GFR Calculator – Complete Guide to eGFR, CKD Staging and Kidney Function
The GFR Calculator on MyTimeCalculator helps estimate kidney function from serum creatinine, age, sex and body weight. It implements the CKD-EPI 2021 race-free equation recommended by major kidney societies, along with CKD-EPI 2009, MDRD and Cockcroft–Gault creatinine clearance for comparison and legacy use.
The main output is the estimated glomerular filtration rate (eGFR) in mL/min/1.73 m² and the corresponding chronic kidney disease (CKD) stage. Together, these values help categorize kidney function from normal (G1) to kidney failure (G5) when combined with other clinical information such as albuminuria and duration of impairment.
1. What GFR Measures
Glomerular filtration rate is the volume of plasma filtered by the glomeruli per minute. It is a core indicator of how well the kidneys are cleaning the blood. Because measuring true GFR directly is complex and invasive, clinical practiceies on estimated GFR calculated from serum creatinine and patient characteristics.
Creatinine is a breakdown product of muscle metabolism that is cleared almost entirely by the kidneys. When kidney function declines, serum creatinine usually rises and eGFR falls, although theationship is influenced by factors such as muscle mass, diet and certain medications.
2. Equations Used in This GFR Calculator
The calculator supports four commonly used equations. All use serum creatinine in mg/dL internally; values entered in µmol/L are converted automatically.
- CKD-EPI 2021 (race-free, creatinine only): default equation. Uses age, sex and serum creatinine, without a race term, and reports eGFR in mL/min/1.73 m².
- CKD-EPI 2009: earlier equation that includes a race coefficient and is still used in some references and historical lab results.
- MDRD 4-variable: Modification of Diet in Renal Disease study equation with age, sex, serum creatinine and race factor. Reports eGFR in mL/min/1.73 m².
- Cockcroft–Gault: calculates creatinine clearance in mL/min using age, weight, sex and serum creatinine. If height is provided, the result can be indexed to 1.73 m² for comparison with eGFR values.
3. CKD-EPI 2021 Race-Free Equation
CKD-EPI 2021 was designed to remove race from the equation while keeping good accuracy for a wide range of patients. In simplified form, the calculator uses:
where Scr is serum creatinine in mg/dL, k is 0.7 for females and 0.9 for males, and α is −0.241 for females and −0.302 for males. The result is reported in mL/min/1.73 m².
4. MDRD, CKD-EPI 2009 and Cockcroft–Gault
The MDRD 4-variable equation estimates eGFR as:
with a race factor of 1.212 or 1.178 for patients identified as Black, depending on the lab implementation, and 1.0 otherwise. CKD-EPI 2009 uses a similar structure with a two-slope model for creatinine and a race term. Both equations are included for compatibility with older lab reports and studies.
The Cockcroft–Gault formula estimates creatinine clearance (CrCl) in mL/min:
Because Cockcroft–Gault uses actual body weight and does not normalize to a standard body surface area, results can differ from eGFR even when kidney function is stable.
5. CKD Stages Based on eGFR
The calculator maps eGFR to widely used CKD stages for adults:
- G1: eGFR ≥ 90 mL/min/1.73 m² (normal or high, CKD only if other markers of kidney damage are present)
- G2: eGFR 60–89 (mildly decreased)
- G3a: eGFR 45–59 (mild to moderate decrease)
- G3b: eGFR 30–44 (moderate to severe decrease)
- G4: eGFR 15–29 (severely decreased)
- G5: eGFR < 15 (kidney failure)
A single eGFR measurement is not enough to diagnose CKD. Chronic kidney disease is defined by reduced GFR or markers of kidney damage present for at least three months, usually combined with other clinical data such as albuminuria and imaging findings.
6. How to Use the GFR Calculator Step by Step
- Enter the patient’s age in years, ensuring they are at least 18 years old.
- Select sex at birth and, if you plan to use MDRD or CKD-EPI 2009, choose the appropriate race factor.
- Enter serum creatinine and choose whether it is reported in mg/dL or µmol/L.
- Enter body weight in kilograms if you want to calculate Cockcroft–Gault creatinine clearance.
- Optionally enter height in centimeters if you wish to approximate body surface area for CrCl indexing.
- Choose the desired equation from the dropdown; CKD-EPI 2021 is usually preferred for adults in general practice.
- Click calculate to view eGFR or creatinine clearance, CKD stage and a brief interpretation.
7. Limitations and When Results May Be Misleading
Creatinine-based GFR estimates can be inaccurate in several situations. Examples include extremes of muscle mass, severe obesity or cachexia, pregnancy, acute kidney injury, limb amputation, very high or low protein intake, and certain medications that affect creatinine handling. In children and adolescents, specialized pediatric equations such as Schwartz are used instead of adult formulas.
For borderline cases or when treatment decisions have major consequences, clinicians may confirm kidney function with additional tests, including cystatin C-based equations or measured GFR using clearance of exogenous markers.
GFR Calculator FAQs
Frequently Asked Questions
Answers to common questions eGFR, CKD-EPI equations, MDRD, Cockcroft–Gault and how to interpret this calculator’s results.
eGFR from CKD-EPI or MDRD is an estimate of kidney filtration normalized to a standard body surface area of 1.73 m². Cockcroft–Gault creatinine clearance uses age, weight, sex and creatinine to estimate clearance in mL/min without normalization. Drug dosing recommendations often reference Cockcroft–Gault, while CKD staging and lab reports usually use eGFR in mL/min/1.73 m².
For adults in general practice, CKD-EPI 2021 race-free is typically preferred because it avoids race adjustments while maintaining good accuracy. CKD-EPI 2009 and MDRD are helpful for comparing with older lab reports or research that used those formulas. Cockcroft–Gault is often used for drug dosing decisions where creatinine clearance is specified in guidelines.
Creatinine reflects muscle mass as well as kidney function. In very muscular individuals, creatinine may be higher and eGFR lower than true GFR. In frail or low-muscle individuals, creatinine may be low and eGFR artificially reassuring. Diet, hydration and medications can also shift creatinine levels independently of GFR, so results must be interpreted in clinical context.
No. The adult CKD-EPI, MDRD and Cockcroft–Gault equations are not validated in children, adolescents or pregnancy. Specialized pediatric and obstetric formulas are recommended in those settings. This calculator is intended for adults aged 18 years and older and does not replace specialist assessment.
No. CKD-EPI and MDRD equations already report eGFR normalized to 1.73 m² and do not require height or weight. Height is only used when you want to estimate body surface area to index Cockcroft–Gault creatinine clearance to 1.73 m² for easier comparison with eGFR values.
No. CKD is defined by persistent reduction in GFR or other markers of kidney damage for at least three months. A single low eGFR may reflect temporary changes such as dehydration, acute illness or lab variation. Repeated measurements, urine tests and clinical assessment are needed before labeling someone with chronic kidney disease.
This tool is for educational use only and does not replace clinical judgment. Results are not a diagnosis and are not suitable for children, pregnancy or acute kidney injury.