Atomic Mass Unit To Kilogram Converter

1. Keep the direction on u to kg, or switch to kilograms to atomic mass units.
2. Type the mass in u or Da, for example 12 for a carbon-12 atom or 50000 for a 50 kDa protein.
3. Read the kilograms in scientific notation. The cards add grams, kilodaltons and the molar mass that matches that particle mass.

The unified atomic mass unit, symbol u, is a unit of mass for atoms and molecules. It equals 1/12 of the mass of a free carbon-12 atom, at rest and in its ground state. NIST lists the dalton (Da) and the unified atomic mass unit as two names for the same unit.

The unit is not part of the SI, but it is accepted for use with it. Its value in kilograms is not fixed by definition. It is measured, and CODATA publishes a recommended value. The 2022 CODATA value is 1.66053906892 x 10^-27 kg, with a standard uncertainty of 0.00000000052 x 10^-27 kg.

Chemists and physicists usually write u. Biochemists write Da and kDa for proteins, so a 50 kDa protein has a mass of 50,000 u. Many textbooks still write amu; on this page amu, u and Da mean the same unit.

Do not confuse the unit with the number beside an element on a periodic table. That number is a relative atomic mass, a pure ratio with no unit. A mass in u uses the same number with a unit attached.

You multiply the mass in u by the CODATA value of one u in kilograms.

1. Multiply the number part: for 12 u, 12 x 1.66053906892 = 19.92646882704.
2. Keep the power of ten: 19.92646882704 x 10^-27 kg.
3. Normalize so one digit sits before the point: 1.992646882704 x 10^-26 kg.

The Link to Molar Mass

Multiply the u value by the Avogadro constant and you get molar mass. The Avogadro constant is exactly 6.02214076 x 10^23 per mole. The product of the two constants is the molar mass constant, Mu = 1.00000000105 x 10^-3 kg/mol in CODATA 2022.

So a particle of 18 u has a molar mass of about 18 g/mol. This bridge lets a chemist go from one molecule to grams on a balance. The grams to milligrams converter helps when that weighed amount is small.

Carbon-12 atom. By definition it is exactly 12 u. 12 x 1.66053906892 x 10^-27 = 1.9926468827 x 10^-26 kg. Its matching molar mass is 12.0000000126 g/mol, not exactly 12.

A protein. A 50 kDa protein is 50,000 u. One molecule is 8.3026953446 x 10^-23 kg, and its molar mass is about 50,000 g/mol, which is 50 kg/mol.

Counting molecules. How many of those protein molecules are in 1 mg? 1 mg is 10^-6 kg. Divide by one molecule: 10^-6 / 8.3026953446 x 10^-23 = about 1.2044 x 10^16 molecules.

Reverse. A particle of 1 x 10^-26 kg is 1 x 10^-26 / 1.66053906892 x 10^-27 = 6.02214075367 u. For more practice with powers of ten, see the grams to kilograms converter.

The factor is a measured constant, so the value you use and the digits you keep both matter.

Which CODATA Release You Use

CODATA revises its recommended values over time. An older NIST table still shows the 2014 value, 1.660539040 x 10^-27 kg. It differs from the 2022 value by about 1.7 parts in 100 million, which matters only in precision work.

Rounded Constants

Many textbooks use 1.66 x 10^-27 kg. That is 0.032 percent low. Using 1.6605 x 10^-27 kg cuts the error to 0.0024 percent.

Uncertainty of the Constant

The 2022 value has a relative standard uncertainty of 3.1 x 10^-10. Digits beyond about the tenth significant figure are not meaningful, even though the tool shows 12.

Isotopes and Averages

A single atom or isotope has one mass in u. A periodic-table atomic weight is an average over natural isotopes, so it describes a typical sample, not one atom.

The atomic mass unit and the dalton are the same unit. Molar mass is a different quantity that uses almost the same number.

Before the 2018 SI revision, one mole was defined by the atoms in 0.012 kg of carbon-12. Under that rule, Mu was exactly 1 g/mol. Now the Avogadro constant is fixed instead, and Mu is measured. It is 1.00000000105 g/mol, just over 1 part per billion above 1.

Physics Problems

Formulas for momentum, kinetic energy or mass defect need SI units. A particle mass given in u must become kilograms first.

Chemistry and Molar Mass

Stoichiometry links one molecule to a weighed sample. Converting u to kg, or to g/mol, shows how the per-particle mass scales up to a mole.

Biochemistry and Mass Spectrometry

Protein and DNA masses come in Da or kDa. Converting one molecule to kilograms, then dividing a sample mass by it, counts the molecules in a sample.

Values below use the CODATA 2022 constants, rounded to 12 significant figures. Molar mass uses Mu = 1.00000000105 g/mol per u.

1. Using the Gram Value as Kilograms

1 u is 1.66053906892 x 10^-24 g but 10^-27 kg. Mixing the two powers gives an answer 1,000 times off.

2. Confusing U with Atomic Number

The atomic number counts protons. Carbon has atomic number 6 but carbon-12 has a mass of 12 u.

3. Treating Molar Mass as Exactly the U Value

Since the 2018 SI revision, Mu is 1.00000000105 g/mol, not exactly 1. The gap is about one part per billion, so it rarely matters outside metrology.

4. Using 1/NA Grams as the U

1 / 6.02214076 x 10^23 gives 1.66053906717 x 10^-24 g. That is close, but it is not the CODATA value of the u.

5. Reporting Too Many Digits

The constant is known to about 10 significant figures. Writing 15 digits claims precision nobody has. The scientific notation calculator helps round long values cleanly.

The tool is as accurate as the CODATA 2022 constant, a relative uncertainty of 3.1 x 10^-10.

One atomic mass unit is 1.66053906892 x 10^-27 kg, the 2022 CODATA recommended value. Its standard uncertainty is 0.00000000052 x 10^-27 kg.

Yes. The dalton (Da) and the unified atomic mass unit (u) are two names for the same unit, 1/12 of the mass of a carbon-12 atom.

One kilogram is about 6.02214075367 x 10^26 u. That is the inverse of the CODATA value of one u.

One gram is about 6.02214075367 x 10^23 u. This is very close to, but not exactly, the Avogadro number.

Since the 2018 SI revision, the Avogadro constant is fixed and the u is measured. The two differ by about 1 part per billion.

One u is 1.66053906892 x 10^-24 g. Multiply the kilogram value by 1,000.

A kilodalton (kDa) is 1,000 Da. Proteins are often described in kDa, so a 66.5 kDa protein has a mass of 66,500 u.

amu is still common in classrooms. Current NIST tables use u and Da, and this converter treats all three as one unit.

Multiply the mass in u by 1.00000000105 to get g/mol. For everyday chemistry the number in u and the number in g/mol are the same.

No. The converter runs in your browser and nothing is sent to our servers. Anything you choose to Save stays in this browser only.

• CODATA 2022 value of the atomic mass unit in kilograms (NIST Reference on Constants, Units and Uncertainty).
• Avogadro constant, exact since the 2018 SI revision (NIST Reference on Constants, Units and Uncertainty).
• Molar mass constant, CODATA 2022 (NIST Reference on Constants, Units and Uncertainty).
• Non-SI units accepted for use with the SI, including the dalton and u (NIST).
• The redefinition of the mole and the earlier carbon-12 definition (NIST).