Objectives: The aim of the study was to develop a new whiteness index which correlates to perception of tooth whiteness. Methods: Psychophysical experiments were conducted by a panel of observers under a standard viewing condition as well as under typical clinical viewing conditions. Various existing whiteness and yellowness indices were compared with regard to their ability to measure the perceived whiteness of human teeth. The Pearson coefficient of determination and '% wrong decision' method were used to determine the best index for tooth whiteness measurement. Results: A new whiteness formula (WIO) was developed by optimising the original CIE whiteness formula (WIC) and it was found that WIO gave the best performance for predicting tooth whiteness based on the laboratory results. A total of 88 NHS patients repeated the same visual experiments under typical clinical viewing conditions. Conclusion: The results confirmed the finding that the new whiteness formula is appropriate for the prediction of tooth whiteness. © 2009 Elsevier Ltd. All rights reserved.