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Grains to Grams Converter
↔ Convert g to gr instead

Common Conversions

gr g
0.5 0.0324
1 0.0648
2 0.1296
5 0.324
7.5 0.486
10 0.648
15 0.972
15.432 1
20 1.296
50 3.2399
100 6.4799

Why this conversion matters in chemistry

The grain is a tiny, precise mass — exactly 64.79891 mg by US and UK definition. It survives mostly in pharmacy history (a 5-grain aspirin tablet is the 325 mg dose still on shelves), in propellant chemistry (small-arms powder loads quoted in grains), and in apothecary references that predate metric adoption. Multiplying by 0.06479891 lands a grain count in grams for any modern stoichiometric calculation. The conversion is exact because the grain itself is defined against the kilogram, not measured against it.

Formula

g = gr × 0.06479891

Worked Examples

1 gr = 0.0648 g

One grain — about 64.8 mg, the foundational unit behind the apothecary system.

5 gr = 0.324 g

A 5-grain aspirin tablet — the 325 mg adult dose, with the small rounding difference baked into pharmaceutical convention.

10 gr = 0.648 g

Ten grains, about two-thirds of a gram — useful as a quick reference for the apothecary scale.

15.432 gr = 1 g

Exactly one gram expressed in grains — the conversion's inverse anchor.

Frequently Asked Questions

How do I convert grains to grams?
Multiply by 0.06479891. So 5 grains becomes 0.324 g, or 324 mg. The factor is exact through the legal definition of the grain.
What is 1 grain in milligrams?
Exactly 64.79891 mg, normally rounded to 65 mg in chemistry contexts. Older pharmaceutical practice often approximated it as 60 mg, which is where the 5-grain → 300 mg historical dosing came from.
Why is 5 grains of aspirin called 325 mg instead of 324 mg?
The 325 mg figure is a rounded pharmaceutical standard for tableting. The exact conversion is 5 × 64.79891 = 323.99 mg, but tablet presses produce reproducible 325 mg doses, and the rounded value sticks.
Is the grain still a recognized unit?
Yes — the grain is legally defined in the US and UK as exactly 64.79891 mg. It still appears in some pharmacopoeial references and in propellant chemistry, though SI metric units are preferred everywhere else.