HOW TO USE
How To Calculate eGFR
NephroCalc estimates kidney function (eGFR) from standard lab values. It uses the CKD-EPI 2021 equation (the current race-free standard) plus three additional formulas for comparison. Results appear instantly as you type — nothing is saved or sent anywhere.
1
Enter the patient details
| Input | What to enter | Typical values |
|---|---|---|
| Sex | Male or Female | Affects all equations |
| Age | 18–100 (slider) | — |
| Serum creatinine | From the blood test | 0.6–1.3 mg/dL (60–115 µmol/L) |
| Cystatin C (optional) | Toggle on if available | ~0.6–1.2 mg/L |
| Weight & height | For Cockcroft-Gault & BSA | e.g. 70 kg, 170 cm |
Important: match the creatinine unit to your lab report using the mg/dL ⇄ µmol/L toggle — the app converts automatically.
2
Read the primary result
The big number is your eGFR via CKD-EPI 2021, in mL/min/1.73m². Beside it, the color-coded CKD stage badge and gauge show where the result falls:
| Stage | eGFR range | Meaning |
|---|---|---|
| G1 | ≥ 90 | Normal / high |
| G2 | 60–89 | Mildly decreased |
| G3a | 45–59 | Mild–moderate |
| G3b | 30–44 | Moderate–severe |
| G4 | 15–29 | Severely decreased |
| G5 | < 15 | Kidney failure |
3
Compare all four estimates
CKD-EPI 2021 (creatinine)
The recommended standard. Race-free, most accurate for staging.
CKD-EPI 2012 (cystatin C)
Uses the alternate marker. Only shown if you toggled cystatin C on.
MDRD Study
An older legacy equation, kept for reference and comparison.
Cockcroft-Gault (CrCl)
Creatinine clearance in mL/min — used mainly for drug dosing.
The equations can differ slightly. Rely on CKD-EPI 2021 for staging.
4
Read the interpretation & BSA
The Interpretation panel explains the result in plain language. The BSA box shows body surface area (Mosteller formula) and the unindexed GFR — the raw value without the 1.73m² normalization, which is useful for very large or very small patients.
!
When to be careful
•Extremes of muscle mass (bodybuilders, frail elderly)
•Acute kidney injury — eGFR assumes stable function
•Pregnancy, amputation, or significant edema
•Values ≥ 60 — many labs just report “≥ 60” since estimates get imprecise there
✓
Quick example
Male · 60 years · Creatinine 1.0 mg/dL
eGFR ≈ 86 mL/min/1.73m²
Stage G2 — mildly decreased.
NephroCalc
Patient details
Estimated GFR (CKD-EPI 2021)
—
CKD stage
—
—
All estimates
CKD-EPI 2021 (creatinine)
—
Recommended standard · race-free
CKD-EPI 2012 (cystatin C)
—
Enter cystatin C to calculate
MDRD Study equation
—
Legacy 4-variable method
Cockcroft-Gault (CrCl)
—
Creatinine clearance, mL/min
Interpretation
—
Not a substitute for clinical judgment. eGFR is an estimate based on standard equations and may be inaccurate in certain patients (e.g. extremes of muscle mass, acute kidney injury, pregnancy, amputation, or edema). Always interpret results in clinical context and confirm with a healthcare professional. eGFR ≥ 60 is reported as “≥ 60” by many labs.
The Number on My Blood Test That I Couldn’t Stop Googling
It started, as these things usually do, with a routine checkup.
Everything was fine. Cholesterol fine, blood sugar fine, blood pressure fine. The doctor moved down the list, nodding, flipping pages, and then she said the thing every doctor says when they’re not worried: “one of your kidney numbers is a little low, but I’m not concerned.”
And just like that, I was very, very concerned.
She’d already moved on to something else — vitamin D, I think, the eternal villain of every blood panel — but I was stuck on that one sentence. A little low. Not concerned. I smiled and nodded and heard absolutely nothing else she said for the rest of the appointment.
The thing nobody tells you about “a little low”
The number was my eGFR. Estimated glomerular filtration rate. Which is a very intimidating way of saying “how well your kidneys are filtering your blood.” And here’s the thing about kidney numbers that I genuinely did not understand before all this: they can be a little off without it meaning anything at all.
