Atomic Mass Units to Kilograms Converter
Common Conversions
| amu | kg |
|---|---|
| 1 | 1.66054e-27 |
| 2 | 3.32108e-27 |
| 4 | 6.64216e-27 |
| 12 | 1.99265e-26 |
| 16 | 2.65686e-26 |
| 28 | 4.64951e-26 |
| 32 | 5.31373e-26 |
| 56 | 9.29902e-26 |
| 100 | 1.66054e-25 |
| 200 | 3.32108e-25 |
| 1000 | 1.66054e-24 |
| 10000 | 1.66054e-23 |
Why this conversion matters in chemistry
De Broglie wavelength calculations hits this regularly. A neutron at thermal energy (kT at 300 K, about 25 meV) carries 1.008 u of mass — 1.675 × 10⁻²⁷ kg — and lands at a wavelength of about 1.8 Å, the right scale to diffract off atomic spacings in a crystal. The ratio of 1.66054 × 10⁻²⁷ kg per u was a measured constant before 2019; the SI redefinition that fixed Avogadro's number at exactly 6.02214076 × 10²³ /mol made the equality exact. It comes up when atomic-scale physics has to come out in SI kilograms.
Formula
Where the factor comes from
This pair joins two mass scales that are now realized independently. Since 2019 the kilogram follows from a fixed value of the Planck constant, reached through a Kibble balance or a silicon sphere. The dalton still points at carbon-12, one twelfth of a free atom's ground-state mass. Nothing in the SI forces those two routes to agree at any particular number, so their ratio has to be measured. The current value, 1.66053907 × 10⁻²⁷ kg per u, carries a relative uncertainty near three parts in 10¹⁰ and shifts slightly with each CODATA adjustment. Reaching it by dividing 10⁻³ kg/mol by Nₐ works to nine digits, because the molar mass constant stays within about one part in 10⁹ of 1 g/mol — but that near-equality is now a finding, not a definition.
Precision and significant figures
Six figures on the factor cover any use a chemist has for it; the digits past that belong to metrology and move between CODATA releases. Instrumental reality sets the ceiling well before the constant does — an FT-ICR or orbital-trap analyzer quoting single-ppm mass accuracy works at 10⁻⁶, more than three decades coarser than the factor's uncertainty, so rounding to 1.6605 × 10⁻²⁷ costs nothing real. Care pays downstream instead. Kilogram-scale particle masses feed expressions where the mass sits under a square root or multiplies constants carrying their own digits: de Broglie wavelengths, root-mean-square speeds, kinetic energies. Carry one guard figure through and round at the end.
Worked Examples
The conversion anchor — one atomic mass unit in SI kilograms.
One carbon-12 atom — the calibration anchor for the entire u scale.
Avogadro's number of u — exactly one gram, by construction of the mole.
One oxygen-16 atom — useful for any per-atom mass calculation in SI units.
Common mistakes
Leaving mass in grams in SI formulas
Constants written in joules and joule-seconds decompose into kg·m²·s⁻², so every mass in the same expression has to be in kilograms. A gram figure is numerically a thousand times the kilogram one, so feeding it to λ = h/mv shrinks the wavelength by that same thousand — a thermal neutron's 1.8 Å collapses to 0.0018 Å, far below any lattice spacing it could diffract from.
Particle mass and molar mass in one formula
Root-mean-square speed is √(3RT/M) with M the molar mass in kg/mol, or √(3kT/m) with m the single-particle mass in kg. The forms are equivalent only when R and k each keep their matching mass quantity. Pairing R with a per-atom kilogram mass inflates the speed by roughly √Nₐ, about 10¹², putting the answer well past light speed.
Tabulated u values include the electrons
Atomic mass tables list neutral-atom masses, electrons and their binding energy included. Converting straight to kilograms and calling the result a nuclear mass overstates it — hydrogen's atomic 1.00783 u against the proton's 1.00728 u — which matters once the kilogram figure heads into E = mc² and a binding-energy calculation. Subtract Z electron masses first.