Dosage Calculation Formulas: The Complete Cheat Sheet

Oral doses

The desired-over-have formula covers every tablet and liquid question: divide what is ordered by the strength you have, and multiply by the volume (or tablet) that strength comes in. Keep the units of "desired" and "have" identical before dividing — converting after dividing is where errors breed.

IV rates

Pump rates are volume over time in hours. Gravity drips multiply by the tubing drop factor and divide by minutes. Units-based drips (heparin, insulin) divide the ordered units per hour by the bag concentration in units per mL.

Weight-based doses

Multiply the ordered dose per kilogram by the patient's weight in kilograms — converting from pounds first by dividing by 2.2. For daily totals divided across doses, compute the full day first, then divide by doses per day (q6h = 4, q8h = 3, q12h = 2).

Critical-care drips

For mcg/kg/min orders: dose × weight × 60 gives mcg per hour; divide by the bag concentration in mcg/mL for the pump rate. The two universal slips are forgetting the ×60 and leaving the concentration in mg instead of mcg.

Reconstitution

After mixing a powder vial, the concentration that matters is total drug over final total volume — not the diluent volume you added. Then it is desired-over-have as usual. Exam questions deliberately print the diluent volume hoping you divide by it.

Pediatric specials

BSA dosing multiplies the ordered mg/m² by the child's body surface area. Maintenance fluids follow the 100-50-20 rule: 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10, 20 mL/kg for every kilogram after that.

Safe-dose-range questions ask you to compute both ends of a mg/kg/day range and judge whether an order falls inside — the arithmetic is easy; reading which end the question wants is the trap.

Print it, then practice past it

A cheat sheet gets you oriented; automaticity gets you passed. Every formula above is drilled by topic in our free practice app, with worked steps on every question and diagnosis of exactly which formula you misapplied when you miss.