This paper develops a methodology for estimating the costs of delay of harvesting wheat and demonstrates the approach using data for Southern Queensland. Costs associated with delay in harvesting wheat stem from quality downgrading after rain on the mature crop, yield loss due to ageing and labour costs related to the delay period itself. The most difficult of these costs to predict is loss due to quality downgrading. This loss is hypothesized to depend on the probability of various rain related events given a range of harvest starting dates and harvest durations. A probability analysis was used to relate rainfall events and loss so that expected delay costs, given a range of harvesting durations and starting dates, could be specified. The variability of delay costs from year to year was considered so that the risk associated with decisions based on expected costs could be specified. An optimal harvesting capacity for a given area and rainfall site in Southern Queensland was suggested after taking account of the other delay costs and the capital cost of various harvesting capacities.