Grains — the oldest weight unit still on modern spec sheets
One grain = 64.8 mg. Originally the mass of a single barleycorn. Now measuring bullets, arrows and gunpowder charges.
The grain is the oldest weight unit still in commercial use in the English-speaking world. It survived because ballistics — bullet weight, powder charge, projectile drop calculations — was worked out in grains from the 17th century onward, and rewriting a century of load data in milligrams offered no benefit to anyone.
The exact factors
1 grain (gr) = 64.79891 mg = 0.06480 g (exact) > 1 g = 15.4324 gr > 1 avoirdupois lb = 7,000 gr > 1 troy oz = 480 gr > 1 pennyweight = 24 gr
Bullet weights
A 9 mm Luger typically fires a 115 gr (7.45 g) bullet at 380 m/s. A .308 rifle round fires a 168 gr (10.9 g) match bullet. The grain is universal in North American ammunition catalogues; European catalogues typically list grams alongside. Reloaders work exclusively in grains for both powder charges and projectile masses.
Arrows and archery
Modern arrow shafts and arrow points are sold in grains. A hunting arrow assembly is typically 400–500 grains total. Kinetic energy formulas in archery use grains directly: KE (ft·lb) = mass (gr) × velocity² (fps) / 450,240.
Historic apothecary use
Pre-metric pharmacies dispensed in grains. 'One-quarter grain morphine' meant 16.2 mg. This convention persisted in the US until the 1970s and still appears in period medical fiction. Modern US pharmacies dispense in milligrams exclusively.
Worked example — a 55-grain .223 bullet
55 gr × 64.8 mg = 3,564 mg = 3.56 g. Fired at 3,240 fps, kinetic energy = 55 × 3,240² / 450,240 = 1,282 ft·lb = 1,738 J. Every load-data book in the US works out this calculation in grains and feet-per-second — converting to SI first would double the reader's work.
The five-second recap
1 gr = 64.8 mg. 7,000 gr = 1 lb. Bullets, powder charges and arrows are all in grains. Ammunition catalogues rarely convert.
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FAQ
Why is a pound 7,000 grains?+
Historical accident. The Roman pound was 5,760 grains; the merchant pound was 7,200; the avoirdupois pound settled on 7,000 in the 16th century.
Are grains used in medicine today?+
Only in a few legacy references. Aspirin was 'five grains' = 325 mg per tablet historically; modern labelling is mg exclusively.
How much powder in a rifle load?+
Typically 25–50 grains (1.6–3.2 g) for a rifle cartridge. Reloading manuals list charges to 0.1 grain resolution.
This article was written by The Turbo Unit Converter engineering desk (Historical metrology) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.