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Short answer: 1 kDa = 1,000 Da = 1,000 g/mol. A dalton and a gram per mole are numerically identical, so converting a molecular weight is a change of scale only: multiply kDa by 1,000 to get g/mol, and divide g/mol by 1,000 to get kDa. A 66 kDa protein therefore has a molar mass of about 66,000 g/mol.
Is a dalton the same as a gram per mole?
Numerically yes; dimensionally no. The dalton (Da) is a unit of mass for a single particle. NIST’s guide to the SI lists the dalton and the unified atomic mass unit (u) as alternative names and symbols for the same unit (NIST SP 811, chapter 5, read 2026-11-07). Its size is tied to the atomic mass constant, mu = 1.660 539 068 92 × 10-27 kg in the 2022 CODATA adjustment (NIST CODATA value, read 2026-11-07).
Molar mass, in g/mol, is the mass of one mole of entities. A mole contains exactly 6.02214076 × 1023 elementary entities (NIST, SI unit of amount of substance, read 2026-11-07). Multiplying the mass of one particle in daltons by that number of particles gives, to within a tiny fraction of a part per billion, one gram per mole: 1 Da × NA ≈ 1 g/mol. That is why the two numbers match for any molecule, and why a protein of “66 kDa” and a molar mass of “66,000 g/mol” describe the same figure.
Convert kDa and Da to g/mol (and back)
The rule never changes: g/mol = Da, and kDa = Da / 1,000.
| You have | Operation | Result |
|---|---|---|
| 66 kDa | × 1,000 | 66,000 g/mol (also 66,000 Da) |
| 25,000 Da | no change in value | 25,000 g/mol (also 25 kDa) |
| 150,000 g/mol | no change in value; ÷ 1,000 for kDa | 150,000 Da = 150 kDa |
| 0.5 kDa | × 1,000 | 500 g/mol |
These are arithmetic examples of the unit conversion, not measured masses of particular proteins.
Da to g/mol, dalton to g/mol converter, g/mol to Da, g/mol to kDa
All of these calculators and converters do the same thing. Da to g/mol and g/mol to Da: keep the number. kDa to g/mol: multiply by 1,000. g/mol to kDa: divide by 1,000. You do not need a dedicated tool; a spreadsheet cell or a hand calculation is enough, and the only error to avoid is the factor of 1,000.
How do I convert nanomoles to micrograms?
Multiply the amount in nanomoles by the molar mass in g/mol: the product is in nanograms. Divide by 1,000 for micrograms.
µg = nmol × (g/mol) / 1,000
Worked example: 10 nmol of a 66 kDa protein is 10 × 66,000 = 660,000 ng = 660 µg. The same chain works for picomoles (pmol × g/mol gives pg) and for the reverse direction: nmol = ng / (g/mol). For mg/mL working solutions and concentrations in micromolar, see the protein molarity calculator guide.
How do I calculate grams from moles?
grams = moles × molar mass (g/mol). Moles from grams is the inverse: moles = grams / (g/mol). For a macromolecule, take the molar mass in g/mol from its kDa value as above. For a small compound, add the atomic weights of its atoms; the guide to calculating molar mass walks through that.
What is the molar mass of oxygen?
It depends on whether you mean the atom or the molecule. NIST publishes the standard atomic weight of oxygen as an interval, [15.99903, 15.99977] (NIST atomic weights and isotopic compositions, read 2026-11-07), so a single atom is about 16 g/mol and the diatomic molecule O2, which is what the gas is, is about twice that, roughly 32 g/mol. State which one a calculation uses.
Common misunderstandings
- kDa versus kD versus KD. kDa is a mass unit. A capital K and D in KD usually denotes a dissociation constant, which has concentration units and is unrelated to mass. Check the context before converting.
- Per-particle versus per-mole. Da describes one particle; g/mol describes a mole of them. Only the numbers coincide, so keep the unit label when you write a result.
- Average versus monoisotopic mass. Mass spectrometry reports can quote either. The difference does not change the Da-to-g/mol rule, but make sure the value you convert is the one your calculation needs. Sequence-based tools such as ProtParam predict a mass from sequence alone.
- Subunit versus complex. A homodimer of two 30 kDa chains is 60 kDa as a complex. Use the mass of the species actually in your tube. Methods that measure intact complexes, such as mass photometry, report the assembled mass.
- Gel estimate versus true mass. A band position on a gel gives an apparent size. Modifications and tags change the true mass, and some change migration without changing mass equally, so a gel-derived kDa is an estimate, not a molar mass for precise molar calculations.
- mg/mL is g/L. Dividing a concentration in g/L by the molar mass in g/mol gives mol/L, which is the step many errors hide in. See moles to molarity.
FAQ
Is Da g per mol?
Numerically, yes: 1 Da corresponds to 1 g/mol. They are different quantities (mass of a particle versus mass per mole), but the number is the same.
How do I convert kDa to g/mol?
Multiply by 1,000. 1 kDa is 1,000 g/mol.
How do I convert g/mol to Da or kDa?
g/mol to Da keeps the number. For kDa, divide by 1,000.
How do I convert nanomoles to micrograms?
Multiply nmol by the molar mass in g/mol and divide by 1,000.
How to calculate g from mol?
Multiply moles by molar mass in g/mol.
What is the molar mass of oxygen?
About 16 g/mol for the atom and about 32 g/mol for O2; see the NIST atomic weight interval above.
Sources
- NIST, Guide for the Use of the International System of Units (SP 811), chapter 5: units outside the SI, read 2026-11-07.
- NIST, CODATA atomic mass constant, 2022 values, read 2026-11-07.
- NIST, SI units: amount of substance, read 2026-11-07.
- NIST, Atomic weights and isotopic compositions: oxygen, read 2026-11-07.
