Molecular Weight Calculator (Molar Mass)

What is molecular weight?

Molecular weight (molar mass) is the total mass of one mole of a compound, found by summing the atomic mass of every atom named in its chemical formula:

$MW = \sum (\text{atoms} \times \text{atomic mass})$

Enter a formula above — element symbols, subscripts for atom counts, and grouping with () or [] — and the calculator parses it, looks up each element's IUPAC standard atomic mass, and reports the total in grams per mole (g/mol).

Accepted formula syntax

• Element symbols are case-sensitive: Na (sodium) is valid, NA is not.
• Subscripts after a symbol give its atom count; no digit means 1 atom (NaCl = 1 Na + 1 Cl).
• Groups in () or [] followed by a number multiply every atom inside by that number: Ca(OH)2 means 1 Ca plus 2 of the OH group (2 O, 2 H).
• Nesting is unlimited: Fe2(SO4)3 multiplies the sulfate group by 3 inside a formula that also carries an Fe subscript.

Worked example 1: water (H2O)

1. Count atoms: 2 hydrogen, 1 oxygen.
2. Look up masses: H = 1.008, O = 15.999.
3. Multiply and add: 2 × 1.008 + 1 × 15.999 = 2.016 + 15.999 = 18.015 g/mol.

Worked example 2: table salt (NaCl)

1. Count atoms: 1 sodium, 1 chlorine.
2. Look up masses: Na = 22.990, Cl = 35.45.
3. Multiply and add: 22.990 + 35.45 = 58.44 g/mol.

Worked example 3: calcium hydroxide (Ca(OH)2) — a nested group

1. Calcium: 1 × 40.078 = 40.078.
2. The OH group, multiplied by 2: (15.999 + 1.008) × 2 = 17.007 × 2 = 34.014.
3. Add: 40.078 + 34.014 = 74.092 g/mol.

From molar mass to percent composition

Once you have the total molar mass, you can find what share of that mass comes from each element — its percent composition — by dividing that element's subtotal by the molar mass and multiplying by 100. For water: hydrogen's share is 2.016 ÷ 18.015 × 100 ≈ 11.19%, and oxygen's share is 15.999 ÷ 18.015 × 100 ≈ 88.81%. The two shares always add up to 100%, and the percentage calculator handles that final division-and-multiply step if you want it worked out separately.

Common mix-ups

• Forgetting the group multiplier applies to every atom inside. In Fe2(SO4)3, the ×3 covers all 4 oxygens and the sulfur in each sulfate group — 3 S and 12 O total, not 3 O.
• Mixing up case-sensitive symbols. Co (cobalt) and CO (carbon + oxygen) parse as completely different things; a lowercase-second-letter typo changes the whole formula.
• Using atomic number instead of atomic mass. Oxygen's atomic number is 8, but its atomic mass is 15.999 — the calculator always uses mass, since that's what determines a mole's weight.

Where this shows up

Molar mass is the bridge between a lab-bench mass in grams and a chemically meaningful count of particles in moles — it's the same relationship the ideal gas law calculator uses when it converts between an amount of gas in moles and other measurable quantities. Very small atomic-scale masses or huge molar quantities (a mole is about 6.022 × 10²³ particles) are often easier to read and compare in scientific notation than written out in full.