Milligrams to Grains Converter
Convert between milligrams and grains using the exact legal definition — one grain is 64.79891 milligrams, no rounding in the constant itself.
Milligrams and grains converter
1 gr = 64.79891 mg · 1 mg = 0.0154323584 gr
This direction is exact — multiplying by a finite constant cannot lose a digit.
How many milligrams are in a grain?
One grain is exactly 64.79891 milligrams. That is a definition, not a measurement: the international pound is defined as exactly 0.45359237 kg and as exactly 7,000 grains, so a grain is 453.59237 g ÷ 7,000 = 0.06479891 g. Going the other way, 1 mg is about 0.0154 grains.
1 grain = 64.79891 mg
What a grain is
An apothecary unit that never quite went away
The grain is one of the oldest mass units still in everyday use. It was originally the weight of a single barleycorn, and it survived the move to the metric system because it was written into pharmacy, ammunition and precious-metal practice long before the gram arrived.
It is the smallest unit in the troy and apothecary systems, and unusually it has the same value in all of them — troy, apothecary and avoirdupois grains are identical. That is why a grain converts cleanly to milligrams with one constant, with no need to ask which system you meant.
You will still meet grains in three places. Older medicine labels and prescriptions use them: aspirin is classically 5 grains, which is why the modern tablet is 325 mg rather than a round 300. Ammunition uses them for both bullet and powder mass. And gemstone and precious-metal work uses grains alongside carats and troy ounces.
Everywhere else the milligram has replaced it. The conversion below is the bridge between the two.
Conversion chart
Milligrams and grains, both directions
Milligrams to grains
Common milligram values converted to grains. Divide milligrams by 64.79891 to get grains. These results are rounded — the division does not terminate.
| Milligrams | Grains |
|---|---|
| 1 mg | 0.0154323584 gr* |
| 5 mg | 0.0771617918 gr* |
| 10 mg | 0.1543235835 gr* |
| 15 mg | 0.2314853753 gr* |
| 25 mg | 0.3858089588 gr* |
| 30 mg | 0.4629707506 gr* |
| 50 mg | 0.7716179176 gr* |
| 60 mg | 0.9259415012 gr* |
| 65 mg | 1.0031032929 gr* |
| 100 mg | 1.5432358353 gr* |
| 125 mg | 1.9290447941 gr* |
| 250 mg | 3.8580895882 gr* |
| 300 mg | 4.6297075059 gr* |
| 325 mg | 5.0155164647 gr* |
| 500 mg | 7.7161791765 gr* |
| 600 mg | 9.2594150118 gr* |
| 1,000 mg | 15.4323583529 gr* |
Grains to milligrams
Common grain values converted to milligrams. Multiply grains by 64.79891 to get milligrams. These results are exact.
| Grains | Milligrams |
|---|---|
| 0.25 gr | 16.1997275 mg |
| 0.5 gr | 32.399455 mg |
| 1 gr | 64.79891 mg |
| 1.5 gr | 97.198365 mg |
| 2 gr | 129.59782 mg |
| 3 gr | 194.39673 mg |
| 4 gr | 259.19564 mg |
| 5 gr | 323.99455 mg |
| 7.5 gr | 485.991825 mg |
| 10 gr | 647.9891 mg |
| 15 gr | 971.98365 mg |
| 20 gr | 1,295.9782 mg |
| 30 gr | 1,943.9673 mg |
| 60 gr | 3,887.9346 mg |
| 100 gr | 6,479.891 mg |
* Rounded. Dividing by 64.79891 does not terminate, so milligram-to-grain results are correct to the digits shown rather than exact.
Precision
One direction is exact, the other is not
Grains to milligrams: exact
Multiplying by 64.79891 is multiplication by a finite decimal, and a finite decimal times a finite decimal always terminates. So 5 grains is exactly 323.99455 mg, with no rounding anywhere in the calculation.
That holds at any size. 7,000 grains — one pound — comes out at exactly 453,592.37 mg, which is the definition the constant came from in the first place.
Milligrams to grains: rounded
Dividing by 64.79891 does not terminate. 100 mg is 1.5432358353… grains and the digits continue forever, so any answer has to stop somewhere.
This calculator divides to twelve decimal places using long division on the digits, then rounds half-up. No floating-point error creeps in underneath — the digits it shows are right — but the result is a rounded one, and the page says so rather than calling it exact.
Where you will meet it
What grains are still used for
Medicine labels
Aspirin's classic 5-grain dose is 323.99455 mg, rounded to the 325 mg you see on a modern bottle. Thyroid tablets and some older prescriptions are still written in grains.
Ammunition
Bullet and powder masses are quoted in grains almost universally, and the numbers are small enough that the milligram equivalent is the useful one for a scale.
Gems and precious metals
Grains appear alongside carats and troy ounces in jewellery and bullion work, where the unit predates the metric system by centuries.
Reloading and lab scales
Many precision scales switch between grains and milligrams directly, which is exactly the conversion this page performs.
Key facts
The numbers worth remembering
- 1 grain = 64.79891 mg, exactly, by international definition.
- 1 grain = 0.06479891 g — the same constant, three places left.
- 7,000 grains = 1 pound = 453.59237 g, exactly.
- 1 mg ≈ 0.0154 grains, so a grain is roughly 65 times heavier than a milligram.
- A 5-grain aspirin is 323.99455 mg, which is why the tablet is sold as 325 mg.
- Troy, apothecary and avoirdupois grains are all the same size — unlike ounces.
Questions
Milligrams and grains: common questions
How many milligrams are in a grain?
Exactly 64.79891 milligrams. It is a defined value, not a measured one — the pound is defined as exactly 0.45359237 kg and as exactly 7,000 grains, which fixes the grain at 453.59237 g ÷ 7,000.
1 gr = 64.79891 mg
How many grains are in a milligram?
About 0.0154323584 grains. This direction does not divide evenly, so the answer is rounded — a milligram is roughly one sixty-fifth of a grain.
1 mg ≈ 0.0154323584 gr
How many grains is 30 mg?
30 mg is 0.4629707506 grains, or a little under half a grain. Rounded, because dividing by 64.79891 never terminates.
30 mg = 0.4629707506 gr
How many milligrams is 5 grains?
Exactly 323.99455 mg. This is the classic aspirin dose, which is why a standard aspirin tablet is labelled 325 mg — it is the 5-grain dose rounded to a sensible printed number.
5 gr = 323.99455 mg
Is a grain the same as a gram?
No, and the similar names cause real mistakes. A grain is 64.79891 mg, which is about one fifteenth of a gram. A gram is roughly 15.43 grains. They differ by more than a factor of fifteen, so mixing them up on a medicine or powder measurement is a serious error.
1 g ≈ 15.4323584 gr
Why is a grain 64.79891 mg and not a round number?
Because the grain came first and the metric definition had to fit it. When the international yard and pound agreement of 1959 fixed the pound at exactly 0.45359237 kg, the grain — already defined as 1/7000 of a pound — inherited whatever value that produced. The awkward digits are the price of making an old unit exactly compatible with the metric system rather than approximately compatible.
453.59237 g ÷ 7000 = 0.06479891 g
Are troy grains and apothecary grains different?
No. The grain is the one unit shared identically across the troy, apothecary and avoirdupois systems — all three use 64.79891 mg. This is unusual: troy and avoirdupois ounces and pounds are all different sizes from each other. If a source says 'grains', no further qualification is needed.
Need the metric side instead?
The main calculator converts micrograms, milligrams and grams with the same exact engine — and that direction is a pure decimal-point shift, so every answer there is exact.