Dosage Calculations with D/H × Q (Tablets and Liquids)

The D/H × Q formula (“desired over have, times quantity”) tells you how much of a medicine to give. D is the dose ordered, H is the strength you have, and Q is the amount that strength comes in: 1 tablet, or so many mL.

For learning only, not for patient care. With a real patient, always follow the order, the drug’s label and your facility’s policies, and double-check every dose with a pharmacist or a second nurse.

Try it with your own problem ↓

The steps

Read D (the order), H (the strength on the label) and Q (what H comes in).
Put D and H in the same unit: 1 g = 1,000 mg and 1 mg = 1,000 mcg.
Divide D by H, then multiply by Q: amount = D ÷ H × Q.
Round the way you can give it: liquids to a tenth of a mL, tablets to whole or half tablets (halves only if the tablet is scored).
Check: the amount × the strength should give back the dose ordered.

A worked example

The order is 375 mg. The label says 250 mg per 5 mL. How many mL do you give?

D = 375 mg, H = 250 mg, Q = 5 mL. The units already match.
D ÷ H = 375 ÷ 250 = 1.5.
× Q: 1.5 × 5 mL = 7.5 mL.
Check: 7.5 mL × 250 mg per 5 mL = 7.5 × 50 = 375 mg. ✓

Give 7.5 mL.

Why it works

D ÷ H says how many “have” portions fit into the dose you want. Here 375 mg is one and a half times 250 mg, so you need one and a half of the 5 mL portions the label describes. Multiplying by Q turns “how many portions” into an amount you can measure.

Try it with your own problem

Open The Dose Calculator (free) with this example already filled in. It shows every step, and you can type over it with your own numbers.

Work it out: ordered 375 mg, available 250 mg per 5 mLOpens The Dose Calculator with this example

Or try these

More nursing math, every step written out: open The Dose Calculator.

Common mistakes

Not matching the units. An order in g and a label in mg must be put in one unit first, or the answer is off by 1,000 times.
Flipping the fraction. It is desired over have: 375/250, not 250/375. A dose bigger than the strength needs more than one Q.
Forgetting Q. With a liquid, D ÷ H counts 5 mL portions, not mL. Multiply by Q.
Splitting a tablet that is not scored. Only scored tablets can be halved.

Questions people ask

How many tablets for 0.5 g if the tablets are 250 mg?

Change 0.5 g to 500 mg first. 500 ÷ 250 × 1 tablet = 2 tablets.

Can I use ratio and proportion instead?

Yes. 250 mg : 5 mL = 375 mg : x mL gives 250x = 1,875, so x = 7.5 mL. It is the same calculation.

What is dimensional analysis?

Writing the problem as a chain of fractions so the units cancel: 375 mg × 5 mL/250 mg = 7.5 mL. The mg cancel and leave mL.

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