Cash flow forecasting is an indispensable tool for construction companies, and is essential for the survival

of any contractor at all stages of the work. The time available for a detailed pre-tender cash flow forecast is often

limited. Therefore, contractors require simpler and quicker techniques which would enable them to forecast cash flow

with reasonable accuracy. Forecasting S-curves in construction in developing countries like Iran in compare with

developed countries has many difficulties. It is because of uncertainty and unknown situation in nature of construction

industry of these countries. Based on knowledge of authors there is a little attempt for cash flow forecasting in

construction industry of Iran. As a result authors produced An S-curve equation for construction project from historical

data which has reasonable accuracy. A sample of 20 completed projects was collected and classified in to the three

different groups. In order to model S-curves for each group, a simple and reliable method of S curve fitting has been

used. S-curves were fitted into each group by using different techniques. Errors incurred when fitting these curves were

measured and compared with those associates in fitting individual projects. At the end, accuracy of each model has

been calculated and an equation has been proposed to forecast S-curves.