Drought is a major cause of economic loss to the world’s wheat growers; estimated at up to US$20 billion (£12.5 bn) in 2000. Film antitranspirants are polymers applied to foliage to reduce water loss and have recently been shown to increase droughted wheat yield. This increase is linearly related to the drought severity (soil moisture deficit [SMD]) at the time of application. This paper demonstrates how this linear relationship can be used to calculate an economic threshold SMD, above which an economic yield response should be obtained, from spray cost and expected grain price. This will enable agronomists and growers to make a clear decision on the cost-effectiveness of spraying to protect from drought damage. Sensitivity analysis shows that using the correct spray decision threshold SMD is especially important when the wheat grain price is expected to be low.