g/mol to Kilodaltons Converter
Common Conversions
| g/mol | kDa |
|---|---|
| 100 | 0.1 |
| 500 | 0.5 |
| 1000 | 1 |
| 5000 | 5 |
| 10000 | 10 |
| 25000 | 25 |
| 50000 | 50 |
| 100000 | 100 |
| 150000 | 150 |
| 500000 | 500 |
| 1000000 | 1000 |
| 5000000 | 5000 |
Why this conversion matters in chemistry
An expressed-protein construct gets a molar mass from a sequence-derived calculation in g/mol; the SDS-PAGE gel running next door has its ladder labelled in kDa. A 66,430 g/mol BSA-sized construct lands at 66.4 kDa, right between the 50 and 75 kDa standard bands. The ratio of 1000 falls cleanly out of 1 kDa = 1000 Da and the 1 Da = 1 g/mol identity. The conversion is a routine unit conversion that takes a calculated molecular weight from sequence-based protein analysis into the kDa form gel filtration columns, MWCO membranes, and Western-blot ladders are calibrated against.
Formula
Where the factor comes from
One of the two steps here is free. A mass in daltons and a molar mass in g/mol share a numerical value, so a sequence-derived 66430 g/mol is 66430 Da without arithmetic; the kilo prefix then supplies the only calculation there is, dividing by exactly 10³ to give 66.430 kDa. SI prefixes are defined as exact powers of ten, so that step introduces nothing at all. Worth noticing is which side of the pair takes the prefix. kg/mol is perfectly legal and turns up in polymer work, but biochemistry prefixes the per-particle unit instead, because gel ladders, size-exclusion columns and ultrafiltration membranes are calibrated against particles and labelled to match. The dalton to g/mol correspondence underneath is the measured part of the chain, and at a part in 10⁹ it never surfaces.
Precision and significant figures
Two or three significant figures is usually the honest ceiling, and the reason has nothing to do with the arithmetic. A sequence-derived molar mass is excellent — a polypeptide of known composition computes to within a few daltons out of tens of thousands — but nothing downstream reads at that resolution. A ladder places a band to a few percent at best. Size-exclusion columns report a hydrodynamic estimate that can sit tens of percent away for anything non-globular. Membrane cutoffs are nominal ratings describing a retention curve, not a threshold. Writing 66.430 kDa beside a gel image claims a resolution the gel does not have; keep the spare digits in the sequence calculation and round to 66 kDa on the figure.
Worked Examples
Bovine serum albumin — the calibration anchor for many protein-quantitation curves.
An IgG antibody — the reference molecular weight for any antibody-based assay.
The factor anchor — about a 9-residue peptide expressed at the kDa scale.
A small protein — about the size of an antibody light chain or chymotrypsin.
Common mistakes
Apparent gel mass read as true mass
SDS-PAGE separates by the mobility of a protein-detergent complex, which tracks mass only where SDS binding is uniform. Strongly acidic proteins, glycoproteins, membrane proteins and heavily phosphorylated species migrate anomalously and can appear tens of percent away from their sequence mass. A kDa value read off a ladder is an apparent mass, and the word belongs in the caption.
Sequence mass omits everything added later
A mass computed from the coding sequence describes the unmodified chain. Glycosylation can add tens of kDa, each disulfide bond removes about 2 Da, and signal peptide cleavage, N-terminal processing and an affinity tag all move the number. When a measured kDa disagrees with a calculated one, the gap is often a real mass difference rather than an arithmetic error.
Subunit mass mistaken for assembly mass
An IgG is quoted near 150 kDa as the assembled molecule, but a reducing gel shows heavy chains around 50 kDa and light chains around 25 kDa. Multimeric enzymes behave the same way. Deciding whether a kDa figure refers to the monomer or to the complex is not optional, since the two differ by the number of subunits.