A general transformation formula is presented for interval traits in pigs such as weaning-to-first-service interval or farrowing interval. In the logarithmic transformation, only observations that are greater than the median of the trait as originally recorded are transformed. The transformation reduces considerably the skewness and the kurtosis of the trait so that the distribution of transformed data approaches normality. In analysis of over 9,000 records for weaning-to-first-service interval and over 12,000 records for farrowing interval, heritability estimates for the transformed data were higher than for the data as recorded. Therefore, selection on the transformed trait should be more effective than selection on the original trait. The transformation formula is likely to be appropriate to other livestock species and traits.