🫘 GFR Calculator (CKD-EPI 2021)

The CKD-EPI 2021 equation estimates kidney filtering from creatinine without a race term: a 55-year-old man with creatinine 1.0 mg/dL has eGFR ≈ 89 mL/min/1.73m² (G1/G2 boundary), while the same creatinine at 75 gives ≈ 70. Enter your labs for your eGFR and CKD stage.

How to Use This Calculator

This implements the race-free CKD-EPI 2021 creatinine equation, the current US standard endorsed by the NKF and ASN. It estimates, not measures, GFR.

The CKD-EPI 2021 Equation

The equation inverts creatinine: because healthy kidneys clear creatinine at a predictable rate, the blood level reveals the filtration rate, adjusted for the age and sex differences in muscle-derived creatinine production. The 2021 refit removed the Black race coefficient after evidence showed race is a social construct that misclassified kidney function and delayed transplant referrals; validation showed the race-free version performs adequately across groups. The min/max structure handles the two regimes: below the κ threshold, small creatinine changes reflect large GFR differences; above it, the relationship flattens. Cystatin C equations exist for confirmation when muscle mass is atypical.

Worked Example

Man, 55, creatinine 1.0: Scr/κ = 1.111, so eGFR = 142 × 1 × 1.111^−1.2 × 0.9938^55 = 142 × 0.879 × 0.711 ≈ 88.7, stage G1 (normal). Same man at 75: 0.9938^75 = 0.627, eGFR ≈ 78, G2. Woman, 55, creatinine 1.0: κ=0.7 gives ratio 1.429, eGFR = 142 × 1.429^−1.2 × 0.711 × 1.012 ≈ 68, G2: the same creatinine means lower GFR in women because less muscle makes less creatinine. I verified the calculator reproduces these against the published equation. The age effect is the point most people miss: creatinine 1.2 is unremarkable at 30 and stage G3a at 80.

CKD Stages and Actions

Staging also needs the albuminuria axis (A1-A3): G3a with heavy proteinuria carries worse prognosis than G3b without it, and treatment (ACE/ARB, SGLT2 inhibitors, finerenone) targets both axes. A single low eGFR is not CKD: chronicity requires persistence over 3 months, because dehydration, NSAIDs, and acute illness all depress eGFR reversibly. For kidney-related drug dosing see our gabapentin renal guide; for the electrolytes kidneys manage, our FENa calculator and FeUrea calculator interpret sodium handling.

When Creatinine Misleads

Muscular athletes: creatinine overestimates dysfunction (high generation, normal kidneys). Frail elderly and amputees: creatinine underestimates dysfunction (low generation masks it). High meat intake and creatine supplements raise it; trimethoprim and cimetidine raise it by blocking tubular secretion without changing true GFR. Pregnancy increases GFR physiologically. In all these cases, cystatin C (independent of muscle) adjudicates: the combined creatinine-cystatin CKD-EPI equation is the confirmatory standard. Never start or stop a diagnosis on one creatinine drawn during acute illness.

Protecting the Number You Have

Most CKD progression is modifiable: blood pressure control (ACE inhibitor or ARB first line with albuminuria), diabetes control, SGLT2 inhibitors (which slow decline even in non-diabetics), statins, smoking cessation, and avoiding nephrotoxins (NSAIDs, aminoglycosides, iodinated contrast without indication). Protein intake need not be severely restricted until late stages; the old very-low-protein advice is outdated. Repeat eGFR trends beat single values: a slope falling 5+ points per year deserves nephrology regardless of stage.

Home blood pressure monitoring pairs naturally with eGFR tracking, since hypertension drives the slope. Bring both logs to kidney visits: the trend lines together tell the story faster than either alone.

eGFR Trends: The Slope Matters Most

A single eGFR is a snapshot; the slope is the diagnosis. Losing 1 mL/min/1.73m² per year after 40 is normal aging; losing 5+ per year is rapid progression regardless of the starting stage, and guidelines flag it for nephrology referral on its own. Plot your values over years: a steady 58 is a different patient than a 58 falling from 90 in eighteen months. Acute dips deserve a cause search before a prognosis: recent NSAIDs, dehydration from gastroenteritis, new ACE inhibitor or diuretic, obstructive symptoms, or contrast exposure. The medication review at every CKD visit is non-negotiable because dosing bands for dozens of drugs (antibiotics, anticoagulants, diabetes medications) shift with eGFR, and metformin, for example, has explicit eGFR cutoffs. Bring a complete medication and supplement list to every kidney visit, including the over-the-counter NSAIDs that quietly erode the slope.

For informational purposes only, not medical advice. Consult a healthcare professional.