In order to estimate the salt and potassium intake in a population and to compare their annual trends, we developed a simple method to estimate population mean levels of 24-h urinary sodium (24HUNaV) and potassium (24HUKV) excretion from spot urine specimens collected at any time. Using 591 Japanese data items from the INTERSALT study as a gold standard, we developed formulas to estimate 24-h urinary creatinine (24HUCrV), 24HUNaV and 24HUKV using both spot and 24-h urine collection samples. To examine the accuracy of the formulas, we applied these equations to 513 external manual workers. The obtained formulas were as follows: (1) PRCr (mg/day) = -2.04 x age + 14.89 x weight (kg) + 16.14 x height (cm) -2244.45; (2) estimated 24HUNaV (mEq/day) = 21.98 x XNa (0.392); (3) estimated 24HUKV (mEq/day) = 7.59 x XK(0.431); where PRCr = predicted value of 24HUCr, SUNa = Na concentration in the spot voiding urine, SUK = K concentration in the spot voiding urine, SUCr = creatinine concentration in the spot voiding urine, XNa (or XK) = SUNa (or SUK)/SUCr x PRCr. In the external group, there was a significant but small difference between the estimated and measured values in sodium (24.0 mmol/day) and potassium (3.8 mmol/day) excretion. In every quintile divided by the estimated 24HUNaV or 24HUKV, the measured values were parallel to the estimated values. In conclusion, although this method is not suitable for estimating individual Na and K excretion, these formulas are considered useful for estimating population mean levels of 24-h Na and K excretion, and are available for comparing different populations, as well as indicating annual trends of a particular population.