Atomic Mass Units to Kilograms Converter

The mass of a single hydrogen atom is 1.67 × 10⁻²⁷ kg — a number so small it defies human intuition. Atomic mass units (u, also called daltons, Da) provide a manageable scale where the same atom weighs 1.008 u. The calculator for atomic mass units to kilograms converts between these two scales in either direction, for any mass from a single proton to a macromolecule.

The Exact Conversion Factor

The atomic mass unit (u) is defined as exactly 1/12 of the mass of a ¹²C atom. Its value in SI units was determined by the 2018 CODATA recommended values:

1 u = 1.66053906660 × 10⁻²⁷ kg (uncertainty: ±0.00000000050 × 10⁻²⁷ kg)

Equivalently: 1 u = 1 Da (dalton) = 1 g/mol / Nₐ = 1.66054 × 10⁻²⁷ kg, where Nₐ = 6.02214076 × 10²³ mol⁻¹ is Avogadro's number. The reciprocal: 1 kg = 6.02214 × 10²⁶ u. Use this online calculator for any mass conversion between these scales. The atomic mass calculator computes the weighted average mass in atomic mass units from isotope data.

Key Particle Masses in Both Units

The masses of fundamental particles in atomic mass units and kilograms:

• Proton: 1.007276 u = 1.67262 × 10⁻²⁷ kg
• Neutron: 1.008665 u = 1.67493 × 10⁻²⁷ kg
• Electron: 0.000549 u = 9.10938 × 10⁻³¹ kg (about 1/1836 of a proton)
• Hydrogen atom: 1.007825 u = 1.67353 × 10⁻²⁷ kg
• Carbon-12: exactly 12.000000 u = 1.99265 × 10⁻²⁶ kg
• Water molecule (H₂O): 18.01528 u = 2.99151 × 10⁻²⁶ kg
• Antibody IgG: ~150,000 u (150 kDa) = 2.49 × 10⁻²² kg

Dalton and Kilodalton: Common Usage in Biochemistry

In biochemistry and molecular biology, the dalton (Da) and kilodalton (kDa) are the preferred units for macromolecular masses: 1 Da = 1 u; 1 kDa = 1,000 u. Protein molecular weights are routinely expressed in kDa — a small protein might be 15 kDa, a typical enzyme 50–100 kDa, a large antibody 150 kDa. Mass spectrometry (ESI-MS, MALDI-MS) measures protein masses directly in daltons with sub-dalton precision, enabling protein identification, modification detection, and quality control in biopharmaceutical manufacturing. The amino acid molecular weight calculator computes protein molecular weights that are directly expressible in daltons.

Nuclear Physics: Mass Excess and Binding Energy

In nuclear physics, the mass excess Δ expresses how much a nuclide's actual mass differs from its mass number in unified atomic mass units: Δ = (actual mass in u − mass number in u) × 931.5 MeV/u. Converting between u and kg is essential when computing binding energies: E_binding = (Z × m_proton + N × m_neutron − m_nucleus) × c² where c = 3 × 10⁸ m/s. The nuclear binding energy per nucleon peaks around iron (Fe-56) and determines stellar nucleosynthesis — elements heavier than iron require energy to form, which is why they are produced only in supernova explosions rather than in normal stellar burning. The nuclear binding energy calculator applies these mass-to-energy conversions directly.