Drug Dosage Calculation Basics: Desired Over Have and mg/kg

This post explains the arithmetic taught in nursing school dosage courses. In “desired over have,” you divide the dose desired by the dose on hand and multiply by the quantity it comes in. For weight-based doses, you convert to kilograms first. It is practice math, not dosing advice.

The formula

Desired over have (formula method)

Amount to give = D / H x Q

• D is the dose desired (ordered).
• H is the dose on hand (the strength on the label).
• Q is the quantity the dose on hand comes in (tablets, mL).

OpenStax shows the formula method with an example: 500 mg ordered and 250 mg tablets on hand gives 2 tablets. StatPearls shows the same idea as drug ordered / drug available x total mL, and gives 0.2 mg from a 1 mg in 10 mL vial as 2 mL.

Weight-based dose

Dose = dose per kg x weight in kg. Body weight must be in kilograms, so pounds are converted first, using 1 kg = 2.2 lb.

Dimensional analysis

Also called the factor-label method. You write the known values as fractions so the unwanted units cancel and the wanted unit is left. OpenStax notes it lets you solve problems with the same method rather than memorizing separate formulas.

Units

1 mg = 1,000 mcg. When the order and the supply use different units, convert one so they match before dividing.

Using it in NurseTools

The Drug Dosage tool is in clinical review and is not available yet. It is locked in the app and will not be available at launch. Drug dosage tools also need the approval route Apple requires for dosage calculators. NurseTools itself is coming soon to the App Store, and nothing is downloadable today.

The app shows a release-gates timeline: source and rights audit, clinical editorial review, independent testing, and regulatory and App Store review. A tool that has not passed all of these stays locked. No payment or setting unlocks a tool that has not passed review. You can turn on the “Notify me when available” switch to hear about it.

When the app is out, it will not need an account, will never ask for patient data and will work offline. Only the BMI and MAP tools are planned for launch.

Worked example

This is a made-up practice example with a fictional drug. “Drug X” is not a real medication, and the numbers are not a real order or dose.

Practice problem: Drug X is ordered at 15 mcg/kg as a single dose for a child who weighs 66 lb. The label reads 0.5 mg per 2 mL. How many mL are measured?

1. Weight in kg. 66 lb / 2.2 = 30 kg.
2. Dose in mcg. 15 mcg/kg x 30 kg = 450 mcg.
3. Match the units. The label is in mg. 450 mcg / 1,000 = 0.45 mg.
4. Desired over have. D = 0.45 mg, H = 0.5 mg, Q = 2 mL. 0.45 / 0.5 x 2 = 0.9 x 2 = 1.8 mL.
5. Round. OpenStax says not to round until the final answer, and that doses are typically rounded to the nearest hundredth for children unless agency policy says otherwise. Here the answer is 1.80 mL, written as 1.8 mL.
6. Sense check. The dose 0.45 mg is a little less than the 0.5 mg on hand, so a little less than the 2 mL quantity (1.8 mL) is reasonable.
7. Write it safely. Write “0.45 mg” with the leading zero, never “.45 mg”.

Limits and common pitfalls

• Independent double checks. The OpenStax dosing chapter advises asking whether the dose makes sense and having a colleague or pharmacist check the math when in doubt. StatPearls likewise says a second opinion should be sought when in doubt.
• mg vs mcg. The ISMP list tells readers to write mcg instead of the microgram symbol, which can be mistaken for mg.
• Leading zero. ISMP lists a missing leading zero (.5 mg) as mistaken for 5 mg and advises writing 0.5 mg. ISMP marks this as on The Joint Commission’s “Do Not Use” list.
• Trailing zero. A trailing zero (1.0 mg) can be mistaken for 10 mg. ISMP advises no trailing zeros for doses expressed in whole numbers. This is also on The Joint Commission’s list.
• Write out “units.” ISMP lists “U” for units as mistaken for zero or 4, with a possible 10-fold overdose or greater.
• Decimal slips. StatPearls warns that one wrong decimal place can become a ten-fold error.
• Pounds vs kilograms. Convert first and check the units cancel.
• Orders and policy come first. Real doses come from the order, the label, a drug reference and your facility’s policy, never from a practice example.