I am working a hotel project with 6090 sq.m greenery + 14210 sq.m pavement and willing to get 3 points by Percentile of rainfall events (Option 1) - Path 2 98th Percentile.

Information I currently have is recent 10 years rainfall of 98th precentile is 109.21mm. 

And my calculation of rainwater tank will be:

Greenery area: 6090sq.m x 0.10921m x 0.4 (runoff coefficients) = 266 cubic meter

Pavement area: 14210sq.m x 0.10921m x 0.95 (runoff coefficients) = 1474.28 cubic meter

So total Rain water tank will be 1741 cubic meter which is huge compare to the site~!

Can anyone correct me how to calculate the tank size or give me example to follow?

Many thanks,