Rational Method

Rational method is in use for computing peak flows since late 18th century. The main reasons for its existence for such a long duration are its ease of use and its applicability for sizing pipes and other drainage conveyances for urban site developments.
Peak flow is often required for sizing the pipes and other structures (for example, several stormwater treatment technologies) whose design is primarily governed by maximum flow and not the runoff hydrograph.
The rational formula for estimating peak runoff rates is shown below:
Q = C*I*A
As you can see, to estimate peak discharges, all you need to know is:
Rainfall intensity, I (in/hr), for the storm duration equal to 'tc-time of concentration'
Drainage area, A (acres)
Weighted runoff coefficient, C (Most texts on hydrology or manuals from regulatory agency provides this value).
The output is the peak flow, Q, in cfs.
(The units are different here but it does not matter as they cancel each other out to give the value of, approximately 1.0)

Rational method was primarily developed for sizing small pipes for conveying small storms (upto 10 year storm events). For applying rational method for larger storms, following multiplier should be used:

Storm Event	Multiplier
25	1.1
50	1.2
100	1.25

As you can see the limitation of Rational method is that it cannot provide runoff hydrograph, which is necessary for sizing flood control and other stormwater runoff facilities whose design is based on the volume. One such variation of Rational method, known as 'Modified Rational Method' is used by several professionals. Although, experts disagree on its applicability and one should check with regulatory agency if it's use is accepted in that jurisdiction.