1. Volume over time. mL/hour = volume ÷ hours. Drops per minute = (volume in mL × drop factor) ÷ (time in minutes), which is the same as (mL/hour × drop factor) ÷ 60. The drop factor is printed on the giving-set packet: 20 drops/mL for a standard macro set, 15 or 10 for other macro sets, 60 for a microdrip or burette.

2. Dose to rate. Concentration = amount of drug ÷ final volume in the bag. Then mL/hour = (dose per hour for the whole patient) ÷ concentration. For mcg/kg/min, the hourly dose is dose × weight × 60. Units are kept in micrograms throughout, so a bag labelled in milligrams is converted (1 mg = 1000 mcg) before dividing.

3. Time to empty. Hours remaining = volume left ÷ rate, converted to hours and minutes and to a wall-clock finish time from the moment you press the key. That clock is a projection from now, not from when the infusion actually started.

Worked examples. Volume: 500 mL over 4 hours on a 20 drops/mL set = 125 mL/hour; 125 × 20 ÷ 60 = 41.67, so 42 drops/minute — and on a gravity set that rounding is a real 0.8 % rate error, worth knowing on a narrow-therapeutic-index drug. Dose: 400 mg in 50 mL = 8 mg/mL = 8000 mcg/mL; 70 kg at 5 mcg/kg/min = 5 × 70 × 60 = 21 000 mcg/hour; 21 000 ÷ 8000 = 2.63 mL/hour, and the 50 mL bag lasts 19.0 hours. Empty: 320 mL at 125 mL/hour = 2.56 hours = 2 hours 34 minutes.

Where it goes wrong. Gravity sets do not hold the calculated rate — the drip rate drifts with cannula position, bag height, patient movement and venous pressure, and a rate calculated to two decimal places will still be wrong at the bedside within minutes. Anything with a narrow therapeutic index goes on a volumetric pump. Concentration is the most common source of a real error: this tool takes the final volume in the bag including the drug's own volume, so a 50 mL syringe with 4 mL of drug added is 54 mL, not 50 mL. The time-to-empty clock starts when you look at it, so it assumes no bolus, no second line into the same cannula and no rate change. Drops/min is shown rounded because you cannot count a fraction of a drop, but the unrounded figure is given alongside so you can see the size of the rounding.

Disclaimer. Arithmetic for qualified clinicians double-checking their own working — not medical advice and not a substitute for independent checking of a prescription or an infusion. Always verify against the prescription, the product label and your local IV administration standard. Last reviewed: 2026-09-21.