Molecular Weight Calculator
The molecular weight calculator is a simple tool that computes the mass of a given molecule without needing to look up the periodic table manually. Select each element in your chemical formula, enter its atom count, and instantly get the molecular weight in unified atomic mass units (u / Da) together with the molar mass in g/mol — or switch to the American system to see results in lb/mol and oz/mol.
How to Use the Molecular Weight Calculator
- Choose your unit system: Metric (g/mol, Da) or American (lb/mol, oz/mol).
- Select the first element from the dropdown and enter how many atoms of that element the molecule contains.
- Click "+ Add Element" to add more rows — repeat for every element in your formula (up to 10).
- Press "Calculate" to get the results. The full chemical formula, molecular weight, molar mass, and percent elemental composition are shown instantly.
Results are displayed in unified atomic mass units [u] = [Da] (daltons). The molar mass equals the same numerical value in g/mol because 1 u ≡ 1 g/mol.
Molecular Weight vs. Molar Mass
Although these terms are often used interchangeably, they describe different things:
- Molecular weight (MW) — the mass of a single molecule, expressed in daltons (Da) or unified atomic mass units (u). Example: one water molecule has MW = 18.015 u.
- Molar mass (M) — the mass of one mole (6.022 × 10²³ molecules) of a substance, expressed in g/mol. Example: molar mass of water = 18.015 g/mol.
Because the numerical values are the same, this calculator displays both side by side.
How to Calculate Molecular Weight
The general formula is:
MW = Σ (counti × atomic massi)
Example — glucose (C₆H₁₂O₆):
- C: 6 × 12.011 = 72.066 u
- H: 12 × 1.008 = 12.096 u
- O: 6 × 15.999 = 95.994 u
- MW = 180.156 u
Example — water (H₂O):
- H: 2 × 1.008 = 2.016 u
- O: 1 × 15.999 = 15.999 u
- MW = 18.015 u
Unit Systems
| Unit | System | Value for H₂O |
|---|---|---|
| Da (u) | Universal | 18.015 Da |
| g/mol | Metric | 18.015 g/mol |
| lb/mol | American | 0.039724 lb/mol |
| oz/mol | American | 0.635588 oz/mol |
Periodic Table — Curiosities
The periodic table (Mendeleev table) now contains 118 confirmed elements. The last four — nihonium (Nh), moscovium (Mc), tennessine (Ts), and oganesson (Og) — were officially named in 2016. This calculator includes all 118 elements with their IUPAC standard atomic weights.
Four elements are named after American places or scientists:
- Lawrencium (Lr, 103) — after Ernest Lawrence, inventor of the cyclotron
- Seaborgium (Sg, 106) — after nuclear chemist Glenn T. Seaborg
- Tennessine (Ts, 117) — after the state of Tennessee
- Californium (Cf, 98) — after the state of California
Related Calculators
- Mole Calculator — convert grams ↔ moles
- Molarity Calculator — find molar concentration
- Avogadro's Number Calculator — moles ↔ particles
- Percent Yield Calculator — reaction efficiency