It all depends on what you referring to there. Are you calculating by looking at the drives through Windows Explorer? When a drive is first partitioned and formatted you lose some of the advertised space since drive capacities are rounded off to whole numbers.
An 80gb drive should see roughly 74-76gb approximate when ready for use. A 500gb sata drive here saw 465gb once the single partition was created on it and then formatted. Drives seem like they lose space until you calculate heads, sectors, cylinders to find that an 80gb drive should be seeing some odd number like 80,000,303,403 bytes pn the post screen when first starting the system up if the logo is disabled and post tests are shown.