The Cockcroft-Gault formula is used to calculate

Creatinine Clearance.

In men, this is:
GFR= (140 - age) x weight (kg) / (72 x serum creatinine)

In women, multiply this result by .85

The formula gives the Glomerular Filtration Rate, in ml/hour. In other words, it estimates the rate at which plasma ultrafiltrate is produced by the kidneys.

This is a quick and dirty estimate, and gives a good estimate only when creatinine clearance is stable. If the creatinine concentration is rising, applying this formula would be highly inaccurate.

For most purposes, however, the Cockcroft-Gault formula is the most common formula used by physicians to estimate creatinine clearance, and thus Glomerular Filtration Rate.