Basic medication math: dose ordered vs dose on hand, weight-based dosing
Interactive · Dosage Math Trainer
Fresh practice problems with fully worked solutions.
Protocol orders 0.02 mg/kg of a medication for a 40 kg patient. What is the total dose in mg?
Practice tool for exam preparation only — always follow your protocols, drug references, and medical direction when dosing a real patient.
Two numbers drive every drug calculation: the dose ordered (what the patient needs) and the dose on hand (what's in the vial or bag in front of you). Your job is to turn "give 20 mg" into "draw up this many mL." One formula does it.
The formula
Volume to give = (dose ordered ÷ dose on hand) × volume on hand
Or in the shorthand you'll hear: Desired over Have, times the volume it's in.
Your vial says 50 mg in 10 mL. Order is 20 mg.
(20 mg ÷ 50 mg) × 10 mL = 0.4 × 10 = 4 mL
The milligrams cancel — mg over mg — and you're left with mL, which is what you wanted. If the units don't cancel to a volume, you set it up upside down.
Watch the units before you plug in
The single most common mistake is mixing units — the order is in mcg, the vial is labeled in mg. Convert first, then divide.
Order: 250 mcg. On hand: 1 mg in 1 mL.
- Convert the vial to mcg: 1 mg = 1,000 mcg, so it's 1,000 mcg in 1 mL.
- (250 mcg ÷ 1,000 mcg) × 1 mL = 0.25 mL
Get both numbers into the same unit before the division, every time.
Weight-based dosing
Most EMS drugs are ordered per kilogram — a set amount of drug for every kg the patient weighs. Two steps:
- Get the weight in kg. If it's in pounds, divide by 2.2.
- Multiply the per-kg dose by the weight to get the total dose ordered.
Order: 0.1 mg/kg. Patient: 176 lb.
- 176 lb ÷ 2.2 = 80 kg
- 0.1 mg/kg × 80 kg = 8 mg total
Now 8 mg is your "dose ordered." If the vial is 10 mg in 5 mL, run it through the same formula:
(8 mg ÷ 10 mg) × 5 mL = 4 mL
Put the whole chain together
A weight-based problem is just three moves in a row: convert the weight, calculate the total dose, then convert dose to volume. Do them one at a time, label every number with its unit, and estimate the answer before you commit. The math itself is small — multiplication and division you already know. The discipline is keeping the units straight and not skipping the sanity-check.
Key takeaways
- Volume to give = (dose ordered ÷ dose on hand) × volume on hand; the drug units cancel and leave a volume.
- Convert order and vial into the same unit before dividing — mismatched mcg/mg is the top error.
- Weight-based dosing is two steps: weight to kg (÷ 2.2 from pounds), then per-kg dose × weight = total dose.
- Chain it — convert weight, find total dose, then find volume — one labeled step at a time, with an estimate to check against.
That's one skill down — the course drills every section your entrance exam throws at you.
Enroll to unlock all 38 lessons, a graded check after every module, and lifetime access.
Enroll for $29One-time payment · lifetime access