Atomic Mass Unit Calculator: Convert u to kg & g/mol

This atomic mass unit calculator converts between atomic mass units (u, also called daltons, Da) and kilograms or grams per mole (g/mol), and it can also convert a given mass in u to the equivalent mass in grams or kilograms. You enter a value in atomic mass units (or in another unit), select the output unit, and the calculator returns the converted mass using the exact definition 1 u = 1.66053906660 × 10�²� kg. The tool also shows the molar mass in g/mol, which is numerically identical to the mass in u per particle.

Formula

The atomic mass unit (u) is defined as exactly one-twelfth of the mass of an unbound neutral carbon-12 atom in its ground state. In SI units:

1 u = 1.66053906660 × 10�²� kg (CODATA 2018 recommended value).

To convert a mass in u to kilograms, multiply by that factor:

m(kg) = m(u) × 1.66053906660 × 10�²�

To convert to grams, multiply by 10�³ after converting to kg, or directly use the factor 1.66053906660 × 10�²� g.

The molar mass in grams per mole (g/mol) is numerically equal to the mass in u per atom or molecule. For example, a carbon-12 atom has a mass of 12 u, so one mole of carbon-12 atoms has a mass of 12 g. This is not a coincidence—the mole is defined so that this holds exactly for carbon-12.

Worked Example

Using the default value of 12 u (the mass of a carbon-12 atom):

1. Convert to kilograms: 12 × 1.66053906660 × 10�²� = 1.99264687992 × 10�²� kg.
2. Convert to grams: 12 × 1.66053906660 × 10�²� = 1.99264687992 × 10�²³ g.
3. Molar mass: 12 u corresponds to 12 g/mol.

The calculator shows these results with 10 decimal places in scientific notation, matching the above.

Why Use Atomic Mass Units?

Atomic and molecular masses are incredibly small when expressed in kilograms. A single proton weighs about 1.6726 × 10�²� kg, which is unwieldy. The atomic mass unit gives chemists and physicists a convenient scale where a proton is roughly 1 u, a neutron is roughly 1 u, and an electron is about 0.00054858 u. This makes it easy to compare masses of atoms and molecules without juggling exponents.

The unit is also central to the definition of the mole. One mole of any substance contains Avogadro’s number (6.02214076 × 10²³) of particles. Because of the way the mole is defined, the mass of one mole of a substance in grams is numerically equal to the average mass of one particle in u. This is why the molar mass of water is 18.015 g/mol—each water molecule has a mass of about 18.015 u.

How to Use the Calculator

Enter any positive number in the “Mass in atomic mass units� field. The calculator will instantly show:

• Mass in kilograms (scientific notation)
• Mass in grams
• Molar mass in g/mol (same numeric value as the input in u)

For example, if you enter 1, you get the mass of a single proton or hydrogen atom (approximately 1.0078 u, but the calculator uses 1 u exactly). If you enter 56, you get the mass of an iron-56 atom (about 55.9349 u, but again the calculator uses the exact number you type).

The calculator is unit-agnostic: it does not know whether you are entering the mass of an atom, a molecule, or a made-up value. It simply applies the conversion factor. For real atoms, you would look up the actual mass in u from a periodic table or a mass table.

Common Mistakes and Misconceptions

One frequent error is confusing atomic mass (in u) with mass number (the total number of protons and neutrons). The mass number is an integer, while the atomic mass is a weighted average of isotopes. For example, carbon has a mass number of 12, but the average atomic mass of natural carbon is 12.011 u because of the presence of carbon-13 and carbon-14.

Another mistake is mixing up “atomic mass� and “molar mass.� They are numerically equal but have different units: atomic mass is in u per particle, while molar mass is in grams per mole. The calculator shows both so you can see the equivalence.

A third issue is rounding. The conversion factor has 12 significant figures, but many textbooks use 1.66054 × 10�²� kg. For most purposes, that is precise enough. However, if you are doing high-precision work, use the full CODATA value.

Limitations of the Formula

The calculator uses the exact definition of the atomic mass unit based on carbon-12. It does not account for:

• Isotopic abundance—if you want the average mass of an element, you need a weighted average of its isotopes.
• Binding energy—the mass of a nucleus is slightly less than the sum of its protons and neutrons; this is already included in the measured atomic mass.
• Electron mass—the atomic mass includes electrons, but the calculator just uses the number you enter.

For molecules, the mass in u is the sum of the atomic masses of the constituent atoms. The calculator does not compute molecular masses from formulas; you must enter the total mass in u yourself.

Alternatives and Related Units

Besides the atomic mass unit (u), you may encounter the dalton (Da), which is exactly the same unit. The unified atomic mass unit is also called the dalton in biochemistry. The symbol “amu� is sometimes used, but the official SI name is “unified atomic mass unit� with symbol “u�.

Another related unit is the electronvolt (eV), often used in particle physics. The mass-energy equivalence gives 1 u ≈ 931.494 MeV/c². This calculator does not convert to eV, but you can do that separately if needed.

For everyday chemistry, you rarely need to convert u to kg directly—you usually work with g/mol. But in physics, especially in nuclear reactions and mass spectrometry, the kilogram conversion is essential.

Frequently Asked Questions

What is the difference between atomic mass and mass number?

Atomic mass is the actual mass of an atom expressed in u, which includes protons, neutrons, and electrons, and reflects the natural isotopic mix. Mass number is simply the total count of protons and neutrons in a specific isotope. For example, carbon-12 has a mass number of 12, but the atomic mass of natural carbon is 12.011 u.

Why is 1 u equal to 1 g/mol?

Because the mole is defined as the amount of substance containing exactly 6.02214076 × 10²³ elementary entities, and the gram is defined relative to the kilogram. The numerical equality is a consequence of the definition: one mole of carbon-12 atoms weighs exactly 12 grams, and each carbon-12 atom weighs 12 u. So the ratio of grams to u is exactly Avogadro’s number, making the numbers equal.

How do I convert from u to kg without a calculator?

Multiply the value in u by 1.66053906660 × 10�²�. For example, 4 u × 1.66053906660 × 10�²� = 6.6421562664 × 10�²� kg. For grams, multiply by 1.66053906660 × 10�²�.

Can I use this calculator for molecular masses?

Yes, if you know the molecular mass in u (e.g., water is about 18.015 u), you can enter that number. The calculator will give the mass in kg and the molar mass in g/mol. It does not compute the molecular mass from a chemical formula.

What is the most accurate value of 1 u in kg?

The CODATA 2018 recommended value is 1.66053906660 × 10�²� kg, with a relative uncertainty of 1.5 × 10�¹�. The calculator uses this value. Older textbooks may use 1.66054 × 10�²� kg, which is fine for most purposes.

Related

• Bond Order Calculation: MO Diagram Formula and Examples
• Bond Order Calculation: The Quick MO Theory Trick
• Electrolysis Calculation: Faraday’s Law Explained

in u | Mass in atomic mass units (u) | u | 12 | 0
out kg | Mass in kilograms | kg | u * 1.66053906660e-27 | 10
out g | Mass in grams | g | u * 1.66053906660e-24 | 10
out gmol | Molar mass (g/mol) | g/mol | u * 1.0 | 6