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. [REVIEW: confirm typical drop-factor values].

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. [REVIEW: confirm clinical use cases].

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. [REVIEW: confirm tolerance vs 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.