Your eGFR bounces around. It’s affected by how hydrated you are that day. By whether you worked out the day before. By your muscle mass, your age, your sex, and about a dozen other things that have nothing to do with your kidneys being sick. It’s not a diagnosis. It’s a rough sketch, drawn in pencil.
But when it’s your number, on your lab report, none of that matters. You don’t want nuance. You want someone to tell you if you should be scared.
And the internet, bless it, is terrible at this. Type “low eGFR” into a search engine and you will be diagnosed with every kidney disease ever discovered by 9:15 in the morning. The forums are full of people catastrophizing. The medical sites are either too dense to parse or too vague to be useful. There is this enormous gap between “here’s a scary number” and “here’s what that number actually means,” and I spent way too many late nights falling into it.
So I built the thing I wished I’d found
I’m not a doctor. Let me say that clearly, right up front, because it matters. I’m a person who got a slightly-off blood test result, went down a rabbit hole, and came out the other side having learned enough to realize how badly this stuff is usually explained.
I built NephroCalc. It’s an eGFR calculator — you type in your age, your sex, and your creatinine level, and it tells you your estimated kidney function. That part isn’t new. There are a hundred calculators that do this.
But I built it to answer the question that the other calculators never quite answer, which is: should I actually be worried about this?
So it does a few things differently.
It shows you the stage, and then it tells you what the stage means. Every eGFR falls into a stage, from G1 to G5. Most calculators will happily show you “G3a” and leave you to Google what that is. Mine explains it in plain English. Not medical-journal English. Just English. “Mild-to-moderate reduction. This is usually where kidney disease is formally diagnosed.” That sentence, right there, would have saved me about three weeks of anxiety.
It compares all the formulas, because they disagree. This was the part that confused me the most. Different calculators gave me slightly different numbers, and I didn’t understand why. The answer is that there are several ways to estimate kidney function — CKD-EPI, MDRD, Cockcroft-Gault — and they don’t all agree. They’re estimates, not measurements, and the differences are usually noise. NephroCalc shows you all of them at once, so you can see that “85 versus 88” isn’t your kidneys getting worse, it’s just two equations doing their best.
It’s honest about when it’s wrong. Most tools won’t tell you this, but eGFR is genuinely unreliable in certain situations. If you’re very muscular, or very frail, or pregnant, or you just had an acute illness, the number can mislead you. I put that warning right on the page, because I’d rather you know the limits than feel falsely reassured — or falsely terrified.
What I actually learned from all of this
Here’s the part that surprised me, and the part I think is worth sharing.
The more I understood my number, the less scary it became. Not because the number got better — it didn’t, it’s about the same — but because “a little low” stopped being this vague, ominous threat and became a specific, understandable thing. It had a stage. It had a range. It had a plain-language meaning. And once it had a meaning, I could stop spiraling and actually do something about it — drink more water, get re-tested in a few months, keep an eye on it instead of losing sleep over it.
That’s the whole point of the calculator, really. Not to give you a number — your lab already gave you a number. It’s to give you the context that turns a scary number into something you can actually understand.
Before you use it
A few honest caveats, because I refuse to be one of those sites that makes you more anxious than you started.
This is an estimate. It is not a diagnosis. It cannot see your kidneys. It doesn’t know your medical history, your medications, or the thousand other things your actual doctor knows. If your eGFR is low, or trending down, the calculator will help you understand the numbers — but the person who interprets them should be a real, licensed human who went to medical school, not a webpage built by a worried guy with a spreadsheet habit.
And if your lab report says something concerning, the move isn’t to calculate it yourself and hope for the best. It’s to talk to your doctor. Bring the number, sure. Ask about it. But let them do the diagnosing.
The calculator is for the part that comes after the doctor visit — the part where you’re home, it’s 11 p.m., and you just want to understand what that number on your chart actually means. That’s the moment I built it for. Because that’s the moment I needed it.
NephroCalc uses the CKD-EPI 2021, MDRD, Cockcroft-Gault, and cystatin C equations. It is an educational tool, not medical advice, and it runs entirely in your browser — your health data never leaves your device.