Enter an indexed GFR (mL/min/1.73m²) and BSA into the calculator to convert to an absolute (unindexed) GFR in mL/min. This calculator can also solve for any variable if the other two are known. For a creatinine clearance estimate from serum creatinine, use the Cockcroft–Gault tab.
References / Guidelines (for further reading)
- National Kidney Foundation (NKF): eGFR and kidney function overview pages.
- KDIGO: chronic kidney disease guideline overview materials.
- NIDDK or Mayo Clinic: patient education on eGFR/creatinine testing.
- Medication labeling (FDA/your country’s regulator): renal dosing sections often specify whether to use Cockcroft–Gault CrCl vs eGFR.
- All Health and Medical Calculators
- All Medical and Health Unit Converters
- BSA Calculator – Body Surface Area
- GFRC Weight Calculator
- Urine K To Creatinine Ratio Calculator
Indexed GFR To Absolute (Unindexed) GFR Formula
The following formula is used to convert a BSA-indexed GFR (mL/min/1.73m²) into an absolute (unindexed) GFR (mL/min) using the patient’s body surface area (BSA). This conversion is not the same as a measured urine creatinine clearance.
GFR_{absolute} = (GFR_{indexed} * BSA) / 1.73Variables:
- GFRabsolute is the absolute (unindexed) GFR (mL/min)
- GFRindexed is the BSA-indexed GFR (mL/min/1.73m²)
- BSA is the body surface area (m²)
To calculate the absolute (unindexed) GFR, multiply the indexed GFR by the body surface area, then divide by 1.73 (the standard BSA used for indexing).
| GFR (mL/min/1.73m²) | Absolute GFR at BSA 1.50 m² (mL/min) | Absolute GFR at BSA 1.73 m² (mL/min) | Absolute GFR at BSA 2.00 m² (mL/min) |
|---|---|---|---|
| 15 | 13.0 | 15 | 17.3 |
| 20 | 17.3 | 20 | 23.1 |
| 25 | 21.7 | 25 | 28.9 |
| 30 | 26.0 | 30 | 34.7 |
| 35 | 30.3 | 35 | 40.5 |
| 40 | 34.7 | 40 | 46.2 |
| 45 | 39.0 | 45 | 52.0 |
| 50 | 43.4 | 50 | 57.8 |
| 55 | 47.7 | 55 | 63.6 |
| 60 | 52.0 | 60 | 69.4 |
| 65 | 56.4 | 65 | 75.1 |
| 70 | 60.7 | 70 | 80.9 |
| 75 | 65.0 | 75 | 86.7 |
| 80 | 69.4 | 80 | 92.5 |
| 85 | 73.7 | 85 | 98.3 |
| 90 | 78.0 | 90 | 104.0 |
| 95 | 82.4 | 95 | 109.8 |
| 100 | 86.7 | 100 | 115.6 |
| 110 | 95.4 | 110 | 127.2 |
| 120 | 104.0 | 120 | 138.7 |
| Formula: Absolute GFR (mL/min) = Indexed GFR (mL/min/1.73m²) × (BSA (m²) / 1.73). Values rounded to one decimal place. | |||
What Is Indexed GFR vs. Creatinine Clearance?
GFR (glomerular filtration rate) describes how much fluid the kidneys filter per unit time. Many lab-reported eGFR values are indexed to a standard body surface area of 1.73 m² (reported as mL/min/1.73m²) so results can be compared across people of different sizes.
Creatinine clearance (CrCl) is different: it is typically measured from urine collection (e.g., 24-hour urine) or estimated from serum creatinine using formulas such as Cockcroft–Gault. CrCl and GFR are related but not interchangeable; CrCl often overestimates true GFR due to creatinine tubular secretion.
How to Convert Indexed GFR to Absolute (Unindexed) GFR
The following steps outline how to convert a BSA-indexed GFR to an absolute (unindexed) GFR.
- Determine the indexed GFR in mL/min/1.73m².
- Determine the patient’s BSA in m².
- Use the formula from above: Absolute GFR = (Indexed GFR × BSA) / 1.73.
- Calculate the absolute (unindexed) GFR in mL/min.
- Check your answer with the calculator above.
Example Problem:
Use the following variables as an example problem to test your knowledge.
Indexed GFR = 80 mL/min/1.73m²
BSA = 1.8 m²
Absolute (unindexed) GFR = (80 × 1.8) / 1.73 = 83.24 mL/min (≈ 83.2 mL/min)
