Dosage Calculation Formulas

volume (mL) = dose (mg) ÷ strength (mg/mL)

The relationship the whole site runs on. Neither quantity converts to the other without the strength.

strength (mg/mL) = amount (mg) ÷ volume (mL)

How most oral liquids are labelled: 250 mg in 5 mL is 50 mg/mL.

strength (mg/mL) = percentage × 10

A percentage is grams of drug in 100 mL, so 1% is 10 mg/mL.

strength (mg/mL) = 1000 ÷ the second number

1:1000 is one gram in 1000 mL, which is 1 mg/mL.

volume (mL) = dose (units) ÷ strength (units/mL)

International units measure activity, not mass, so they never convert to milligrams.

1 g = 1000 mg = 1,000,000 mcg

Fixed for every drug and every product, unlike the dose-to-volume relationship.

dose (mg) = mg/kg × weight (kg)

Whether the mg/kg figure is per dose or per day changes the answer by the number of doses.

dose per administration = daily total ÷ doses per day

Applies only when the order is written per day.

tablets = dose ÷ strength of one tablet

A result that is not a whole or half tablet means the stock strength cannot make the dose.

mL/hr = volume (mL) ÷ time (hr)

What a pump is programmed with. The giving set is irrelevant to it.

time (hr) = volume (mL) ÷ rate (mL/hr)

The same relationship read the other way.

gtt/min = volume (mL) × drop factor (gtt/mL) ÷ time (min)

The drop factor is a property of the giving set and is printed on its packet.

mL/hr = (dose rate × weight) ÷ bag strength (mcg/mL) × 60

A dose rate and a pump rate are different quantities. Only the second goes into a pump.

BSA (m²) = √(height (cm) × weight (kg) ÷ 3600)

Mosteller RD. Simplified calculation of body-surface area. N Engl J Med. 1987;317(17):1098.

BSA (m²) = 0.007184 × height^0.725 × weight^0.425

Du Bois D, Du Bois EF. A formula to estimate the approximate surface area. Arch Intern Med. 1916;17:863-71.

BSA (m²) = 0.024265 × height^0.3964 × weight^0.5378

Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area. J Pediatr. 1978;93(1):62-6.

BSA (m²) = 0.0235 × height^0.42246 × weight^0.51456

Gehan EA, George SL. Estimation of human body surface area from height and weight. Cancer Chemother Rep. 1970;54(4):225-35.

CrCl (mL/min) = (140 − age) × weight (kg) ÷ (72 × Scr in mg/dL), × 0.85 if female

Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron. 1976;16(1):31-41.

142 × min(Scr/κ,1)^α × max(Scr/κ,1)^−1.200 × 0.9938^age × 1.012 if female

Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations without race. N Engl J Med. 2021;385:1737-49.

corrected Ca (mg/dL) = measured Ca + 0.8 × (4.0 − albumin in g/dL)

Payne RB, Little AJ, Williams RB, Milner JR. Interpretation of serum calcium in patients with abnormal serum proteins. BMJ. 1973;4(5893):643-6.

corrected gap = gap + 2.5 × (4.0 − albumin in g/dL)

Figge J, Jabor A, Kazda A, Fencl V. Anion gap and hypoalbuminemia. Crit Care Med. 1998;26(11):1807-10.