelfth the mass of one atom of carbon-12.

When you calculate molecular weight, you sum the atomic masses of every atom in the formula. For water (H2O):

• Hydrogen: 1.008 amu × 2 = 2.016
• Oxygen: 15.999 amu × 1 = 15.999
• Total MW: 18.015 g/mol

The calculator also expresses each element's contribution as a percentage. For water, hydrogen makes up about 11.2% and oxygen about 88.8% of the total mass. This composition data is useful for stoichiometry, assay calculations, and preparing solutions.

Molecular Weight Definition: Molecular Mass vs. Molar Mass vs. Formula Weight

These terms overlap, but they are not identical. Here is a quick breakdown:

• Molecular mass (molecular weight, MW): The sum of atomic masses in a single molecule. Expressed in amu or Da (daltons). Strictly applies to covalent molecules.
• Molar mass: The mass of one mole of a substance, expressed in g/mol. Numerically equal to the molecular weight but with different units.
• Formula weight: The same calculation, but used for ionic compounds (like NaCl) that don't form discrete molecules. The term avoids implying a molecular structure where one doesn't exist.

In practice, most people use "molecular weight" and "molar mass" interchangeably. The calculator returns values in g/mol, which works for both.

Relative Molecular Mass and Molar Weight

Relative molecular mass (Mr) is a dimensionless number. It compares the mass of a molecule to one twelfth the mass of one atom of carbon-12. Numerically, it matches the molecular weight, but it carries no units.

"Molar weight" is an informal synonym for molar mass. You may see it in older texts or product catalogs. It means the same thing: the mass per mole.

The Mole and Grams in Chemistry

A mole is 6.022 × 10²³ particles (Avogadro's number). One mole of water weighs 18.015 grams. One mole of sodium chloride weighs 58.44 grams.

This connection between moles and grams is the foundation of nearly every chemistry calculation. When you know the molar mass, you can convert between mass and moles in a single step:

• Moles = mass (g) ÷ molar mass (g/mol)
• Mass (g) = moles × molar mass (g/mol)

The molecular weight calculator gives you the molar mass value you need for these conversions.

Calculate Molar Mass for Protein, Peptide, DNA, and RNA

Biological macromolecules are too large to represent with a simple molecular formula. A typical protein might have a molecular weight of 10,000 to 200,000 Da or more.

For proteins, peptides, DNA, and RNA, molecular weight is usually calculated from the amino acid or nucleotide sequence rather than an atom-by-atom formula. Each residue has a known average mass, and the total is adjusted for water lost during bond formation.

Linear Sequence Mass Calculator for Biology

A linear sequence mass calculator works differently from a chemical formula tool:

1. You enter the one-letter or three-letter amino acid sequence (for proteins and peptides) or the nucleotide sequence (for DNA and RNA)
2. The tool sums the residue masses
3. It subtracts water molecules lost to peptide bond or phosphodiester bond formation
4. It returns the estimated molecular weight in Da or kDa

Key considerations for biology calculations:

• Proteins and peptides: Average residue mass is roughly 110 Da, but the exact value depends on the specific amino acids. Post-translational modifications (glycosylation, phosphorylation) change the mass.
• DNA: Each nucleotide averages about 330 Da (for double-stranded DNA, roughly 660 Da per base pair).
• RNA: Each nucleotide averages about 340 Da.
• Recombinant proteins: Verify the expressed sequence, not just the gene sequence. Tags, signal peptides, and cleavage sites all affect the final MW.

This formula-based molecular weight calculator handles standard chemical compounds. For protein, peptide, DNA, or RNA mass calculations, use a dedicated sequence-based tool from a biochemistry resource.

Chemical Formula Examples: Salt, Solution, and Common Molecules

Here are some common formulas you can enter directly into the calculator:

For ionic compounds like salt, remember the calculator returns the formula weight. The result is still the value you need for preparing a solution or doing a stoichiometry problem.

Molarity Calculator: Concentration, Volume, and Dilute Stock Solution

Once you know the molecular weight, you can calculate molarity. Molarity (M) is the number of moles of solute per liter of solution. It is the most common way to express concentration in chemistry and biology.

Calculate Concentration From Mass and Volume

The formula is straightforward:

Molarity (M) = mass (g) ÷ (molar mass (g/mol) × volume (L))

Steps:

1. Find the molar mass using the molecular weight calculator
2. Weigh out the desired mass of your compound
3. Dissolve it in the target volume of solvent
4. Divide to get concentration in mol/L

Example: Dissolve 5.84 g of NaCl (MW 58.44) in 1 liter of water.

Molarity = 5.84 ÷ (58.44 × 1) = 0.1 M

This is a planning estimate. When preparing a solution for an assay or synthesis, account for solubility limits, purity of the compound, and the final volume after dissolving.

How to Dilute a Stock Solution

The dilution equation connects two concentrations and two volumes:

C1 × V1 = C2 × V2

• C1 = concentration of the stock solution
• V1 = volume of stock you need to take
• C2 = desired final concentration
• V2 = desired final volume

Example: You have a 10 mM stock solution and need 50 mL of a 1 mM solution.

V1 = (C2 × V2) ÷ C1 = (1 × 50) ÷ 10 = 5 mL

Take 5 mL of your stock and add solvent to a final volume of 50 mL.

This calculation assumes ideal dilution behavior. For organic solvents or highly concentrated solutions, volume changes on mixing may affect accuracy.

Weight and Mass Calculation Limitations

This molecular weight calculator is a quick reference tool. Keep these limitations in mind:

• Standard atomic weights are averages across isotopes. If you work with isotopically enriched materials, the actual mass will differ.
• Hydrates and salts must be entered with the correct formula, including any molecules of crystallization. The anhydrous MW is not the same as the hydrated form.
• Structural isomers share the same molecular formula and therefore the same molecular weight. The calculator cannot distinguish between them.
• Purity matters. The MW calculation assumes 100% pure compound. Adjust for purity when preparing real solutions.
• Macromolecules (proteins, DNA, RNA, polymers) are better served by sequence-based or domain-specific tools. A chemical formula approach becomes impractical above a few thousand Da.
• This tool does not replace professional judgment. For pharmaceutical formulations, clinical assays, or regulatory submissions, verify all values against authoritative references.

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