GFR Calculator

Last updated: April 2026

Age

yrs

Sex

Serum creatinine

mg/dL

Creatinine unit

Estimated GFR

98 mL/min/1.73m^2

CKD stage: G1

Normal or high kidney filtration. Kidney damage markers may still matter.

Details

Creatinine used1 mg/dL

EquationCKD-EPI 2021 creatinine equation

Age40

Sexmale

Kappa / alpha0.9 / -0.302

Medical warning

This calculator estimates eGFR and is not a medical diagnosis. Kidney function should be interpreted by a healthcare professional, especially if results are low, changing, or accompanied by symptoms.

CKD staging usually requires repeat results and clinical context, including urine albumin and other findings.

Related calculators:

Body Surface Area Calculator
BMI Calculator
Healthy Weight Calculator
Water Intake Calculator

What is GFR?

GFR estimates how well the kidneys filter blood.

What is eGFR?

eGFR is estimated from blood creatinine, age, and sex.

How is eGFR calculated?

This calculator uses the CKD-EPI 2021 creatinine equation without a race coefficient.

What do GFR stages mean?

G1 is 90 or higher, G2 is 60 to 89, G3a is 45 to 59, G3b is 30 to 44, G4 is 15 to 29, and G5 is below 15.

Limitations of eGFR

eGFR is not a diagnosis by itself. It can be affected by muscle mass, diet, pregnancy, acute illness, and some medicines. CKD assessment often needs repeat tests and urine albumin.

About this calculator

The GFR Calculator estimates kidney filtration rate from age, sex, and serum creatinine using the CKD-EPI 2021 creatinine equation. It helps users understand an eGFR number, but kidney function should be interpreted by a healthcare professional with urine tests, repeat results, symptoms, and medical history. Use this expanded guide when you need more than a quick result. It explains the assumptions behind the GFR Calculator, the records to gather, and the decisions the estimate can support. It is especially useful for people trying to understand an eGFR estimate from creatinine, age, and sex before discussing results with a clinician. The strongest use of the page is scenario comparison: change one input at a time, compare the output, and keep a note of which assumption changed.

CKD-EPI 2021 eGFR method

The calculator converts creatinine to mg/dL if needed, applies sex-specific kappa and alpha values, uses the CKD-EPI 2021 creatinine equation, then groups the result into broad G categories. The calculator result depends on the quality of the inputs and on the rule set or formula selected in the calculator above. For practical use, treat the output as a structured estimate: start with the core inputs, review the main outputs, then test the decision points that matter most to your situation. Key decisions include whether the unit was entered correctly, how the stage label should be treated, what questions to ask about a lab result.

creatinine mg/dL = creatinine umol/L / 88.4
eGFR = 142 x min(Scr/k,1)^alpha x max(Scr/k,1)^-1.2 x 0.9938^age x sex multiplier
female multiplier = 1.012; male multiplier = 1
better estimate = accurate inputs + correct rule set + realistic assumptions
scenario difference = revised result - original result

How to use the GFR calculator

Enter age in years.
Choose sex because the equation uses sex-specific constants.
Enter serum creatinine from a blood test.
Choose the correct creatinine unit, mg/dL or umol/L.
Review the eGFR and stage label only as educational context.
Gather the main inputs first: age, sex, serum creatinine.
Check supporting records such as blood test result and creatinine unit before relying on a final number.
Enter one realistic scenario first, using conservative assumptions where the future is uncertain.
Review the main outputs: eGFR, G stage, interpretation label.
Run at least one alternative scenario so you can see which input changes the answer most.
Compare the result with NHS kidney disease diagnosis guidance and clinical lab reporting or the relevant contract, bill, statement, or professional document.
Keep the calculation date and assumptions with your notes so you can revisit the estimate when rates, rules, or circumstances change.

Worked example

Creatinine in umol/L

Input: Female, age 60, creatinine 88.4 umol/L

Calculation: 88.4 umol/L converts to 1.0 mg/dL before CKD-EPI 2021 is applied.

Result: The calculator estimates eGFR at about 65, stage G2.

Unit mistake scenario

Input: A UK creatinine result in umol/L is accidentally entered as mg/dL.

Calculation: The calculator would treat the value as far too high.

Result: The eGFR would be badly wrong, so unit checking is essential.

Repeat-result scenario

Input: A user has one lower result during illness.

Calculation: The calculator can estimate the number but cannot decide whether it is temporary.

Result: The user should discuss repeat testing and clinical context with a healthcare professional.

Why eGFR needs clinical context

A single eGFR result is not the whole kidney picture. Healthcare professionals may review repeat results, urine albumin, blood pressure, diabetes status, medication, hydration, body size, and symptoms before explaining what a result means.

