Dosing Conversion: Simple Rules to Convert Medication Doses

Dosing conversions are one of the most common chores in medicine and at home when you need to change units, concentrations, or dose forms. A clear method prevents dosing mistakes and keeps patients safe. Below are practical rules you can use immediately, with examples you’ll actually meet.

Quick Conversion Rules

Always convert everything to the same unit first. If you have mg and need mcg, convert mcg to mg or vice versa before calculating. Use decimals instead of fractions when possible. Remember that 1 mg = 1000 mcg and 1 g = 1000 mg. For liquids, concentration is key: tablets are simple, but syrups and injections require math based on mg per mL.

Use the "Desired over Have" formula for tricky problems. Write what you want on top and what you have on the bottom, then multiply by the volume. For example, if you need 150 mg and you have a solution of 50 mg/mL: (150 mg ÷ 50 mg/mL) = 3 mL. This formula works for mg/kg, infusion rates, and reconstitution steps.

When dosing by weight, calculate mg/kg carefully. Multiply the patient's weight in kg by the dose in mg/kg to get the total mg. If weight is in pounds, divide by 2.2 to convert to kg first. Always double-check decimal placement. A misplaced zero can harm.

Common Examples

Pediatric liquid: If a child needs 5 mg/kg and weighs 18 kg, total dose = 90 mg. If syrup is 10 mg/5 mL, then give (90 mg ÷ (10 mg/5 mL)) = 45 mL. Break the steps: total mg, concentration per mL, then volume. Don’t skip steps.

Tablet splitting: If a prescription calls for 7.5 mg and tablets are 15 mg, split the tablet. Use a pill cutter for accuracy. If you can't split evenly, ask the pharmacist for a different strength or formulation.

IV infusion rates: Use mcg/kg/min and convert to mL/hr with concentration details. Example: medication ordered at 1 mcg/kg/min for a 70 kg patient = 70 mcg/min = 4200 mcg/hr = 4.2 mg/hr. If your bag concentration is 2 mg/mL, set pump to (4.2 mg/hr ÷ 2 mg/mL) = 2.1 mL/hr.

Reconstitution: Check vial label for mg per vial and the diluent volume. If vial contains 500 mg and you add 10 mL, concentration is 50 mg/mL. Record the final concentration before drawing doses.

Safety tips: Write every step down and show your work. Use calculators and bedside apps but verify the result manually. When in doubt, call the pharmacist or prescriber. Label syringes and bags clearly with concentration and time prepared. For high-risk meds, get a second check.

Tools that help: unit converters, dosing charts, and trusted apps. Practice with real examples until the steps feel automatic. Good conversion habits save time and prevent errors.

If a dose ever seems odd, stop and ask. Pharmacists want calls, not surprises. Keep a small conversion cheat sheet where you store common concentrations and conversions for medicines you use often. That saves time and reduces mistakes. Share it with caregivers regularly.