Protein mass to moles calculator (µg ↔ pmol)
Maintained by the Talindrew team · Last updated
To convert protein mass to moles, divide micrograms by the molecular weight in kilodaltons and multiply by 1,000: pmol = µg × 10³ / kDa. Ten micrograms of a 50 kDa protein is 200 pmol: about 1.2 × 10¹⁴ molecules (pmol × 6.022 × 10¹¹).
Result
pmol = µg × 10³ / kDa (Promega BioMath). Molecules = pmol × 6.022 × 10¹¹ (Avogadro). 10 µg of a 50 kDa protein is 200 pmol ≈ 1.2 × 10¹⁴ molecules.
Keep this calculation and use it on a real construct
Free account. You land on this calculator with your numbers already filled in, next to your sequences, cloning wizards and lab notebook.
What it looks like
Formula
pmol = µg × 10³ / kDa · µg = pmol × kDa / 10³ · molecules = pmol × 6.022 × 10¹¹
Source: Promega BioMath protein molar conversions; Avogadro constant (SI 2019)
Worked example
Given: 10 µg of a 50 kDa protein
- 1.pmol = 10 × 10³ / 50 = 200 pmol.
- 2.Molecules = 200 × 6.022 × 10¹¹ = 1.20 × 10¹⁴.
10 µg at 50 kDa = 200 pmol ≈ 1.2 × 10¹⁴ molecules
How the calculation flows
Units & constants
| Identity | 1 kDa = 1,000 g/mol; pmol = µg·10³/kDa follows directly |
|---|---|
| Avogadro constant | 6.02214076 × 10²³ mol⁻¹ (exact, SI 2019) |
| MW source | Sequence-derived (ProtParam / in-app protein tools) beats gel estimates |
| Typical uses | Enzyme:substrate molar ratios, binding stoichiometry, standards for quantitative MS |
| Price | Free |
When do you need protein amounts in moles?
Whenever stoichiometry matters: setting an enzyme-to-substrate molar ratio, saturating a binding partner, loading equimolar amounts of two proteins of different sizes, or preparing an internal standard. Equal masses of a 25 kDa and a 100 kDa protein differ four-fold in molar amount: mass-based loading silently biases any molar comparison.
Where should the molecular weight come from?
Compute it from the sequence when you have one: tools like ExPASy ProtParam (or Talindrew's protein properties, which implement the same method) give the exact average mass including tags. SDS-PAGE apparent MW can run 10–20% off for acidic, proline-rich or glycosylated proteins, and using it doubles as an error source in the conversion.
Frequently asked questions
6.022 × 10¹¹. Avogadro's number scaled by 10⁻¹². A picomole of anything is about six hundred billion molecules, a useful sanity anchor for single-molecule and qPCR-adjacent arithmetic.
Yes, use ~150 kDa for whole IgG. 1 µg of IgG is 6.7 fmol (0.0067 pmol × 10³/150), which is why molar excesses in antibody labelling reactions consume more reagent than intuition suggests.
Related calculators & guides
Use this calculator inside the app
Free to use. No download required. Sign in and you land right back on this calculator.