GFR Calculator
Estimate glomerular filtration rate with the race-free CKD-EPI 2021 equation and read the KDIGO stage it implies.
GFR Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Indexed to 1.73 m²; not directly comparable to absolute clearance without de-indexing.
- Unreliable at extremes of muscle mass, where cystatin C is preferred.
- Cannot stage CKD alone — requires albuminuria and persistence beyond three months.
In short: An 82-year-old woman with a serum creatinine of 1.0 mg/dL has a CKD-EPI 2021 eGFR of 56.2 mL/min/1.73m² — KDIGO stage G3a. The 2009 equation would have said 52.0, and eGFR alone cannot stage chronic kidney disease without a urine albumin result.
Formula
CKD-EPI 2021. No race coefficient. Result is indexed to 1.73 m² body surface area.
Worked Example
- Take the creatinine ratio. Scr ÷ κ = 1.0 ÷ 0.7 = 1.4286, which exceeds 1.
- Apply the upper spline. 1.4286−1.200 = 0.6432, since the ratio is above 1.
- Apply the age term. 0.993882 = 0.6018.
- Combine with the female factor. 142 × 0.6432 × 0.6018 × 1.012 = 56.2 mL/min/1.73m².
- Read the stage. 56.2 falls in G3a — but staging is incomplete without urine albumin.
Two things about this result deserve more attention than the number itself. First, the 2009 equation would have reported 52.0 for the same blood sample; the 2021 revision raises it by 4.2, which is enough to move some people into a milder stage without any change in their kidneys. That is a deliberate correction — the old equation's race coefficient reported systematically higher values for Black patients and could delay referral — but it means historical results are not directly comparable. Second, an eGFR alone does not stage chronic kidney disease. KDIGO staging is two-dimensional, pairing the G category with an albuminuria category, and requires the abnormality to persist beyond three months. A single number, taken once, stages nothing.
Strengths & Limits Of This Model
Where this engine is strong
- Uses the current race-free CKD-EPI 2021 equation.
- Reports the 2009 result so historical values can be reconciled.
- States explicitly that the KDIGO stage is incomplete without urine albumin.
Where it stops
- Cannot be used for drug dosing where the label specifies CrCl.
- Cannot substitute for cystatin C where muscle mass is atypical.
Practical Use Cases
Understanding a lab report
The eGFR your laboratory prints is this equation, indexed to an average-sized adult.
Comparing with an older result
If the older value used the 2009 equation it is not directly comparable to this one.
Checking whether a dose should change
Most labels specify CrCl, not eGFR. Use the Creatinine Clearance Calculator.
Interpreting eGFR at extremes of muscle mass
Creatinine comes from muscle, so cystatin C is the better marker in wasting or in bodybuilders.
Methodology & Editorial Standards
Glomerular filtration rate is estimated using the CKD-EPI 2021 creatinine equation, which uses serum creatinine, age and sex and deliberately excludes race. The 2021 refit followed the NKF-ASN task force conclusion that race is a social rather than biological variable and that its inclusion, which raised reported eGFR for Black patients, risked delaying referral and transplant listing. The engine reports the 2009 result alongside so that historical values can be reconciled, since the revision typically raises eGFR by around four units for people previously classified as non-Black. The headline figure is indexed to 1.73 square metres of body surface area to permit comparison between individuals, and the engine also reports the de-indexed absolute value, which is the more relevant quantity when body size departs markedly from average. The KDIGO G category is stated, together with an explicit caveat that staging is two-dimensional and incomplete without a urine albumin-to-creatinine ratio and persistence of the abnormality beyond three months. Because creatinine derives from skeletal muscle, the engine identifies the circumstances in which cystatin C or the combined equation is the more reliable marker. Estimating equations assume steady state and are invalid in acute kidney injury. This calculator is an educational estimate and is not medical advice, diagnosis, treatment or a prescription. It performs arithmetic on values you enter and cannot assess your clinical condition. Never start, stop, change or skip a dose of any medicine on the basis of a number produced here. Dose selection belongs to a prescriber working from the approved product label, your full medication list and your current clinical state. If you are acutely unwell, dehydrated, vomiting, or your kidney function is changing, estimating equations are unreliable and urgent clinical review is required. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
GFR Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is eGFR?
An estimate of how much blood your kidneys filter per minute, normalised to a body surface area of 1.73 m².
What is the CKD-EPI 2021 equation?
The current race-free standard, using creatinine, age and sex. It replaced the 2009 equation and its race coefficient.
Why was race removed?
It is a social construct, not a physiological variable, and its use reported higher eGFR for Black patients, risking delayed care.
Does the new equation change my result?
Usually it raises it by around 4 mL/min for those previously classified as non-Black, which can shift the reported stage.
What is stage G3a?
An eGFR of 45 to 59. It indicates mild to moderate reduction, but staging needs albuminuria too.
Can eGFR alone diagnose kidney disease?
No. CKD requires abnormality persisting over three months plus a urine albumin measurement.
Why does my eGFR differ from my creatinine clearance?
Different quantities and units. eGFR is indexed to 1.73 m²; CrCl is absolute mL/min.
Should medication be dosed from eGFR?
Only if the label says eGFR. Most existing labels specify creatinine clearance instead.
What is de-indexing?
Multiplying eGFR by your own BSA and dividing by 1.73, giving absolute mL/min for your body.
When is cystatin C better?
At extremes of muscle mass — bodybuilders, amputees, frail elderly, cirrhosis. It does not depend on muscle.
Does eGFR fall with age?
Yes, by roughly 1 mL/min per year after about 40. Age-related decline is not automatically disease.
Is a single low reading a problem?
Not necessarily. Dehydration and acute illness lower it temporarily. Confirmation needs a repeat test.
What is albuminuria staging?
A1 under 30, A2 30 to 300, A3 above 300 mg/g. It pairs with the G category to give prognosis.
Can eGFR improve?
Yes, if the cause is reversible — dehydration, obstruction, or a medication effect that is corrected.
Does high protein intake affect it?
It can raise creatinine transiently. Very high creatine supplementation can also mislead the estimate.
Why does my lab report a different number?
Check which equation and which assay. Some laboratories still report the 2009 or MDRD equations.
Is this gfr calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.