Dose Stock Calculator — Desired Over Have (D ÷ H × Q)

Select tablet or syrup mode, enter the prescribed dose and the strength of your available stock, and find out exactly how many tablets or how many milliliters to administer. This dose stock calculator removes the arithmetic from medication preparation and reduces the risk of dispensing errors.

The Same Formula Under Four Different Names

Search for how to work out a dose from stock and you will meet the same equation wearing several different labels. It gets taught as "desired over have", written on whiteboards as D ÷ H × Q, called "need over have times volume" in some nursing programmes, and printed in textbooks as stock required over stock strength. They are one formula.

Amount to give = (Dose you want ÷ Strength you have) × Quantity that strength comes in

The three inputs map onto the prescription and the label like this:

Q is the part people drop, because for tablets it is 1 and multiplying by 1 feels invisible. The moment the stock is a liquid, Q becomes 5 mL or 10 mL and omitting it produces an answer that is wrong by that factor.

Reading the Three Numbers Off a Real Label

A bottle reading "Amoxicillin 250 mg/5 mL — 100 mL" contains four numbers and only two of them belong in the formula. The 250 mg is H. The 5 mL is Q. The 100 mL is the bottle size, which tells you how many doses you can get out of it but plays no part in working out a single dose. The drug name is not a number at all, but checking it against the prescription is the step that catches the errors arithmetic cannot.

If the prescription asks for 400 mg:

(400 ÷ 250) × 5 = 1.6 × 5 = 8 mL

A useful habit is to state the answer with its unit attached from the start — "8 mL", not "8". A bare number carries no protection against being read as 8 tablets, 8 mg, or 8 spoonfuls by whoever picks the note up next.

Tablets, and What to Do With a Fraction

For solid dose forms Q is one tablet, so the formula collapses to a division:

Tablets = Dose required ÷ Strength per tablet

A 75 mg prescription against 25 mg tablets is three tablets. A 30 mg prescription against the same stock is 1.2 tablets, and that is where the arithmetic stops being useful and a decision starts.

Splitting is not a purely mechanical question. A scored, immediate-release tablet generally divides acceptably. A film-coated, enteric-coated, or modified-release tablet does not: breaking the coating on a modified-release product can release the whole dose at once instead of over twelve hours, converting a routine dose into an overdose. Capsules cannot be split at all. When the arithmetic produces a fraction the formulation will not support, the answer is a different preparation, not a sharper knife.

Liquids: the Per-5-mL Problem

Oral suspensions are almost universally labelled per 5 mL because that is a spoon, not because it is a convenient unit for calculation. Two equally valid routes exist and mixing them is the classic error.

Route one — keep Q as 5 mL. Use the label as it is written and let the formula handle it: (Dose ÷ 250) × 5.

Route two — convert to mg/mL first. Divide the labelled strength by 5, then divide the dose by the result: 250 ÷ 5 = 50 mg/mL, so a 400 mg dose is 400 ÷ 50 = 8 mL.

Both give 8 mL. What produces a five-fold error is converting the strength to mg/mL and then still multiplying by 5, or leaving the strength as 250 and forgetting to multiply. Pick one route and use it every time rather than choosing per problem.

Measure the result with an oral syringe rather than a kitchen spoon. Household teaspoons vary by a factor of roughly two between the smallest and largest in an average drawer, which on a paediatric dose is the difference between a therapeutic and a doubled dose.

Vials and Injections

Injectable stock uses the same three terms, with Q as the vial volume:

Volume to draw = (Dose required ÷ Vial strength) × Vial volume

Drawing 60 mg from a vial labelled 80 mg in 2 mL: (60 ÷ 80) × 2 = 1.5 mL.

Two label conventions cause trouble here. Some vials are labelled as a total (80 mg/2 mL); others as a concentration (40 mg/mL). They describe the same vial. If the label gives a concentration, Q is 1 mL and the calculation is a plain division. Reading 40 mg/mL as though it were 40 mg in the whole vial halves every dose drawn from it.

Percentage-strength solutions are the second. A 2% solution is 2 g per 100 mL, which is 20 mg/mL. Treating the 2 as milligrams per millilitre understates the strength tenfold. Convert any percentage to mg/mL before it goes anywhere near the formula — the IV calculator covers this in the context of infusion rates.

Running the Formula Backwards

The same relationship answers a question that comes up at every discharge: will this bottle last?

Doses in the bottle = Bottle volume ÷ Volume per dose
Days of supply = Doses in the bottle ÷ Doses per day

A 100 mL bottle giving 8 mL per dose holds 12 full doses. At three times daily that is four days — short of a five-day course, and the kind of thing better noticed at the counter than at the weekend. Fractional doses at the end of a bottle count as unusable; twelve and a half doses is twelve.

Checking Your Own Answer

Estimation before calculation catches most order-of-magnitude errors. If the dose required is larger than the stock strength, the answer must be more than one unit of stock. If it is smaller, the answer must be less than one. A 400 mg dose from a 250 mg/5 mL bottle must therefore land between 5 and 10 mL, which makes 8 mL believable and instantly rules out 1.6 mL or 80 mL.

• Ratio first: is the dose bigger or smaller than the stock strength, and does your answer sit on the right side of one unit?
• Units attached: mL for liquids, tablets for solids, stated out loud with the number.
• Q accounted for: 1 for tablets, 5 mL for a per-5-mL suspension, the vial volume for injections.
• Percentages converted to mg/mL before use, never entered as themselves.
• The drug name on the label read against the prescription, not just the strength.

Where the prescription gives a rate per kilogram rather than a finished dose, work that out first with the dosage calculator, then bring the resulting milligrams back here to convert into tablets or millilitres.

Stock Dose Questions, Answered