Solid (weight) conversions

Convert between metric and avoirdupois/apothecary weight units, including grains.

What is this for?

Weight conversion sits underneath almost every weight-based dose. Patients report weight in pounds; drug references specify mg/kg. Grains persist in a handful of legacy products — aspirin 5 gr, phenobarbital 1/2 gr, thyroid desiccated 1 gr — and remain examinable.

How to use it

Enter the amount and select the unit you are converting from.
Select the destination unit under "Convert to".
Use the equivalents table when you need several units at once, such as building a compounding worksheet.

Worked example

A patient weighs 176 lb. Convert to kilograms for weight-based dosing.

Answer: 176 lb ÷ 2.2 = 80 kg. This is the total body weight (TBW); check whether the drug should be dosed on TBW, IBW, or AdjBW before proceeding.

Clinical pearls & pitfalls

Divide pounds by 2.2 to get kilograms. Multiplying by 2.2 instead is one of the most common — and most dangerous — arithmetic errors in weight-based dosing, producing a dose roughly 4.8-fold too high.
Always confirm whether a recorded weight is a measured weight or a stated/estimated one before dosing a narrow-therapeutic-index drug on it.
A grain is 65 mg by pharmacy convention but 64.8 mg by exact definition; aspirin 5 gr is labelled 325 mg, which reflects the exact value.

Assumptions & limitations

Uses conventional equivalents (1 kg = 2.2 lb, 1 lb = 454 g). The exact values are 2.20462 lb and 453.592 g. For a 100 kg patient the convention introduces roughly a 0.2% error, which is not clinically material but will differ from a calculator using exact factors.
Converts mass only. It does not convert mass to volume, which requires density or specific gravity.

References

National Institute of Standards and Technology. Special Publication 811: Guide for the Use of the International System of Units. link
United States Pharmacopeia–National Formulary. General Notices, Weights and Measures.