CrCl (mL/min) = (Urine Cr × Urine Volume) / (Serum Cr × Collection Time in minutes). For example: urine Cr 100 mg/dL, volume 1500 mL, serum Cr 1.0 mg/dL, 24h collection: (100 × 1500) / (1.0 × 1440) = 104.2 mL/min.

Measured CrCl is preferred when eGFR equations may be inaccurate: extremes of body size (BMI >40 or very low), unusual muscle mass (bodybuilders, amputees, paraplegia), pregnancy, vegetarian/vegan diets, acute kidney injury, and for precise drug dosing (especially cytotoxic chemotherapy like carboplatin).

Expected 24h creatinine excretion: males 20–25 mg/kg/day, females 15–20 mg/kg/day. If measured excretion is significantly below expected, the collection was likely incomplete, and the CrCl will be underestimated.