The protocol
Choose a representative session (same intensity and conditions you care about). Void, weigh in minimal clothing. Track every ounce consumed. Towel sweat from skin and hair identically before weighing out. Math: (before − after) × 16 oz + fluid consumed − urine = total sweat; divide by hours.
The two corruptions
First: unrepresentative conditions — an easy indoor session tells you nothing about a hot race. Second: forgetting consumed fluid, which can mask half your output. A kitchen scale accurate to 0.1 lb and a marked bottle eliminate both errors.
Using the number
Size during-workout drinking to ~75% of your rate (absorption limits), pre-hydrate the gap, and rehydrate at 150% afterward with sodium. Re-test per season: acclimated athletes sweat earlier and heavier — the number improving is fitness, not a bug in the scale.