The vapor lock index is the measure used to control the tendency of gasoline to form vapor lock in most markets outside of US. The measurement for VLI is calculated:

VLI = 10x(Vapor pressure in kPa) +7x(% evaporated at 70 degrees C)

Typical values range from 800 to 1250. Lower VLI is better.

In the US, vapor lock is controlled by the vapor-liquid ratio rather than VLI. Vapor-liquid ratio is the temperature at which gasoline forms a ratio of vapor volume to liquid volume that is equal to 20.

The normal range for vapor-liquid ratio is 35 to 60 degrees C (95 to 140 degrees F). Higher vapor-liquid ratio is better.