Atomic mass unit

The scale atoms are weighed on.

Mass

How it's defined

One twelfth of the mass of a carbon-12 atom. Unlike everything else on this page, its kilogram value is measured, not defined: about 1.66053906892×10⁻²⁷ kg (CODATA) — the number carries uncertainty.

1 u ≈ 1.66053906892×10⁻²⁷ kg — measured, not exact

Convert — honestly

u equals

1.6605e-27 kg

Kilogram · factor exact — the base unit itself

1.6605e-24 g

Gram · factor exact — exact by definition

1.6605e-30 t

Tonne · factor exact — exact by definition

3.6609e-27 lb

Pound · factor exact — exact by the 1959 agreement

Conversions run through the kilogram using the stated factors — no rounding beyond display precision, and every factor says whether it's exact or measured.

Where you meet it

Elementii — every atomic mass on every element page is in this unit ↗ open elementii
Biochemistry, where proteins are quoted in kilodaltons

Other ways to measure mass

kg Kilogram g Gram t Tonne lb Pound

Source

This page is an authored summary of the authoritative definition — checkable one click away.

CODATA recommended values (via NIST)