Your lab report says serum creatinine 1.1 mg/dL, and at your checkup the doctor mentions 'your kidney numbers look fine.' What numbers? Behind that reassurance sits the GFR, the glomerular filtration rate: an estimate of how many milliliters of blood your kidneys filter every minute. It is the single most used screening number for kidney function, and this calculator estimates it from the same three inputs your lab uses.
How the Estimate Is Built
Formula: eGFR is estimated from age, sex, and serum creatinine with a clinical equation such as the CKD-EPI formula: eGFR = 142 x min(Scr/k, 1)^a x max(Scr/k, 1)^-1.200 x 0.9938^Age (x 1.012 for females), where Scr is serum creatinine in mg/dL, k is 0.9 for men and 0.7 for women, and a is -0.302 for men and -0.241 for women.
Creatinine is a waste product your muscles make at a fairly steady rate, and healthy kidneys filter almost all of it out. So when blood creatinine runs high, filtration is probably running low, and the equation inverts that relationship. Age and sex enter because muscle mass, and therefore normal creatinine, differs across them; the 0.9938^Age term quietly lowers the estimate as you get older, which matches real kidney aging. The result is reported per 1.73 square meters of body surface, the standard normalization. The equation applies as a screening estimate for adults with stable kidney function, not during acute illness.
What to Enter
- Your age in years.
- Your sex, since the constants differ.
- Serum creatinine from a recent lab report, in mg/dL (convert from micromol/L by dividing by 88.4).
- Read the eGFR in mL/min/1.73m^2 and discuss it with your doctor.
A Worked Example
A 45-year-old man has a serum creatinine of 1.1 mg/dL. For men, k = 0.9 and a = -0.302.
- Scr/k = 1.1 / 0.9 = 1.222.
- min(1.222, 1) = 1, and 1^-0.302 = 1.
- max(1.222, 1) = 1.222, and 1.222^-1.200 = 0.786.
- 142 x 1 x 0.786 = 111.6.
- 0.9938^45 = 0.756.
- 111.6 x 0.756 = 84 mL/min/1.73m^2.
An eGFR around 84 sits above the 60 threshold where clinicians start paying closer attention, which is the kind of context the raw creatinine number alone never gives you.
Reading the Result Sensibly
Above 90 with no other signs is generally reassuring territory, 60 to 89 calls for watching trends over time, and below 60 is where doctors investigate further. Protein intake and creatine supplements can nudge creatinine, so the protein calculator helps you see whether a high-protein diet might be coloring your labs, and the body surface area calculator shows the 1.73 m^2 reference your result is normalized against.
Mistakes That Distort the Number
- Wrong creatinine units. Labs in many countries report micromol/L. Entering 97 (micromol/L) where the tool expects 1.1 (mg/dL) produces a wildly wrong eGFR. Divide by 88.4 first.
- Testing after extreme exertion. A brutal workout or a huge meat-heavy meal temporarily raises creatinine. Test rested and normally fed for a representative number.
- Treating one reading as a diagnosis. Creatinine wobbles with hydration and muscle mass. Doctors look at trends and confirm with repeat tests, never a single calculator run.
A Serious Caveat
eGFR is a screening estimate, not a diagnosis and not medical advice. Only a clinician can interpret it alongside your history, urine tests, and other labs. If your result is low or falling, see a doctor promptly instead of researching further online, and never change medication based on this number.