Quick answer: GFR (glomerular filtration rate) tells how well your kidneys filter blood. Enter your age, gender, and serum creatinine above to get your estimated GFR (eGFR) with the CKD-EPI 2021 formula, plus your CKD stage from G1 (normal) to G5 (kidney failure). This tool is for information only and is not medical advice.
Last reviewed: October 2026
Key facts
- GFR measures kidney filtering in mL/min/1.73 m2; higher is better.
- 90 or above is normal (G1); below 60 for 3 months or more defines chronic kidney disease.
- The CKD-EPI 2021 formula is the current standard and does not use race.
- eGFR is an estimate: muscle mass, dehydration, and medications can shift it.
A GFR calculator estimates how well your kidneys filter waste from your blood using your age, gender, and serum creatinine level. Enter those three values, and it applies the CKD-EPI 2021 formula to give your estimated GFR and your CKD stage, G1 through G5.
Your glomerular filtration rate is the single most useful number for judging kidney health. Kidneys filter about 180 liters of blood every day, removing waste like creatinine into the urine. When filtration drops, creatinine rises in the blood, so a blood creatinine test plus your age and sex is enough to estimate GFR with the CKD-EPI 2021 equation. This free tool does the math instantly and maps the result to the standard CKD stages.
Medical disclaimer: This calculator is for general information only. It cannot diagnose kidney disease or replace a doctor. Only a clinician with your full lab history can interpret your eGFR correctly.
How to use the GFR calculator
- Enter your age in years (18 or older).
- Select your gender (female or male).
- Enter your serum creatinine in mg/dL from your blood test.
- Click Calculate to see your eGFR and CKD stage.
GFR formula (CKD-EPI 2021)
The 2021 CKD-EPI equation is the current standard recommended by the National Kidney Foundation and the American Society of Nephrology, and it removed the race variable used in older versions. In the formula, Scr is serum creatinine in mg/dL, k is 0.7 for women and 0.9 for men, and a is -0.241 for women and -0.302 for men. The two-sided form handles both low and high creatinine: min(Scr/k, 1) stays at 1 when creatinine is above the threshold, and max(Scr/k, 1) stays at 1 when it is below.
Worked examples
Example 1: 55-year-old woman, creatinine 1.2 mg/dL
k = 0.7 and a = -0.241. Scr/k = 1.2 / 0.7 = 1.714, so min = 1 and max = 1.714. eGFR = 142 x 1 x 1.714-1.200 x 0.993855 x 1.012 = 142 x 0.524 x 0.710 x 1.012 = 53 mL/min/1.73 m2 (rounded). That falls in the 45 to 59 band, so the CKD stage is G3a (mild to moderate decrease).
Example 2: 45-year-old man, creatinine 1.0 mg/dL
k = 0.9 and a = -0.302. Scr/k = 1.0 / 0.9 = 1.111, so min = 1 and max = 1.111. eGFR = 142 x 1 x 1.111-1.200 x 0.993845 = 142 x 0.881 x 0.756 = 95 mL/min/1.73 m2 (rounded). That is at or above 90, so the CKD stage is G1 (normal or high).
CKD stages by GFR
| Stage | GFR (mL/min/1.73 m2) | What it means |
|---|---|---|
| G1 | 90 or above | Normal or high |
| G2 | 60 - 89 | Mildly decreased |
| G3a | 45 - 59 | Mild to moderate decrease |
| G3b | 30 - 44 | Moderate to severe decrease |
| G4 | 15 - 29 | Severe decrease |
| G5 | Below 15 | Kidney failure |
What affects your eGFR result
Several things move creatinine and therefore the estimate. Dehydration concentrates creatinine and can push eGFR down temporarily. High muscle mass or intense exercise in the days before the test raises creatinine and lowers the estimate, while very low muscle mass does the opposite. Some medications, including trimethoprim and cimetidine, raise creatinine without harming the kidneys. A high-meat meal before bloodwork can also nudge creatinine up. This is why doctors track eGFR over time instead of reacting to a single number.
Tips for a useful result
- Use the creatinine value from a real blood test, in mg/dL, exactly as your lab reported it.
- Be well hydrated and rested before your test for the most representative value.
- Compare results from the same lab over time; different labs can vary slightly.
- A single low reading is not a diagnosis; doctors confirm with repeat tests over at least 3 months.
- Bring this estimate to your doctor and ask what it means for your blood pressure and medication doses.
Common mistakes
- Treating eGFR as exact. It is an estimate; at very high or low muscle mass it can miss.
- Panicking over one reading. A temporary dip after dehydration or exercise usually recovers.
- Ignoring the trend. A slow yearly decline matters more than any single snapshot.
- Using old formulas. The race-free CKD-EPI 2021 equation is the current standard; older MDRD or race-based versions read differently.
Frequently asked questions
What is a normal GFR?
A GFR of 90 or above is considered normal (Stage G1). Chronic kidney disease is defined as a GFR below 60 lasting at least 3 months, or kidney damage signs even with a normal GFR.
What does GFR actually measure?
GFR measures how many milliliters of blood your kidneys filter each minute, adjusted to a standard body surface area of 1.73 m2. The main waste it removes is creatinine, so blood creatinine levels reflect filtration speed.
Is eGFR accurate?
It is a good estimate but not a direct measurement. Muscle mass, hydration, intense exercise, diet, and some medications shift creatinine and the result. Doctors confirm low values with repeat tests before diagnosing anything.
What makes creatinine go up?
Dehydration, high muscle mass, intense exercise, high-meat meals, and medications like trimethoprim can raise creatinine. Rising creatinine lowers the estimated GFR, which is why context matters.
Can I improve my GFR?
You cannot calculate your way to a higher number, but the habits that protect kidneys are well known: control blood pressure and blood sugar, stay hydrated, avoid smoking and excess NSAID use, and keep a healthy weight. Discuss any change with your doctor.
Do I need to fast before a creatinine test?
Most labs do not require fasting for a creatinine test, but a large meat meal beforehand can raise the reading slightly. Ask your lab for its instructions and keep the same routine each time for comparable results.
What eGFR does a 50-year-old man with creatinine 1.2 mg/dL get?
With the CKD-EPI 2021 formula (k = 0.9, a = -0.302 for men): Scr/k = 1.2 / 0.9 = 1.333, so eGFR = 142 x 1.333-1.200 x 0.993850 = 142 x 0.710 x 0.733 = about 74 mL/min/1.73 m2. That falls in the 60 to 89 band, so the CKD stage is G2 (mildly decreased).
How much does 10 years of ageing lower eGFR at the same creatinine?
The CKD-EPI 2021 age factor is 0.9938 per year, so 10 extra years multiply the result by 0.993810 = 0.940. A 50-year-old man with creatinine 1.2 mg/dL has an eGFR of about 74; at 60 with the same creatinine it is 74 x 0.940 = about 69, a drop of roughly 5 points, or about 6%.
Sources
- NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases): kidney disease testing and the role of GFR and creatinine
- National Kidney Foundation: CKD-EPI 2021 race-free eGFR equation and CKD stage classification
Key takeaways
- GFR tells how fast your kidneys filter blood; 90 or above is normal.
- CKD-EPI 2021 is the current standard formula and does not use race.
- Stages run G1 (normal) to G5 (kidney failure); below 60 for 3 months defines CKD.
- eGFR is an estimate: hydration, muscle, and medications affect it.
- This tool is informational; only your doctor can diagnose kidney disease.
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