Dosage Calculation
Work out how much to give from the dose ordered and the strength you have on hand.
Give 10 mL to deliver 500 mg.
To find how much to give, divide the dose ordered by the strength on hand and multiply by the volume that strength comes in. If 500 mg is ordered and the label reads 250 mg per 5 mL, then (500 ÷ 250) × 5 = 10 mL. The same formula works for tablets.
The desired-over-have formula
Almost every basic med-math question reduces to one relationship: what was ordered, what strength you hold, and what volume or tablet count that strength comes in. The formula is usually written D ÷ H × Q — desired over have, times quantity.
The reason it works is that H ÷ Q is just the concentration of the preparation. Dividing the ordered dose by that concentration converts an amount of drug into an amount of liquid or number of tablets. Writing it as a fraction makes the units cancel visibly, which is why programmes teach dimensional analysis alongside it.
Keep the units matching first
The single most common error is a unit mismatch: an order in grams against a label in milligrams, or micrograms against milligrams. Convert before you divide — 1 g = 1000 mg and 1 mg = 1000 mcg. If the order says 0.5 g and the vial says 250 mg per 5 mL, rewrite 0.5 g as 500 mg first, then the arithmetic is straightforward.
Desired and Have must share the same unit (both mg, or both mcg). Quantity is the volume in mL — or the number of tablets — that the stated strength comes in.
Worked example: 500 mg ordered, 250 mg/5 mL on hand
Read the label, match the units, then divide and multiply:
- 1 Write down what was ordered. The order is 500 mg. This is the desired dose.
- 2 Read the strength on the label. The bottle reads 250 mg per 5 mL, so have = 250 mg and quantity = 5 mL.
- 3 Check the units match. Both are milligrams, so no conversion is needed. If the order were 0.5 g, convert it to 500 mg first.
- 4 Divide desired by have. 500 mg ÷ 250 mg = 2. The units cancel, leaving a plain number.
- 5 Multiply by the quantity. 2 × 5 mL = 10 mL. Give 10 mL.
- 6 Sanity-check the answer. The ordered dose is twice the strength on hand, so the volume should be twice 5 mL. It is.
Common dosage conversions
Convert the order and the label to the same unit before applying the formula.
| From | To | Multiply or divide |
|---|---|---|
| 1 g | 1000 mg | Grams → milligrams: × 1000 |
| 1 mg | 1000 mcg | Milligrams → micrograms: × 1000 |
| 1 kg | 2.2046 lb | Kilograms → pounds: × 2.2046 |
| 1 L | 1000 mL | Litres → millilitres: × 1000 |
| 1 tsp | 5 mL | Household teaspoon, for patient teaching |
| 1 tbsp | 15 mL | Household tablespoon, for patient teaching |
Rounding and when the answer looks wrong
Liquid doses are normally rounded to the precision the syringe can measure — often one decimal place for adults, two for paediatric syringes. Tablets are a different matter: a result of 1.5 tablets is only workable if the tablet is scored, and capsules cannot be split at all. If the arithmetic gives an odd fraction of a tablet, that is usually a signal to re-read the order or the label rather than to round.
A result more than about ten times what you expected almost always means a unit slipped — grams read as milligrams, or micrograms as milligrams. Work the sum again from the written order rather than adjusting the answer.
This tool is a study aid for practising calculations. Clinical doses must be calculated and independently verified by a qualified professional against the actual order and product label.