Pharmacology & Dosing Calculators

The pharmacology calculators cover the most common bedside dosing math: IV drip rate (gtt/min) for gravity infusions, IV infusion time from a known volume and rate, and body surface area (BSA) for size-based dose calculations.

Each calculator solves for any variable in its equation and shows the substituted arithmetic step by step. The drip-rate calculator handles the standard tubing calibrations (10, 15, 20, and 60 gtt/mL).

When to use these calculators

IV drip rate is the everyday gravity-infusion calculation — taking a prescribed mL/hr rate and converting it to drops per minute using the tubing's calibration factor. The drop factor is printed on the IV set packaging; macrodrip tubing is typically 10, 15, or 20 gtt/mL and microdrip is 60 gtt/mL.

IV infusion time answers the inverse question: given a volume to infuse and a target flow rate, how long will the bag take? Useful for planning ahead, checking pharmacy preparation timing, and verifying that a programmed pump rate finishes within a clinically appropriate window.

Body surface area is the basis for many chemotherapy and pediatric dosing regimens. The Mosteller formula — used here — gives BSA in m² from height in cm and weight in kg. BSA also feeds the cardiac-index calculation in the Hemodynamics group.

Frequently Asked Questions

How do I calculate IV drip rate?

Multiply the prescribed flow rate in mL/min by the tubing's drop factor in gtt/mL. The result is the drip rate in drops per minute. Always check the IV set packaging for the calibration factor — using the wrong drop factor produces a flow rate that's off by a factor of 2-6x.

What's the difference between macrodrip and microdrip tubing?

Macrodrip tubing delivers larger drops, typically calibrated at 10, 15, or 20 gtt/mL. Microdrip tubing delivers smaller drops at 60 gtt/mL and is used for precise low-rate infusions, common in pediatric and neonatal care. The two cannot be interchanged without recalculating the drip rate.

Which BSA formula does this calculator use?

The body surface area calculator uses the Mosteller formula: BSA = √(Height × Weight / 3600), with height in cm and weight in kg. It's the most common formula in clinical practice because it's simple and well-validated across adult and pediatric populations. Other formulas (Du Bois, Haycock, Boyd) give slightly different values — Mosteller is generally within 5% of those alternatives.

Why does the infusion-time calculator give such different results depending on units?

Pay attention to the units of the rate input. If the prescribed rate is in mL/hr and the volume is in mL, the result is in hours; if the rate is in mL/min, the result is in minutes. The calculator handles the unit conversion internally, but you have to enter values that match the labeled unit on each field.

Should I round drip rate to a whole number?

Yes. Drip rate is set by counting drops over a fixed time interval (usually 15 or 60 seconds), so fractional drops are not physically meaningful. The calculator returns a precise value; round to the nearest whole drop for bedside use, then verify by counting drops in the chamber.