What to check before relying on the result

A useful GFR Calculator result starts with the same evidence you would use if you were checking the answer manually. The calculator can organise the arithmetic, but it cannot know whether a payslip is final, a bill is estimated, a quote excludes fees, or a personal circumstance has changed since the last statement.

Before making a decision, compare the calculator result with the source document that controls the real outcome. For this topic, that usually means checking NHS kidney disease diagnosis guidance and clinical lab reporting. If there is a difference between the calculator and an official statement, contract, assessment, or professional advice, treat the official document as the stronger source.

blood test result: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
creatinine unit: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
previous eGFR results: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
urine albumin result if available: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.

Inputs that usually change the answer

The most important input is not always the largest number on the form. Sometimes a date, threshold, percentage, eligibility flag, or timing assumption changes the result more than the headline amount. This is why scenario testing is more useful than a single calculation.

Input	Why it matters	What to double-check
age	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
sex	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
serum creatinine	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
creatinine unit	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
lab context	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.

How to interpret the output

The output should be read as a decision aid, not just a number. For GFR Calculator, the useful question is often what the result means for timing, affordability, eligibility, comparison, or next steps.

eGFR: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
G stage: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
interpretation label: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
creatinine converted to mg/dL: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.

Scenarios worth comparing

A single estimate is a snapshot. A better approach is to save a base case, then adjust one assumption at a time. This shows whether the result is stable or whether a small change in timing, rate, usage, income, or cost creates a very different answer.

Scenario	Change one assumption	What the comparison shows
Base case	Use the best current evidence.	Shows the result you would expect if nothing important changes.
Conservative case	Use lower income, higher cost, slower growth, or less favourable timing.	Shows whether the decision still works with less optimistic assumptions.
Improved case	Use the realistic upside, such as lower cost, better rate, higher usage, or stronger evidence.	Shows the potential benefit without treating it as guaranteed.

Common mistakes and edge cases

Most errors come from using the right formula with the wrong assumption. Dates can be counted differently, rates can change, official thresholds can move, and real bills or contracts often include conditions that a simple calculator cannot infer automatically.

A single eGFR should not be overinterpreted.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Creatinine depends partly on muscle mass.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Acute illness can change kidney results.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Urine tests and repeat results may matter.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.

Next steps after calculating

Once you have a result, write down the key assumptions and compare them with NHS kidney disease diagnosis guidance and clinical lab reporting. If the number affects a deadline, tax return, benefit claim, employment issue, medical question, finance agreement, or major purchase, use the calculator as preparation for a more formal check.

For lower-stakes use, the next step may simply be comparing two or three scenarios. For higher-stakes use, the next step should be checking the official guidance, speaking to the relevant organisation, or getting qualified advice before acting.

eGFR edge cases

Creatinine can be affected by muscle mass, diet, dehydration, and some medicines.
Acute illness can make a single result less representative.
Very high or low muscle mass can make creatinine-based estimates less reliable.
A single eGFR should not be overinterpreted.
Creatinine depends partly on muscle mass.
Acute illness can change kidney results.
Urine tests and repeat results may matter.

Limitations

This calculator is general information only and is not medical advice. It cannot diagnose chronic kidney disease or decide treatment. This is general information only and is not medical advice. The calculator is designed to support understanding and planning, but it cannot verify documents, predict future rule changes, or account for every exception. Use it as an estimate and check the official source before acting where the result matters.

Discuss abnormal or changing results with a healthcare professional.
Use the unit printed on the blood test report.
Clinical labs and doctors may use additional tests and local reporting rules.
Check NHS kidney disease diagnosis guidance and clinical lab reporting for current rules, rates, definitions, and eligibility where relevant.
Do not rely on a single scenario where income, costs, dates, rates, usage, or health circumstances may change.
Keep records of the inputs used so that the estimate can be reviewed later.

Frequently asked questions

Is eGFR the same as measured GFR?

No. eGFR is an estimate from blood creatinine and demographic inputs.

Does a lower eGFR always mean kidney disease?

Not always. Age, repeat results, urine findings, and clinical context matter.

Why does sex affect the equation?

The CKD-EPI creatinine equation uses sex-specific constants to reflect population differences in creatinine generation.

What does G stage mean?

It is a broad eGFR category, but diagnosis also depends on duration, urine findings, and clinical context.

Can dehydration affect creatinine?

It can affect blood results, which is one reason results need clinical interpretation.

Should I change medication based on this?

No. Discuss medication decisions with a doctor, pharmacist, or qualified clinician.

Related calculators

Body Surface Area Calculator
BMI Calculator
Water Intake Calculator
Healthy Weight Calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.