Estimated glomerular filtration rate (eGFR) is the standard lab measurement used to assess how well the kidneys are filtering waste from the blood, and it’s the number behind every chronic kidney disease (CKD) diagnosis and stage. This GFR calculator uses the 2021 race-free CKD-EPI equations, the current standard recommended by the National Kidney Foundation and American Society of Nephrology, letting you calculate eGFR from creatinine, cystatin C, or both together, then classify the result using the full KDIGO CKD staging and risk grid.
⚠ This is an educational estimate, not a diagnosis. eGFR results can be affected by many factors (muscle mass, diet, pregnancy, acute illness, medications). Always discuss results with a doctor — never adjust treatment based on this tool alone.
How to use this GFR calculator
- Choose an equation tab based on which lab values you have: Creatinine, Cystatin C, or Creatinine + Cystatin C.
- Select your Creatinine Units: mg/dL (US) or µmol/L.
- Enter Age (18+) and Sex.
- Enter Serum Creatinine and/or Serum Cystatin C, depending on the equation tab selected.
- Answer “Standardized Assays” (Yes / No / Not Sure), confirming whether your lab used IDMS-traceable creatinine or standardized cystatin C calibration.
- Optionally enable “Adjust eGFR for Body Surface Area” if you need a BSA-adjusted result for medication-dosing decisions, and enter Height and Weight if so.
- Review your Estimated GFR, and check “Results — All Equations” to compare all three equations side by side using the same shared inputs.
- Under “Is It CKD?”, select the patient’s ACR (Albumin-Creatinine Ratio) range, either a category (A1/A2/A3) or an exact value in mg/g or mg/mmol.
- Review the CKD G-stage, A-category, and the KDIGO Risk Grid combining both.
- Check the Clinical Summary for monitoring frequency and referral guidance based on your specific G/A combination.
- Save your calculation or export it as an Image, PDF, or CSV.
What this GFR calculator does
GFR measures the rate at which the kidneys filter blood, and since it can’t be measured directly without an invasive clearance test, it’s estimated from blood markers instead, hence “eGFR.” This calculator offers all three current CKD-EPI approaches from a single shared set of patient inputs (age, sex, creatinine, cystatin C), so switching between equation tabs recalculates from the same underlying data rather than requiring you to re-enter anything, and the “Results — All Equations” panel lets you see how the three methods compare directly for the same patient.
The three CKD-EPI 2021 equations
| Equation |
Inputs needed |
When it’s preferred |
| CKD-EPI Creatinine (2021) |
Creatinine, age, sex |
Most common; the default when only a basic metabolic panel is available |
| CKD-EPI Cystatin C (2012) |
Cystatin C, age, sex |
More accurate when muscle mass is unusual (very muscular, amputee, or significant muscle wasting), since cystatin C isn’t affected by muscle mass the way creatinine is |
| CKD-EPI Creatinine-Cystatin C (2021) |
Creatinine, cystatin C, age, sex |
Generally the most accurate of the three, especially near-normal GFR, since combining both markers reduces the influence of either one’s individual limitations |
Race was removed from all CKD-EPI equations in the 2021 revision because the original race-adjusted versions were found to systematically overestimate kidney function in Black patients, delaying diagnosis and referral for some patients. The equations used here reflect that correction and are the versions currently recommended by US nephrology guidelines.
Why the equation you choose matters
Creatinine is produced by muscle metabolism, so a person with unusually high or low muscle mass can have a creatinine-based eGFR that doesn’t accurately reflect their true kidney function, a very muscular person’s normal kidney function might look artificially reduced, while someone with muscle wasting might look artificially normal. Cystatin C is produced by nearly all nucleated cells at a fairly constant rate, making it less sensitive to muscle mass, which is exactly why it’s recommended as a confirmatory test when the clinical picture and creatinine-based eGFR don’t seem to match, or when using the combined equation for the most reliable estimate.
Standardized assays and why the calculator asks
The CKD-EPI 2021 equations were developed using standardized (IDMS-traceable, for creatinine) laboratory assays. If your lab result wasn’t measured with a standardized assay, the equation’s accuracy can be affected, which is why this calculator explicitly asks whether your assay was standardized. Most modern US labs use standardized methods by default, but checking your lab report or asking your provider is the reliable way to confirm this if you’re not sure, rather than assuming.
