Daltons to Kilodaltons Converter
Common Conversions
| Da | kDa |
|---|---|
| 100 | 0.1 |
| 500 | 0.5 |
| 1000 | 1 |
| 5000 | 5 |
| 10000 | 10 |
| 25000 | 25 |
| 50000 | 50 |
| 66500 | 66.5 |
| 100000 | 100 |
| 150000 | 150 |
| 500000 | 500 |
| 1000000 | 1000 |
Why this conversion matters in chemistry
Peptides come off a mass spectrometer in daltons — a tryptic fragment around 1500 Da, an intact small protein around 14,000. SDS-PAGE bands and size-exclusion chromatography apparent molecular weights live in kilodaltons — lysozyme at 14.3 kDa, BSA at 66.5, an IgG at 150. The conversion is dividing by 1000, but it earns its keep when a precision MS measurement of a peptide has to land in the same conversation as a gel-band annotation done at three significant figures. Same number, different scale, depending on which technique generated it.
Formula
Where the factor comes from
Among the mass conversions in the dalton family, this is the only one whose factor owes nothing to experiment. Kilo has meant 10³ since the metric prefixes were laid down, and attaching it to the dalton rescales the unit by exactly a thousand without touching the dalton's own definition — one twelfth of a free carbon-12 atom's ground-state mass, a quantity whose value in kilograms is measured but whose relation to the kilodalton is not. Dividing by 1000 is exact in the strict sense: an integer ratio, not a rounded constant. The same prefix rule continues upward, so masses past 10⁶ Da are conventionally written in megadaltons, which is where ribosomes, capsids and large assemblies are usually quoted.
Precision and significant figures
Because 1000 is exact, digits pass straight through: 66,430 Da is 66.430 kDa, not 66.4. Truncating during the conversion is the usual way precision leaks here, and it matters because the two units tend to carry very different measurement pedigrees. A dalton value from a mass spectrometer may be good to a few parts per million. A kilodalton value read off an SDS-PAGE ladder is an interpolation between markers and rarely deserves more than three significant figures, sometimes fewer for glycosylated or unusually charged proteins. Round to match the technique that produced the number, and name that technique when the value is quoted.
Worked Examples
Bovine serum albumin (BSA) — the standard protein-chemistry molecular-weight reference.
Roughly the mass of a small peptide — about nine residues, depending on composition.
An intact IgG antibody — the size that gives a single band on a non-reducing SDS-PAGE gel.
About the size of a small monomeric enzyme like chymotrypsin or trypsin (~23–25 kDa) — the kind that gives a clean single band at the lower end of an SDS-PAGE gel.
Common mistakes
Rounding away digits the measurement earned
Writing 8564.8 Da as 8.6 kDa is a reasonable choice for a figure legend and a loss of three significant figures if the value is headed for an extinction-coefficient or molarity calculation. The conversion costs nothing; the habit of tidying to one decimal in the same keystroke does. Convert first, then round once, at the end.
Gel apparent mass treated as true mass
An SDS-PAGE band yields apparent molecular weight from mobility relative to markers, and heavily glycosylated, strongly acidic or membrane-spanning proteins migrate anomalously, sometimes by tens of percent. A precise dalton measurement converted to kilodaltons that disagrees with the gel is a common outcome, and it usually says something about the protein rather than about the arithmetic.
Which species the number describes
A reduced antibody runs as roughly 50 and 25 kDa chains while the intact molecule sits near 150 kDa, and a homodimer reports half its mass under denaturing conditions. Dividing by 1000 is trivial; attaching the result to the wrong species is not. Record whether the sample was reduced, denatured or native alongside the value itself.