Kilodaltons to g/mol Converter
Common Conversions
| kDa | g/mol |
|---|---|
| 0.1 | 100 |
| 0.5 | 500 |
| 1 | 1000 |
| 5 | 5000 |
| 10 | 10000 |
| 25 | 25000 |
| 50 | 50000 |
| 66.5 | 66500 |
| 100 | 100000 |
| 150 | 150000 |
| 500 | 500000 |
| 1000 | 1000000 |
Why this conversion matters in chemistry
Molarity calculations on a protein stock use this conversion regularly. A 2 mg/mL solution of a 150 kDa antibody is 2 mg/mL ÷ 150,000 g/mol = 1.33 × 10⁻⁵ M, or 13.3 µM. The kDa figure off a Western-blot ladder or a structural-biology PDB entry expands by 1000 to give the g/mol form a molarity equation expects. The factor falls cleanly out of 1 kDa = 1000 Da and the 1 Da = 1 g/mol identity. The conversion is the everyday first step bridging a protein-scale molecular weight in biochemist's units and the per-mole arithmetic a stoichiometry or binding calculation runs in.
Formula
Where the factor comes from
Two relations compose here and only one of them is exact. The first is the prefix: 1 kDa = 1000 Da, an integer by definition. The second is the numerical identity between the dalton and the gram per mole, which since 2019 is no longer definitional. Fixing the Avogadro constant at 6.02214076 × 10²³ mol⁻¹ cut it loose from the old carbon-12 mole, and the molar mass constant — the quantity that had made 1 Da equal 1 g/mol exactly — became something measured. It now sits about one part in 10⁹ away from 1 g/mol, with a relative uncertainty near three parts in 10¹⁰. So the ×1000 is exact while the identity beneath it is an approximation, superb but finite. For protein work the distinction is bookkeeping; it is still worth knowing which half of the factor is which.
Precision and significant figures
The one-part-in-10⁹ gap under the identity will never be the limiting term in a protein calculation, since nothing else in the chain comes within six decades of it. The digits are decided by the kilodalton figure instead. A mass computed from sequence rests on standard atomic weights, which are measured and in several cases published as intervals to cover natural isotopic variation, so five figures is about the ceiling. Everything downstream is coarser: an extinction coefficient predicted from aromatic content typically carries a few percent, and any stock concentration built on it inherits that. Two or three significant figures on the resulting molarity is honest. Carry g/mol at full width and round once, at the answer.
Worked Examples
BSA — the molar mass that anchors many quantitation calibration curves.
IgG antibody — the per-mole figure for any antibody-binding stoichiometry.
The conversion anchor — about a 9-residue peptide expressed in g/mol.
A small protein — chymotrypsin-sized, useful as a low-MW reference.
Common mistakes
The shortcut that hides its units
Dividing mg/mL by kDa returns millimolar directly — 2 mg/mL of a 150 kDa antibody gives 0.0133 mM — which is convenient and also where the units quietly disappear. Two factors of 1000 cancel to make it work, and nothing on the page records that. Write the g/mol value out at least once in the calculation so the cancellation stays visible to whoever checks it.
Molar and percent extinction coefficients
An absorbance at 280 nm becomes a concentration through one of two constants, and the g/mol value is what connects them. The molar coefficient in M⁻¹cm⁻¹ is the absorbance of a 1 g/L solution multiplied by the molar mass; the percent form, quoted for a 1 % w/v solution, is ten times that per-gram figure. Pick the wrong one and the concentration lands a decade or a molar mass away.
A conjugate's kDa is a distribution
PEGylated proteins, polysaccharides and polymer carriers do not have a molar mass; they have a distribution whose number average and weight average differ, sometimes substantially. A single kDa label expanded to g/mol and dropped into n = m/M returns a mole count that depends on which average the label meant. For a defined polypeptide the question never arises; for anything polymeric it decides the answer.