How many milligrams are in one grain?

One grain equals approximately 64.80 milligrams (exact: 64.79891 mg). In pharmacy, this is commonly rounded to 65 mg for clinical calculations.

Is this conversion still used in pharmacy?

Yes. Armour Thyroid and some compounded medications still use grain-based dosing. US pharmacy schools teach the grain-to-milligram conversion as essential knowledge. The conversion factor of 64.8 mg per grain is one of the few imperial-metric relationships that pharmacists must know by heart.

Why is the pharmacy rounding different from the exact value?

Pharmacists round 64.79891 to 65 mg for clinical convenience. The 0.3% difference (0.2 mg per grain) is clinically insignificant for most medications. However, for calculations involving multiple grains, the exact value should be used to avoid compounding the rounding error.

Grains to Milligrams (gr to mg) Converter

1 gr = 64.7989 mg

1 Grain equals 64.7989 Milligrams (1 gr = 64.7989 mg). Convert Grains to Milligrams with formula, table, and examples.

One grain equals approximately 64.80 milligrams. This is the single most important grain conversion in pharmaceutical practice. When pharmacists encounter legacy grain-based prescriptions - still occasionally seen for thyroid medications and some compounded preparations - they convert to milligrams using this factor. The relationship 1 grain = 64.8 mg (often rounded to 65 mg in clinical practice) is memorized by every pharmacy student.

How to Convert Grains to Milligrams

mg = gr × 64.79891

Multiply the value in Grains by 64.79891

Take your value in Grains
Multiply by 64.79891
Read the result in Milligrams

Common Grains to Milligrams Conversions

Grains (gr)	Milligrams (mg)	Status
0.1 gr	6.4799 mg
0.5 gr	32.3995 mg
1 gr	64.7989 mg
5 gr	323.9946 mg
10 gr	647.9891 mg
15 gr	971.9837 mg
20 gr	1,295.9782 mg
50 gr	3,239.9455 mg
100 gr	6,479.891 mg
200 gr	12,959.782 mg
437.5 gr	28,349.5231 mg
480 gr	31,103.4768 mg
500 gr	32,399.455 mg
1,000 gr	64,798.91 mg
7,000 gr	453,592.37 mg

Good to Know About Grains to Milligrams Conversion

The grain is remarkable for being the only unit identical across all three traditional English weight systems (troy, avoirdupois, and apothecary). This universality made it the natural foundation for the entire English measurement tradition. While ounces and pounds changed definition depending on whether you were weighing gold, groceries, or medicine, the grain remained constant: always 64.80 milligrams, everywhere, for everyone.

Grains to Milligrams: What You Need to Know

Armour Thyroid, a natural thyroid hormone replacement, is still prescribed in grain increments: 1/4 grain (about 16.2 mg), 1/2 grain (about 32.4 mg), 1 grain (about 64.8 mg). This makes the grain-to-milligram conversion a living, actively-used pharmaceutical relationship rather than a historical curiosity. Pharmacists dispensing these prescriptions perform this conversion routinely.

What is a Grain? gr

A grain is a unit of mass equal to exactly 64.79891 milligrams. It is the same in the avoirdupois, troy, and apothecaries' systems, derived from the 1959 international agreement defining the pound as exactly 0.45359237 kilograms.

Imperial Troy Apothecaries ammunition weight bullet measurement historical pharmacy

Learn more about Grain →

What is a Milligram? mg

A metric unit of mass equal to one thousandth of a gram, or one millionth of a kilogram. Commonly used in medicine and pharmacology.

Metric medicine pharmacology nutrition

Learn more about Milligram →

Going the other way? Use our Milligrams to Grains converter.

Grains to Milligrams FAQ

One grain equals approximately 64.80 milligrams (exact: 64.79891 mg). In pharmacy, this is commonly rounded to 65 mg for clinical calculations.
Yes. Armour Thyroid and some compounded medications still use grain-based dosing. US pharmacy schools teach the grain-to-milligram conversion as essential knowledge. The conversion factor of 64.8 mg per grain is one of the few imperial-metric relationships that pharmacists must know by heart.
Pharmacists round 64.79891 to 65 mg for clinical convenience. The 0.3% difference (0.2 mg per grain) is clinically insignificant for most medications. However, for calculations involving multiple grains, the exact value should be used to avoid compounding the rounding error.

Non-Frequently Asked Questions About Grains to Milligrams

Questions nobody should ask - but someone did.

Armour Thyroid has been manufactured since 1899, long before metrication. Its dosing tradition in grains is so deeply embedded in clinical practice that changing would require retraining physicians, reprinting prescription pads, and reformulating dosage guidelines. The pharmaceutical industry chose to keep the grain for this one legacy product rather than disrupt a century of clinical experience.
Absolutely. A prescription for '1 grain of thyroid extract' means about 65 milligrams. If misread as '1 milligram,' the patient receives 1/65th of the intended dose - far too little. If misread as '65 grains,' the patient receives 65 times too much - potentially fatal. This ambiguity risk is exactly why modern pharmacy insists on milligram-only notation for new medications.
Not particularly. It does not divide cleanly into halves (32.4 mg), quarters (16.2 mg), or thirds (21.6 mg). Compare this to a hypothetical metric standard of 50 mg or 100 mg, which divide beautifully. The grain's inconvenient milligram equivalent is a direct consequence of its pre-metric origin - it was standardized when nobody was thinking about decimal compatibility.