Creatinine Clearance Calculator

Background.

Look up the kidney dosing for many medicines and the table is organised by creatinine clearance in millilitres per minute, not by the eGFR printed on the laboratory report. That number usually comes from the Cockcroft-Gault equation, published by D.W. Cockcroft and M.H. Gault in the journal Nephron in 1976, which estimates creatinine clearance from four things a clinician always has to hand: age, sex, body weight and serum creatinine.

The point of the equation is that serum creatinine on its own misleads. Creatinine is a waste product of muscle, so how much circulates depends on how much muscle a person has as well as on how well their kidneys clear it. A creatinine of 1.4 mg/dL means something quite different in a 68-year-old woman weighing 62 kg than in a 40-year-old woman of the same weight: Cockcroft-Gault puts the first at 37.64 mL/min and the second at 52.28. The equation folds in the decline in kidney function and muscle with age, scales by body size through weight, and reduces the estimate by 15% for women, as the original paper proposed.

It is an old equation, and it shows. It was built on creatinine methods that predate today's standardised assays, it does not adjust for body surface area as eGFR does, and it is least reliable at the extremes: very old or frail patients, very large or very small bodies, amputees, pregnancy, and anyone whose creatinine is changing day to day. Modern laboratory reporting uses race-free CKD-EPI eGFR equations, which NIDDK describes as estimates rather than precise measures. Cockcroft-Gault survives largely because so many drug labels were written around it.

The result supports clinical judgement and does not replace it. Which weight to enter, whether the creatinine is stable, and how to translate the number into a dose belong to the prescriber and pharmacist, guided by the drug's own label.

What is creatinine clearance calculator?

Creatinine clearance is the volume of plasma cleared of creatinine each minute, in mL/min. Measured directly, it needs a timed urine collection, usually over 24 hours, and a blood creatinine, and the clearance is the creatinine excreted divided by the plasma concentration. The Cockcroft-Gault equation estimates that figure from a single blood test. Because the kidney tubules secrete some creatinine on top of what the glomeruli filter, creatinine clearance runs higher than the true glomerular filtration rate, and the gap widens as kidney function falls. The estimate is an absolute rate for the patient's own body, not a value scaled to a standard body size. That is the property drug dosing cares about: a medicine removed mainly by the kidneys accumulates when clearance falls, so labels lower the dose or lengthen the interval between doses once clearance drops below the levels their studies identified.

How to use this calculator.

1. 01Check that the patient is an adult (the equation requires an age of 18 or over) and that the creatinine is reasonably stable, not climbing or falling from one day to the next.
2. 02Enter age in years and select sex as used by the original equation, which applies its 0.85 factor to females.
3. 03Enter the body weight the drug label or local protocol specifies, whether actual, ideal or adjusted, and its unit. The calculator uses the weight exactly as you enter it, converting pounds to kilograms.
4. 04Enter the serum creatinine and choose its unit: mg/dL as reported in the US, or µmol/L as reported in most other countries.
5. 05Note anything that makes muscle mass unusual, such as an amputation, paralysis, severe wasting or a very muscular build: the equation assumes average muscle for the weight and will misjudge clearance in those patients.
6. 06Read the estimated clearance in mL/min, then apply it through the dosing table in the drug's label or your local guidance, with a pharmacist's review for high-risk medicines.

The formula.

The equation is CrCl = (140 − age) × weight ÷ (72 × serum creatinine), multiplied by 0.85 for women, with weight in kilograms, creatinine in mg/dL and the result in mL/min. Creatinine in µmol/L is divided by 88.4 first, and weight in pounds is divided by 2.2046.

Each term has a job. The (140 − age) factor lowers the estimate by the same amount for each year of age, reflecting both the decline in filtration and the loss of muscle that come with ageing. Proportionally the effect grows with age: going from 68 to 78 cuts the age term from 72 to 62, lowering the estimate by 13.9% with nothing else changed. Weight stands in for muscle mass, and so for how much creatinine the body makes each day. The constant 72 calibrates the units. Serum creatinine sits in the denominator, so the estimate is inversely proportional to it: halve the creatinine and the clearance doubles. That makes the result touchy at low creatinine values, where a difference of 0.1 or 0.2 mg/dL between assays or days shifts it noticeably. The 0.85 factor reflects the original authors' proposal for women, whose lower average muscle mass means less creatinine production at the same kidney function.

The result is not indexed to body surface area. Laboratory eGFR is reported per 1.73 m², a standard body size, while Cockcroft-Gault gives the clearance of the patient's own body, which is what drug elimination depends on. The two diverge most for patients far from average size.

The equation assumes a steady state in which the creatinine made each day equals the creatinine excreted. When kidney function changes quickly, the blood level takes days to catch up, and any single-sample estimate lags behind reality.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Cockcroft, D.W. and Gault, M.H. (1976). "Prediction of creatinine clearance from serum creatinine." Nephron 16(1):31-41.
2. [2]U.S. Food and Drug Administration (2024). "Pharmacokinetics in Patients with Impaired Renal Function: Study Design, Data Analysis, and Impact on Dosing." FDA Guidance.
3. [3]National Institute of Diabetes and Digestive and Kidney Diseases (n.d.). "eGFR Calculators for Adults and Pediatrics." NIDDK.
4. [4]National Institute of Diabetes and Digestive and Kidney Diseases (2025). "eGFR Equations for Adults." NIDDK.
5. [5]Kidney Disease: Improving Global Outcomes (2024). "KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease." KDIGO.
6. [6]National Kidney Foundation (n.d.). "Cockcroft-Gault Equation for Estimating Creatinine Clearance." NKF.