eGFR Calculator (CKD-EPI 2021)

The eGFR Calculator implements the CKD-EPI 2021 (race-free) equation — the current gold-standard formula recommended by KDIGO, NKF, ASN, and the UK NICE guidelines for estimating kidney function in adults. Enter your age, sex and a single serum creatinine measurement from any routine blood test; the tool returns your estimated glomerular filtration rate in mL/min/1.73 m² and the corresponding KDIGO chronic kidney disease stage, along with a full step-by-step breakdown of the calculation.

Everything runs locally in your browser. No data is uploaded, stored or transmitted.

This tool is for educational purposes only and does not constitute medical advice. Always discuss your results with a qualified clinician.

How the CKD-EPI 2021 formula works

Glomerular filtration rate is the gold-standard measure of kidney function, but direct measurement requires inulin or iothalamate infusions — impractical for routine care. Instead, clinicians estimate GFR from serum creatinine, a waste product of muscle metabolism that is freely filtered by the glomeruli. Higher creatinine generally means lower filtration.

The CKD-EPI 2021 creatinine-only equation (Inker et al., NEJM 2021) is:

eGFR = 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^(-1.200)
            × 0.9938^Age × [1.012 if female]

where:

• Scr is serum creatinine in mg/dL
• κ (kappa) is the sex-specific creatinine threshold: 0.7 for females, 0.9 for males
• α (alpha) is the low-creatinine exponent: -0.241 for females, -0.302 for males
• The constant 0.9938 captures the approximately 0.62 % per year age-related decline
• The 1.012 multiplier adjusts for the systematically lower muscle mass — and thus lower creatinine production — in females

The formula splits around the threshold κ because the relationship between creatinine and GFR has different slopes below and above the average. Below the threshold, creatinine is low relative to expectation (higher GFR); above it, creatinine is high (lower GFR). The two power-law segments capture this non-linearity.

Why race-free? The 2009 CKD-EPI and the older MDRD equations included a race multiplier (1.159 for patients self-reported as Black) derived from cohorts where Black patients had, on average, higher measured creatinine for a given GFR — attributed to higher muscle mass. Critics argued this created a circular inequity: if Black patients are systematically assigned a higher eGFR, they are less likely to be referred for nephrology review, transplant evaluation or dialysis initiation at the same clinical threshold. The 2021 revision removed race entirely, accepting a small increase in overall population bias in exchange for greater equity.

Worked example

A 55-year-old female with a serum creatinine of 1.1 mg/dL:

• κ = 0.7, α = -0.241
• Scr/κ = 1.1 / 0.7 = 1.5714 (above 1, so the min-term = 1^(-0.241) = 1.000)
• max(1.5714, 1)^(-1.200) = 1.5714^(-1.200) ≈ 0.5939
• 0.9938^55 ≈ 0.7097
• Sex multiplier = 1.012
• eGFR = 142 × 1.000 × 0.5939 × 0.7097 × 1.012 ≈ 60.7 mL/min/1.73 m²

That places her at Stage G2 (mildly decreased, 60-89) — borderline with G3a, warranting monitoring and albuminuria testing.

Now compare a 68-year-old male with the same creatinine of 1.1 mg/dL:

• κ = 0.9, α = -0.302
• Scr/κ = 1.1 / 0.9 = 1.2222 (above 1, min-term = 1.000)
• max(1.2222, 1)^(-1.200) = 1.2222^(-1.200) ≈ 0.7820
• 0.9938^68 ≈ 0.6518
• Sex multiplier = 1.000
• eGFR = 142 × 1.000 × 0.7820 × 0.6518 × 1.000 ≈ 72.4 mL/min/1.73 m²

Stage G2 as well, but further from the G3a boundary. The same creatinine yields a higher eGFR in the male patient because the male κ threshold is higher (0.9 vs 0.7), meaning 1.1 mg/dL is proportionally less above the expected value.

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