Accurate dry matter mass balance (DMMB) estimates are important if knowledge of nutrient component changes on a mass basis is required for manure handling practices. We compared four methods of estimating DMMB during composting of solid feedlot manure. A manure stockpiling treatment was included for comparison. A method which used carbon (C) concentration to calculate organic matter, and hence ash content, was most accurate and detected treatment effects. One employing changes in measured ash concentrations was also reliable, while one based on weighing material entering and leaving each handling system was prone to errors. A fourth method using changes in phosphorus (P) concentrations was less accurate. Accurate DMMB estimates greatly improve nutrient mass balance calculations which are needed to gauge the success of manure handling practices.