The formulas behind our medical-style calculators, each computed for a worked example and checked against the calculator. These are screening and teaching tools, not diagnoses, and not a substitute for a clinician. Reference ranges differ by lab, guideline and person, and the alcohol estimate must never be used to decide whether it is safe to drive.
Formulas
| Measure | Formula | Example |
|---|---|---|
| Estimated average glucose from A1C | eAG (mg/dL) = 28.7 × A1C − 46.7 |
A1C of 7.0%: 154.2 mg/dL (8.6 mmol/L) |
| Blood pressure category | normal < 120/80; elevated 120-129 and < 80; stage 1 130-139 or 80-89; stage 2 ≥ 140 or ≥ 90; crisis > 180 or > 120 |
118/76 normal, 125/78 elevated, 135/85 stage 1, 145/95 stage 2, 185/125 crisis |
| Cholesterol ratio and non-HDL | total ÷ HDL, total − HDL, LDL ≈ total − HDL − TG/5 (Friedewald) |
Total 200, HDL 50, TG 150: ratio 4.0, non-HDL 150, LDL about 120 mg/dL |
| Pack-years of smoking | packs per day × years |
One pack a day for 20 years: 20 pack-years |
| Creatinine clearance (Cockcroft-Gault) | (140 − age) × kg / (72 × creatinine mg/dL), × 0.85 for women |
Man, 60, 80 kg, creatinine 1.0 mg/dL: 88.9 mL/min |
| Estimated GFR (CKD-EPI 2021) | 142 × min(Scr/κ, 1)^α × max(Scr/κ, 1)^−1.200 × 0.9938^age (× 1.012 for women) |
Man, 50, creatinine 1.0 mg/dL: 91.7 mL/min/1.73 m² |
| Corrected QT interval (Bazett, Fridericia) | QTc = QT / √RR (Bazett), QT / ∛RR (Fridericia), RR in seconds |
QT 400 ms at 75 beats a minute (RR 0.8 s): Bazett 447 ms, Fridericia 431 ms |
| Estimated blood alcohol content (Widmark) | BAC = grams of alcohol / (weight in g × r) × 100 − 0.015 × hours, r = 0.68 men, 0.55 women |
80 kg man, 3 standard drinks (14 g each), 2 hours: about 0.047% |
| IV drip rate | drops per minute = volume (mL) × drop factor / minutes |
1,000 mL over 8 hours at 15 drops/mL: 31 drops a minute, 125 mL an hour |
Try them
A1C to Average Blood Glucose Calculator, Blood Pressure Category Calculator, Mean Arterial Pressure Calculator, Cholesterol Ratio Calculator, Pack-Year Calculator, Creatinine Clearance Calculator, GFR Calculator, QTc Calculator, BAC Calculator and IV Flow Rate Calculator. For body measurements and heart rate see the Health Formulas Cheat Sheet.
Reading the results
- Blood pressure is graded on the higher of the two readings: 135/85 is stage 1 because 135 is in the 130 to 139 band.
- Kidney function is graded by the numbers: an estimated GFR of 90 or more is normal range if there is no other sign of damage, 60 to 89 is mildly decreased, and lower values are staged progressively.
- The Bazett correction over-corrects at fast heart rates; Fridericia is often preferred, and the two differ by 16 ms in the example.
- Blood alcohol varies widely with food, body composition and metabolism; the formula is an average, and the legal limit is a measured value.
- Seek care. Very high blood pressure (a reading above 180 or 120) or symptoms such as chest pain need prompt medical attention.