drainage auto-sizing-levelling and simulation toolbar,
click the 
Quick Storage Estimate... button.
Calculate the storage estimate of a proposed site by running a SWMM drainage simulation on a representation of the site. The site is simulated as a single large catchment area that flows at a restricted rate to an outfall. The volume of water that would have flooded the site is the volume of estimated volume of storage required.
On the drainage auto-sizing-levelling and simulation toolbar,
click the Quick Storage Estimate... button.
After clicking on the Quick Storage Estimate... button you will see the following window appear:
The rainfall data for the storm simulations can be specified using either FSR data (Flood Studies report, M5-60 Ratio R) or FEH99/FEH13 data (Flood Estimation Handbook).
If you know the plan surface are of your site, you can type in the value in the Total Area box. If you don't know the area, you can calculate it by clicking the Pick... button and digitise a site boundary line. After you have drawn the site boundary line, right click and choose Finish to return back to the Quick Storage Estimate window.
The Discharge Rate specifies the rate that water flows to an outfall. You can manually input this or use the value generated from the greenfield runoff calculation. This value can be calculated by going to the Greenfield Runoff window.
If the Discharge Rate is lower than the Minimum Discharge Rate, the Minimum Discharge Rate value will be used in preference to calculate the storage estimate instead.
The % Impermeable (PIMP) value is a percentage of how much of the Total Area will be impermeable, like roads, house roofs and driveways.
The Climate Change % is the projected increase in rainfall intensity and Urban Creep % is the projected increase in impermeable area over the lifetime of the site.
Click the Calculate... button to use the values entered above to see a window showing the estimated storage requirements for your site.