Estimating body weight without a scale involves using mathematical formulas based on body measurements or physical principles like water displacement. One common "anthropometric" method is using a measuring tape to find your waist circumference and height; various online "body mass estimation" calculators use these inputs to provide a rough estimate, though they have a wide margin of error. Another more scientific (but messy) approach is the Water Displacement Method based on Archimedes' Principle: fill a bathtub to the brim, submerge yourself completely, and measure the volume of water that overflows. Since the human body has a density similar to water (1 liter≈1 kg), your weight in kilograms will roughly equal the volume of displaced water in liters. A third, less precise method is the "Balance Method," where you use a see-saw or a sturdy plank on a fulcrum to balance yourself against items of known weight (like several 10kg bags of rice). Note that these are all rough approximations, and a calibrated scale is the only way to get an accurate medical measurement.