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Use of a hydrometer to indirectly measure swine manure total nitrogen (TN) and total phosphorus (TP) was studied using both single and multiple manure sources. It was found that the hydrometer method could be greatly improved in terms of estimation accuracy if separate linear regressions were developed for manure sources from pigs at different growing stages, i.e., gestation, nursery, and finishing. With separate linear equations, highly significant correlation coefficients for TN and TP with manure total solids contents were observed (0.9881 and 0.9935 for gestation; 0.9844 and 0.9873 for nursery; 0.9695 and 0.9628 for finishing). The liquid manure specific gravity (SG) was also found to be well correlated with the TN and TP contents with correlation coefficients of 0.9909 and 0.9873 for gestation, 0.9836 and 0.9843 for nursery, and 0.9634 and 0.9745 for finishing, respectively. The correlation coefficients obtained from this study were much better than those reported by past researchers, which apparently supports the concept of using separate linear equations for different pig growth phases This was also verified by examining the results from the lumped data in this study, in which the correlation coefficients for TN and TP with total solids were 0.8878 and 0.6793, respectively, while those for TN and TP with manure SG were 0.8176 and 0.7022. According to the error analysis, use of separate equations to estimate manure nutrient contents had lower minimum error limits (10% for TN and 11% for TP gestation; 13% for both TN and TP nursery; 17% for TN and 12% for TP finishing) than using combined equations (40% for TN and 64% for TP).