A grain is exactly 64.79891 milligrams.
Grains to Atomic mass units Converter — gr to u
Convert Grain (gr) to Atomic mass unit (u) using the exact conversion factor (1 grain = 3.902282e+22 atomic mass unit). See the formula, worked examples, conversion table, history, and common uses.
Grains to Atomic mass units converter
Converter
Mass/Weight Converter
Convert SI mass, imperial weight, jewelry, atomic, and astronomy mass units.
Context values
Some unit families need one extra value for a physically accurate result.
The unified atomic mass unit is used in atomic physics.
Result = input x 6.479891e-5 / 1.66053907e-27.
Mass uses the kilogram as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Grains to Atomic mass units
Weight conversions between Grain and Atomic mass unit come up constantly in everyday commerce, shipping, and cooking, since US packaging is usually labeled in pounds while most of the rest of the world uses kilograms. Specifically: A grain is exactly 64.79891 milligrams, and the unified atomic mass unit is used in atomic physics.
Formula
atomic mass units = grains × 3.902282e+22
This factor comes from each unit's defined relationship to the category's base unit: 1 grain equals 0.00006479891 base units, and 1 atomic mass unit equals 1.660539e-27 base units, so dividing one by the other gives the direct grain-to-atomic mass unit factor of 3.902282e+22.
Simple example
1 gr × 3.902282e+22 = 3.902282e+22 u
1 grain = 3.902282e+22 atomic mass units.
Real-world example
50 gr × 3.902282e+22 = 1.951141e+24 u
50 grains = 1.951141e+24 atomic mass units.
Conversion table
| Grain (gr) | Atomic mass unit (u) |
|---|---|
| 1 gr | 3.902282e+22 u |
| 5 gr | 1.951141e+23 u |
| 10 gr | 3.902282e+23 u |
| 25 gr | 9.755704e+23 u |
| 50 gr | 1.951141e+24 u |
| 100 gr | 3.902282e+24 u |
Reverse conversion: Atomic mass unit to Grain
3.902282e+22 u × 2.562603e-23 = 1.00000011 gr
3.902282e+22 atomic mass units = 1.00000011 grains.
grains = atomic mass units × 2.562603e-23
For a page dedicated to this direction, see Atomic mass unit to Grain.
Understanding the Grain (gr)
A grain is exactly 64.79891 milligrams.
Understanding the Atomic mass unit
Atomic mass unit (u) measures mass. The unified atomic mass unit is defined as exactly 1/12 the mass of a carbon-12 atom, approximately 1.6605390666 × 10⁻²⁷ kg. It is classified as a Non-SI unit accepted for use with SI (scientific).
Where it's used: Used internationally in chemistry and physics, not a general-public unit
Uses of the Atomic mass unit today
- Expressing atomic and molecular masses in chemistry
- Nuclear and particle physics mass calculations
- Molecular weight tables and mass spectrometry
History of the Atomic mass unit
Atomic mass scales existed before international standardization, using either oxygen-16 or natural oxygen as a reference. In 1961, the International Union of Pure and Applied Chemistry (IUPAC) and physicists agreed on a unified scale based on carbon-12, ending decades of divergent chemistry and physics mass scales.
The unified atomic mass unit (u), also called the dalton (Da) in SI-adjacent contexts, is now tied to the carbon-12 standard and remains the base unit for atomic and molecular mass tables used across chemistry and physics.
Historical origin. Atomic mass unit was not the invention of a single named person; it developed through the process described above.
Atomic mass unit — sources
- BIPM SI Brochure — Non-SI units accepted for use with SI — Lists the unified atomic mass unit and its carbon-12 definition.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many atomic mass units are in 1 grain?
1 grain equals 3.902282e+22 atomic mass units, using the exact defined conversion factor rather than an estimate.
How do I convert grain to atomic mass unit?
Multiply the grain value by 3.902282e+22. The converter above does this instantly to whatever precision you set.
How do I convert atomic mass unit back to grain?
Use the reverse factor: 1 atomic mass unit equals 2.562603e-23 grains. You can also use the swap control in the converter above to flip the direction instantly.
What is a grain?
Grain (gr) is a unit of mass/weight.
What is a atomic mass unit?
Atomic mass unit (u) measures mass. The unified atomic mass unit is defined as exactly 1/12 the mass of a carbon-12 atom, approximately 1.6605390666 × 10⁻²⁷ kg. It is classified as a Non-SI unit accepted for use with SI (scientific).
Is the grain to atomic mass unit conversion exact?
Yes. The unified atomic mass unit is defined as exactly 1/12 the mass of a carbon-12 atom, approximately 1.6605390666 × 10⁻²⁷ kg. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.