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Grams, Milligrams, and Ounces: Comparing Metric and Imperial Weight
· Updated · 3 min read
In short
One ounce is 28.349523125 grams exactly, and one pound is 453.59237 grams. Unlike metric conversions, which only move a decimal point, imperial conversions use irregular factors — 16 ounces to a pound, 14 pounds to a stone — so each one has to be memorised or looked up.
On this page
Converting milligrams to grams takes no arithmetic at all — the decimal point moves three places and you are done. Converting ounces to grams is a different kind of problem entirely, and the contrast is a good way to see what the metric system actually buys you.
The exact factors
Since the international yard and pound agreement of 1959, imperial weight units have been defined in terms of the kilogram. That means the conversion factors are exact, not approximations — they simply are not round.
| Imperial unit | In grams (exact) | In milligrams | Common rounding |
|---|---|---|---|
| 1 grain | 0.06479891 g | 64.79891 mg | ~64.8 mg |
| 1 dram | 1.7718451953125 g | 1,771.85 mg | ~1.77 g |
| 1 ounce (oz) | 28.349523125 g | 28,349.523125 mg | ~28.35 g |
| 1 pound (lb) | 453.59237 g | 453,592.37 mg | ~453.6 g |
| 1 stone | 6,350.29318 g | 6,350,293.18 mg | ~6.35 kg |
| 1 troy ounce | 31.1034768 g | 31,103.4768 mg | ~31.1 g |
Two different ounces
The avoirdupois ounce (28.35 g) is the everyday one used for food and post. The troy ounce (31.10 g) is used for precious metals and is about 10% heavier. A price quoted per ounce of gold means troy ounces.
Why metric conversion is structurally easier
The difference is not that one system is more familiar. It is that metric relationships are powers of ten and imperial ones are not.
| System | Conversion | Factor | Method |
|---|---|---|---|
| Metric | mg → g | 1,000 | Move the decimal point 3 places |
| Metric | g → kg | 1,000 | Move the decimal point 3 places |
| Metric | mcg → g | 1,000,000 | Move the decimal point 6 places |
| Imperial | oz → lb | 16 | Divide by 16 |
| Imperial | lb → stone | 14 | Divide by 14 |
| Imperial | grain → oz | 437.5 | Divide by 437.5 |
Key takeaway
Every metric weight conversion uses the same factor — 1,000 — applied a different number of times. Imperial uses a different factor for each pair of units, and none of them is a power of ten.
There is a practical consequence beyond convenience: metric conversions are exact and reversible, while imperial conversions to metric almost always involve rounding, and rounding compounds if conversions are chained.
Practical conversions
For cooking and everyday use, these approximations are close enough:
- 1 oz ≈ 28 g (28.35 g exactly)
- 4 oz ≈ 113 g (a quarter pound)
- 8 oz ≈ 227 g (half a pound)
- 16 oz = 1 lb ≈ 454 g
- 2.2 lb ≈ 1 kg
| Ounces | Grams | Milligrams |
|---|---|---|
| 0.5 oz | 14.17 g | 14,175 mg |
| 1 oz | 28.35 g | 28,350 mg |
| 2 oz | 56.70 g | 56,699 mg |
| 4 oz | 113.40 g | 113,398 mg |
| 8 oz | 226.80 g | 226,796 mg |
| 12 oz | 340.19 g | 340,194 mg |
| 16 oz (1 lb) | 453.59 g | 453,592 mg |
Notice how quickly the milligram column becomes unusable. Once you are past a few grams, milligrams stop being a helpful unit — which is the same reason nutrition labels switch units partway down the panel.
Where milligrams meet imperial: the grain
There is one imperial unit small enough to overlap the milligram range: the grain, historically the weight of a single barley grain. At 64.79891 mg it sits between a milligram and a tenth of a gram.
The grain survives in a few specialist places — some older medicine formulations, ammunition, and archery — but for nearly all purposes it has been replaced by the milligram, which is easier to work with for exactly the reasons this article is about.
Why medicine is metric everywhere
Even in countries that use imperial units in daily life, medicine, science and pharmaceuticals are entirely metric. Medicine bottles in the United States are labelled in milligrams, not grains or ounces.
The reason is error resistance. A conversion that only moves a decimal point can be checked at a glance, and a check that takes a second gets done. A conversion requiring division by 437.5 does not, and every avoidable calculation in a clinical setting is a place an error can enter.
Grams → Milligrams
28,349.523125mg
28.349523125 g = 28,349.523125 mg
Both sides weigh exactly the same — only the unit changes.
One avoirdupois ounce, converted exactly. Metric conversion stays exact however awkward the starting figure.
Quick facts
- 1 oz = 28.349523125 g exactly = 28,349.523125 mg.
- 1 lb = 453.59237 g exactly.
- A troy ounce (31.10 g) is about 10% heavier than an avoirdupois ounce.
- 1 grain = 64.79891 mg.
- Imperial units have been defined in terms of the kilogram since 1959.
The takeaway
Imperial-to-metric conversions need exact factors and usually a calculator. Metric-to-metric conversions need neither — just the direction and a count of decimal places.
For the metric side, the calculator converts milligrams, grams and micrograms exactly at any magnitude, and the conversion chart lists the values people look up most often.
About this article
Written and maintained by the Milligrams to Grams editorial team. Every conversion figure in this article is generated with the same exact-decimal engine that powers the site's calculator, and is verified by an automated test suite rather than typed by hand. Read more about our methodology and accuracy standards.