Non-SI unit accepted for use with SI
Name Symbol Quantity Value in SI units
dalton Da mass 1.660 539 040 (20) × 10−27 kg
1.660 539 040 (20) yg


The dalton, symbol Da, or unified atomic mass unit, is a non-SI unit of mass accepted for use with the SI.

One dalton is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest.

In SI units, one dalton is 1.660 539 040 (20) × 10−27 kg, or approximately 1.66 yoctograms.

1 \ \text{Da} = \dfrac{1}{12} \ m \mspace{4mu}(^{12} \text{}C)

The dalton is used as a unit of mass for quantities on an atomic or molecular scale. One dalton is approximately the mass of one nucleon (either a single proton or neutron), and is approximately equivalent to 1 g mol-1.

Examples of atomic and molecular masses
Name Symbol/formula Mass in Da Mass in SI units
hydrogen H 1.00 1.66 yg
carbon-12 12C 12.00 19.93 yg
silicon-28 28Si 27.98 46.46 yg
iron-56 56Fe 55.93 92.88 yg
uranium-235 235U 235.04 390.29 yg
water H2O 18.01 29.91 yg
carbon dioxide CO2 44.01 73.08 yg
ethanol C2H5OH 46.07 76.50 yg
benzene C6H6 78.11 129.70 yg
human haemoglobin ≈ 64 000 ≈ 106 000 yg


Prior to 2019, the mole was defined in terms of the kilogram and the dalton, and was equal to

“the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12”.

The numerical value of the mole is now defined exactly, and the value of the dalton in SI units must be obtained experimentally.