Exact Mass Calculator

In short: Enter a chemical formula and calculate monoisotopic exact mass, nominal mass, average molecular weight, and isotope breakdown. It is free, works in your web browser and needs no download.

Enter a chemical formula and calculate monoisotopic exact mass for mass spectrometry and formula checks.

Exact Mass vs Molecular Weight

Exact mass is monoisotopic and is most useful for mass spectrometry. Molecular weight uses standard average atomic weights and is usually better for molarity, percent composition, and solution calculations.

Worked example

For caffeine, C₈H₁₀N₄O₂, the monoisotopic mass uses ¹²C = 12.000000, ¹H = 1.007825, ¹⁴N = 14.003074, and ¹⁶O = 15.994915:

8 × 12.000000 + 10 × 1.007825 + 4 × 14.003074 + 2 × 15.994915 ≈ 194.0804. Its average molecular weight is about 194.19 g/mol, so the two values differ in the first decimal place.

Using it with mass spectra

• In positive electrospray you usually see [M+H]⁺: add the mass of a proton, about 1.00728, giving 195.0877 for caffeine.
• Sodium adducts [M+Na]⁺ add about 22.98922.
• High-resolution instruments report masses to 4 or more decimals, and agreement within a few ppm supports a proposed formula.

Tip: enter formulas with correct capitalisation (Cl, not CL), because CL would be read as carbon plus an invalid symbol. To get a formula from a structure first, use SMILES to Molecular Formula.