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Atomic mass units (u) to Grams Converter

Enter a value in atomic mass units (u) to convert it to grams, kilograms.

Result

2.9915 × 10⁻²³ g
2.9915 × 10⁻²⁶ kg

How it is calculated

The conversion is a single multiplication: grams = u × 1.66053907×10⁻²⁴ and kilograms = u × 1.66053907×10⁻²⁷, where u is a mass in atomic mass units (daltons). The constant is the mass of one atomic mass unit, one twelfth of a carbon-12 atom. Because the values are tiny, the answer is printed in scientific notation with five significant digits, such as 2.9915 × 10⁻²³, where the raised −23 shows how many places the decimal point moves to the left. The tool applies no limit to the input, so zero returns zero and a negative number returns a negative mass, even though a real mass cannot be negative. The same conversion works for atoms, molecules or sub-atomic particles, as long as you enter the mass in u.

Worked example

One water molecule has a molecular mass of about 18.015 u (two hydrogens and one oxygen). Multiply by the constant: 18.015 × 1.66053907×10⁻²⁴ = 2.9915 × 10⁻²³ g. In kilograms the same value is 2.9915 × 10⁻²⁶ kg. So a single H₂O molecule weighs about 3×10⁻²³ g, which is why chemists count molecules in moles (6.022×10²³) rather than singly. Note the exponent: 10⁻²³ means 0.00000000000000000000001.

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Common mistakes

  • Reading the exponent wrongly. 2.9915 × 10⁻²³ is a number with 22 zeros after the decimal point, and not 2.9915 × 10²³.
  • Confusing atomic mass units with grams per mole. The numeric value of a molar mass in g/mol equals the mass of one particle in u, but the converter gives the mass of a single particle, not one mole.
  • Entering the mass number of an element (an integer) when you need the real atomic mass. Chlorine is about 35.45 u on average, not 35 or 37, because natural samples mix isotopes.
  • Typing a space as a thousands separator, as in 1 200. The engine cannot read it and returns no result; type 1200 or 1.2e3.
  • Expecting the same decimal mark on every page. The Turkish page writes 2,9915 × 10⁻²³ with a decimal comma, the English page 2.9915 × 10⁻²³ with a point.

About this calculator

The atomic mass unit (u), also called the dalton, is defined as exactly 1/12 the mass of a neutral carbon-12 atom, equal to 1 u = 1.66053906892×10⁻²⁴ grams — almost exactly 1/NA grams (1/(NA × 1000) kilograms), where NA is Avogadro's number (since the 2019 SI revision the two differ only by parts per billion), because one mole of a substance with molar mass M grams contains NA atoms/molecules each of mass M/NA grams.

Chemists and physicists work in atomic mass units when quoting atomic and molecular masses (an oxygen atom is about 16 u) because the numbers stay conveniently small, then convert to grams or kilograms whenever a calculation needs to interface with macroscopic, SI-based quantities like reaction yields or mass spectrometry calibration.

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Comments (1)

  • Editorial team

    A quick consistency check ties this to molar mass. One water molecule at 18.015 u comes out at 2.9915 × 10⁻²³ g, and multiplying by Avogadro's number, 6.022 × 10²³, takes you back to roughly 18 g per mole. That shows why the molar mass in grams per mole and the mass in u share the same number. Are you converting atoms, molecules or particles, and how do you find the mass?

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