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Water‐extractable P (WEP) in manure is correlated with P concentration in runoff from soils amended with manure and is, thus, an effective indicator of environmental P loss. This study sought to elucidate methodological factors affecting WEP measurement in manure and to quantify errors related to two established methods of manure WEP measurement. Dairy cow (Bos taurus) manure, poultry (Gallus gallus domesticus L.) (layer) manure, and swine (Sus scrofa domestica L.) slurry were used. Varying dry matter/distilled water ratios (1 to 20:200) revealed that greater dilution of manure dry matter increased WEP (mean 1.8–5.4 g kg⁻¹), likely because of the dissolution of calcium phosphates. Increasing shaking time from 1 min to 24 h, increased manure WEP concentration (average 3.7–8.2 g kg⁻¹). Filtration with Whatman 1 paper filters resulted in significantly higher WEP measurements in dairy and poultry manure …