Atomic Mass Unit Converter

The atomic mass unit (amu), also known as the unified atomic mass unit or dalton (Da), is a standard unit of mass used to express the masses of atoms, molecules, and subatomic particles. It is defined as exactly one-twelfth of the mass of a single carbon-12 atom, which means a carbon-12 atom has a mass of exactly 12 amu by definition.

The atomic mass unit provides scientists with a convenient scale for discussing atomic and molecular masses without dealing with extremely small numbers in kilograms. For example, a hydrogen atom has a mass of approximately 1.008 amu, oxygen is about 15.999 amu, and a water molecule (H₂O) is approximately 18.015 amu. These numbers are far more practical than expressing them in kilograms, which would require scientific notation with exponents like -27.

The amu is essential in chemistry for calculating molar masses, stoichiometric ratios, and molecular weights. In nuclear physics, it is used to express the mass of protons, neutrons, and electrons, and to calculate the mass defect and binding energy of atomic nuclei.

The atomic mass unit has exact conversion factors to kilograms and other mass units, based on the defined value of the amu.

Atomic masses are measured relative to carbon-12, which serves as the standard reference. Here are the approximate masses of some common atoms and particles.

• Proton: 1.00728 amu — the nucleus of hydrogen and a building block of all atomic nuclei.
• Neutron: 1.00866 amu — slightly heavier than a proton, found in all atomic nuclei except hydrogen.
• Electron: 0.00054858 amu — approximately 1/1836 the mass of a proton, contributing negligibly to atomic mass.
• Hydrogen (¹H): 1.00783 amu — the lightest and most abundant element in the universe.
• Carbon-12: 12.00000 amu — the exact definition of the atomic mass unit.
• Oxygen-16: 15.99491 amu — the most common isotope of oxygen.
• Uranium-238: 238.05079 amu — the heaviest naturally occurring element.

Converting atomic mass units to other mass units is straightforward:

1. Enter the AMU value: Type the mass in atomic mass units into the input field. You can enter molecular weights, atomic masses, or any subatomic particle mass.
2. Read the kilogram conversion: The primary result shows the equivalent mass in kilograms.
3. View all conversions: The all-conversions panel displays your value in kilograms, MeV/c², daltons, and electron masses simultaneously.
4. Check quick references: The reference table provides common atomic masses for context.

In chemistry, the atomic mass unit is the foundation for calculating molar masses. The periodic table lists the average atomic mass of each element in amu, which equals the mass of one mole of that element in grams. For example, carbon's average atomic mass of 12.011 amu means one mole of carbon atoms weighs 12.011 grams.

Mass spectrometry instruments measure the mass-to-charge ratio of ions and report results in amu (or m/z). This technique is used in drug development, forensics, environmental testing, and protein characterization. The precision of modern mass spectrometers can measure masses to four or more decimal places, enabling identification of unknown compounds.

Nuclear physics uses the amu to calculate binding energy through the mass-energy equivalence principle (E = mc²). The mass defect — the difference between the mass of individual nucleons and the mass of the assembled nucleus — multiplied by c² gives the nuclear binding energy. This calculation explains why nuclear fission and fusion release enormous amounts of energy.