Creatinine clearance calculator: calculate CrCl with the Cockcroft-Gault formula

National guidance has moved. The FDA's 2024 final guidance on pharmacokinetics in patients with impaired renal function prefers estimated GFR for determining renal function in pharmacokinetic studies, which is why the share of product labels stating eGFR thresholds instead of creatinine clearance keeps rising.

The pharmacy side went further. In 2025 the National Kidney Foundation Workgroup for Implementation of Race-Free eGFR-Based Medication-Related Decisions published a consensus in the American Journal of Health-System Pharmacy recommending that health systems, laboratories, electronic health record vendors, compendia and practitioners transition away from Cockcroft-Gault estimated creatinine clearance and toward the race-free eGFR equations, adjusted for the individual patient's body surface area when the purpose is a medication decision.

None of which makes this calculator obsolete, and that is the awkward part. Thousands of approved products carry labeling written when creatinine clearance was the standard, and that labeling has not been rewritten. So a US clinician in 2026 works with both: an eGFR that arrives automatically on the chemistry panel, and a label that states a threshold in mL/min. Knowing which one the specific drug asks for is the actual skill.

Strictly, you do not, because they estimate different physiology. What you can do is remove the normalization. Reported eGFR is indexed to a body surface area of 1.73 square metres, so it is expressed as mL/min/1.73 m2. Multiply that reported value by the patient's own body surface area and divide by 1.73, and you get an absolute mL/min figure that can be compared with a mL/min threshold.

For an average-sized adult the correction barely moves anything. For a 45 kg woman or a 140 kg man it moves a lot, and it moves in the direction that matters: the smaller patient's indexed eGFR reads higher than their true absolute clearance, so an unadjusted comparison can leave a frail patient on a full dose of a renally cleared drug. That is the single most common way this arithmetic goes wrong in practice.

The same trap appears with the drugs where the margin is narrowest. Our reference list of drugs to avoid in renal failure covers the agents where the answer is avoidance rather than adjustment, and the guidance on antibiotics that are safe in renal impairment works through the class most often dosed against a CrCl band.

Creatinine clearance overestimates true glomerular filtration rate as a matter of physiology, because the tubules secrete creatinine on top of what is filtered. Cockcroft-Gault adds its own sources of error. It was derived in 1976 from a small and predominantly male cohort, before creatinine assays were standardized, and it uses weight as a proxy for muscle mass, which is exactly where it breaks in the very obese, the very frail and the critically ill.

Those limitations are the substance of the case for moving to race-free eGFR. They are also a reason to treat any single number, from any equation, as one input rather than an answer. In an unstable patient a creatinine drawn this morning describes the kidney of a day or two ago, and no equation fixes that.