Gutter Flow Calculator

The Gutter Flow Calculator exists because sizing the slope of a gutter is only half the drainage problem — the other half is ensuring the outlets can physically remove water at the rate the roof produces it. In a heavy storm, an under-sized or too-few downspouts will overflow no matter how well the gutter is sloped. This tool combines the roof catchment area with a design rainfall intensity to produce the peak runoff the gutter must handle, then divides that by per-outlet flow capacities to estimate how many downspouts of each size are needed. It is the essential companion to a fall calculation when designing a complete drainage system.

Runoff flow is calculated from roof plan area and rainfall intensity using the rational method:

Q (L/s) = Area (m²) × Intensity (mm/hr) ÷ 3600

In US customary units the equivalent is:

Q (US gpm) = Area (ft²) × Intensity (in/hr) × 0.01044

For example, an 80 m² roof during a 100 mm/hr design storm produces 80 × 100 ÷ 3600 ≈ 2.22 L/s. Dividing by a 4 in (100 mm) round outlet capacity of roughly 3.2 L/s gives approximately 0.7 outlets — so one outlet handles this roof comfortably. The number of outlets is always rounded up to the nearest whole number.

The gutter flow calculator sizes the stormwater a gutter has to handle. It combines the roof plan area that drains into the gutter with a design rainfall intensity to give the peak runoff in litres per second and US gallons per minute, then divides that by the capacity of a single outlet to estimate how many downspouts you need. Use it alongside a fall figure from the gutter fall calculator so both the channel slope and the outlets are sized for the same storm.

These per-outlet capacities are planning ballparks — real capacity depends on outlet shape, the head of water in the gutter, and the downpipe. Always confirm against your local stormwater code and the manufacturer's flow data for a final design.