Your kidneys clear waste from the blood at a rate called the glomerular filtration rate, or GFR. A GFR calculator estimates that rate from a few numbers on your blood work — your serum creatinine, age, body weight, and sex — using the Cockcroft-Gault equation, the formula clinicians have used since 1976 to quick-check kidney function at the bedside.
This tool estimates creatinine clearance (CrCl), which closely tracks GFR and is the figure most people mean when they ask "what is my GFR?" Enter your lab values below and the result updates instantly, along with a CKD stage and a plain-English explanation of what the number does — and does not — mean.
How to Use
- Enter your Serum Creatinine exactly as it appears on the lab report, in mg/dL. A typical adult value runs about 0.6–1.3 mg/dL; higher values mean the kidneys are filtering more slowly.
- Type your Age in years. The Cockcroft-Gault equation assumes adults 18 and older; creatinine naturally rises and estimated clearance falls as people age.
- Enter your Body Weight in kilograms. If your report is in pounds, divide by 2.205 first. The original equation works best with your dry or lean weight rather than an overweight peak.
- Choose your Sex. Female results are multiplied by 0.85 to account for lower muscle mass and creatinine production.
- Read the result card: the headline number is your estimated GFR in mL/min, followed by the CKD stage code (1 = G1 normal at/above 90, 2 = G2 mild 60–89, 3 = G3a 45–59, 4 = G3b 30–44, 5 = G4 severe 15–29, 6 = G5 failure under 15) and how many points you sit below the normal floor of 90 mL/min.
The calculator rejects a creatinine of 0 or less, and any value outside a plausible adult range, rather than producing a nonsense number.
The Cockcroft-Gault Formula
The equation is deliberately simple, which is why it has survived for decades:
CrCl = ((140 − age) × weight in kg) / (72 × serum creatinine)
For females, multiply the whole result by 0.85.
Where:
- 140 − age shrinks as you get older, reflecting the natural decline in filtration.
- weight in kilograms is multiplied in, because larger muscle mass produces more creatinine.
- 72 × serum creatinine is the denominator: the higher the creatinine, the lower the clearance.
- 0.85 is the female adjustment factor.
Worked example for a 50-year-old man weighing 70 kg with a creatinine of 1.0 mg/dL: ((140 − 50) × 70) / (72 × 1.0) = (90 × 70) / 72 = 6,300 / 72 = 87.5 mL/min. That falls in the G2 band, "mildly reduced" — still reassuringly close to normal for a middle-aged adult.
A woman the same age and creatinine, but weighing 60 kg, gets ((90 × 60) / 72) × 0.85 = 75 × 0.85 = 63.75 mL/min. The 0.85 factor lowers her result by 15%, which is why two people with the identical lab number can read differently on this page.
How GFR Maps to CKD Stages
Once the calculator computes clearance, it maps the mL/min figure onto the chronic kidney disease staging system used by nephrologists:
- G1 — 90 or more: normal or high kidney function.
- G2 — 60 to 89: mildly reduced, common with age.
- G3a — 45 to 59: mild-to-moderate reduction.
- G3b — 30 to 44: moderate-to-severe reduction.
- G4 — 15 to 29: severely reduced.
- G5 — under 15: kidney failure.
Staging is only half the picture. CKD is formally diagnosed when GFR stays below 60 for three months or more, or when there is other evidence of kidney damage such as protein in the urine. A single borderline result is worth a repeat test rather than panic.
Cockcroft-Gault vs CKD-EPI
You may also have seen an "eGFR" printed on your lab report. That number usually comes from the newer CKD-EPI 2021 equation, which does not use body weight. This page uses Cockcroft-Gault instead because it still powers most drug-dose adjustments — antibiotics, chemotherapy, and blood thinners are commonly dosed against CrCl — and because it lets you experiment with weight. The two equations usually agree within about 10–15 mL/min in stable adults, but they are not interchangeable for dosing.
Examples
Example 1 — A healthy middle-aged adult. A 50-year-old man weighing 70 kg checks a routine blood test: creatinine 1.0 mg/dL. His estimated GFR is 87.5 mL/min, stage G2, "mildly reduced kidney function." The recommended step is a routine check-up — nothing urgent, just a repeat in a year or two.
Example 2 — The female adjustment in action. A 50-year-old woman, 60 kg, same creatinine of 1.0 mg/dL. Her result is 63.75 mL/min, still G2, but notice it is roughly 15% below the man's figure despite the identical lab value. That gap is the 0.85 sex factor, not a sign her kidneys are failing.
Example 3 — Moderate impairment. A 70-year-old man weighing 60 kg has a creatinine of 2.0 mg/dL. The equation gives ((140 − 70) × 60) / (72 × 2.0) = 4,200 / 144 = 29.2 mL/min, stage G4, "severely reduced kidney function." The calculator flags an urgent nephrology referral and a repeat urine albumin test.
FAQ
What is a normal GFR?
A normal estimated GFR is 90 mL/min or higher (stage G1). From about age 40, filtration naturally declines by roughly 0.5–1 mL/min per year, so a result of 60–89 in a healthy older adult is often stage G2 and not, by itself, alarming. The trend over time matters more than any single reading.
How is GFR different from creatinine?
Creatinine is a waste molecule measured in your blood, in mg/dL. GFR is the rate at which your kidneys clear that waste, in mL/min. A high creatinine usually means low clearance, but raw creatinine also rises with muscle mass, age, and diet — which is exactly why the Cockcroft-Gault equation folds age, weight, and sex into a single estimated rate.
Why does this calculator use weight while my lab report's eGFR does not?
Your printed eGFR typically uses the CKD-EPI equation, which estimates clearance from creatinine, age, and sex only, without weight. Cockcroft-Gault deliberately includes weight, which makes it the standard formula for adjusting drug doses. The two agree within about 10–15 mL/min for most stable adults, but they answer slightly different questions.
What does the 0.85 female factor mean?
On average, women carry less muscle mass than men, so they produce less creatinine at any given level of kidney function. The Cockcroft-Gault equation multiplies a woman's raw result by 0.85 — a 15% reduction — to correct for that. It is a population average, not a diagnosis.
When should I worry about my GFR result?
A single reading below 60 mL/min is worth repeating in three months and discussing with your doctor, especially if you have diabetes, high blood pressure, or a family history of kidney disease. A result below 30, or one that is dropping quickly over time, warrants prompt specialist review. Never adjust prescription drugs based on this page alone.
Can I use this to dose my medication?
Cockcroft-Gault CrCl is widely used to adjust kidney-cleared drugs, but dosing is a clinical decision that must account for your full medication list, liver function, and body composition. This calculator gives an estimate only — always confirm doses with your prescriber or pharmacist.
Disclaimer
This GFR calculator is provided for informational purposes only and is not medical advice. It uses the Cockcroft-Gault equation to estimate creatinine clearance from the numbers you enter; it does not measure your actual kidney function, and results are not normalized to body surface area. It is not a substitute for professional medical diagnosis, treatment, or the specific eGFR reported by your accredited laboratory. Always discuss your kidney function results with a qualified healthcare provider before changing any medication or treatment.