Measurements of underwater spectral irradiance have been used to investigate the behavior of the ratio of total quanta to total energy (Q : W) within the spectral region of photosynthetic activity. In data from a wide range of optical water types, the Q : W ratio varies by no more than ±10% and is 2.5 ± 0.25 × 1018 (quanta sec−1 watt−1). Within this range Q : W varies systematically and thus it may be predicted within ±5%, if either the wavelength of maximum spectral irradiance or the chlorophyll concentration of the water under investigation is approximately known. The value of Q : W may be used to determine the total quanta available for photosynthesis accurately from a measurement of the total energy and vice versa.