Previous research showed a relationship between viability and electrolyte leakage; here, we develop an index of viability based on electrolyte leakage from sample populations of seeds soaked in deionized distilled water. Conductivity of leachates from individual seeds was determined for 10 lots of lettuce (Lactuca sativa L.), each germinating at 99%. Conductivity data for two lots of soybean (Glycine max L.) seeds germinating at 100% and 74%, respectively, were obtained from literature. Cumulative frequency distributions (CFD) with a class interval of one µA, were fitted with a natural logarithmic form of the Richards function, which requires no arbitrary starting values. The procedure provided an effective estimation of slopes [(dCF/dµA)MAX] of hypothetical lines tangent to inflection points of the respective sigmoidal CFD curves. We suggest that this maximum slope, or internal slope can be used as a seed viability index. The index is unaffected by outlier µA readings and reflects the shape of the CFD. It is also a measure of seed-to-seed variability in leachate conductivity. The se of the 10 internal slopes derived from the 10 lettuce seed lots was 3.8. The viability index is sensitive, since nearly a four-fold difference in internal slope was found for the two soybean seed lots. The greater the internal slope, the less the variation among individual seed conductivities and the higher the seed quality.