Body Surface Area adjustment for medication dosing
Standard eGFR results are already normalized to a body surface area of 1.73 m², the average adult BSA, which works well for CKD staging purposes. However, some medication dosing decisions require an eGFR adjusted to the patient’s own actual body surface area rather than the standardized 1.73 m² reference, since drug clearance depends on the patient’s real filtering capacity, not a population average. This calculator’s optional BSA adjustment, using the Dubois method from height and weight, produces that patient-specific figure when a prescriber needs it, while the standard 1.73 m² result remains the correct one for staging CKD itself.
Is it CKD? The two-part definition
Either of the following, present for ≥3 months, indicates CKD:
• eGFR less than 60 mL/min/1.73m²
• uACR ≥30 mg/g (or other markers of kidney damage)
This is a genuinely important point the calculator gets right: a single lab result, even a low eGFR, doesn’t by itself confirm CKD. The KDIGO definition specifically requires either criterion to persist for three months or more, since a temporary dip in eGFR can result from dehydration, an acute illness, or a medication effect rather than chronic kidney disease. This calculator computes your result from the values you enter at this moment; the three-month persistence requirement is something only your medical history and repeat testing can confirm, not something a single calculation can verify on its own.
GFR categories (G-stages)
| Stage |
eGFR range (mL/min/1.73m²) |
Description |
| G1 |
≥90 |
Normal or high (kidney damage with normal GFR, if other markers present) |
| G2 |
60–89 |
Mildly decreased (only CKD if other kidney damage markers are present) |
| G3a |
45–59 |
Mildly to moderately decreased |
| G3b |
30–44 |
Moderately to severely decreased |
| G4 |
15–29 |
Severely decreased |
| G5 |
<15 |
Kidney failure |
Notably, G1 and G2 alone don’t meet the criteria for CKD without evidence of kidney damage (such as an elevated ACR), since a GFR in the normal-to-mildly-reduced range can occur without any underlying kidney disease.
Albuminuria categories (A-stages) and the KDIGO risk grid
| Category |
ACR range |
| A1 |
<30 mg/g (<3 mg/mmol) |
| A2 |
30–300 mg/g (3–30 mg/mmol) |
| A3 |
>300 mg/g (>30 mg/mmol) |
Combining your G-stage and A-category on the KDIGO Risk Grid is more informative than either number alone, since persistent albuminuria is itself an independent risk factor for CKD progression, even at a relatively preserved GFR. This is why the calculator’s grid cross-references both values into a single Low, Moderate, High, or Very High risk classification, along with the guidance a clinician would typically use for monitoring frequency and nephrology referral timing at that specific G/A combination.
A note on accuracy
eGFR results can shift due to muscle mass, diet, hydration status, pregnancy, acute illness, and certain medications, none of which this calculator can account for on its own. This tool produces an educational estimate only, not a diagnosis, and any result, especially one suggesting reduced kidney function, should be discussed with a doctor who can evaluate it alongside your full clinical picture and repeat testing rather than treating a single calculation as conclusive.
Frequently asked questions
Which equation should I use if I only have a creatinine result?
Use the Creatinine tab, the CKD-EPI Creatinine (2021) equation, which is the most commonly used and only requires creatinine, age, and sex. It’s a reliable general-purpose estimate for most people, though the Cystatin C or combined equations may be more accurate if muscle mass is unusually high or low.
Why did the equations remove race as a factor?
The original race-adjusted CKD-EPI equations were found to systematically overestimate kidney function in Black patients, which could delay diagnosis and appropriate referral. The 2021 race-free equations, used by this calculator, correct for this and are now the standard recommended by the National Kidney Foundation and American Society of Nephrology.
Does a single eGFR result below 60 mean I have CKD?
Not necessarily. The KDIGO definition of CKD requires either eGFR below 60 or an ACR of 30 mg/g or higher to persist for at least 3 months. A single low reading can result from temporary factors like dehydration or acute illness, so a diagnosis requires confirming persistence over time, typically through repeat testing with your doctor.
When would I need the Body Surface Area adjustment?
Standard eGFR is normalized to 1.73 m², the average adult body surface area, which is appropriate for CKD staging. A BSA-adjusted result using your actual height and weight is sometimes needed for specific medication dosing decisions, where a prescriber needs your true individual filtering capacity rather than the population-normalized figure. Your prescriber or pharmacist can tell you if this adjustment applies to a specific medication you’re being dosed for.
