Home Improvement
Gutter Capacity Calculator
Enter roof catchment area, rainfall intensity and gutter capacity to see flow in L/s and headroom.
Is my gutter big enough?
Gutter and downpipe sizing starts from catchment area and design rainfall intensity. Peak flow in litres per second is approximately area (m²) × rainfall (mm/h) ÷ 3600, because one millimetre on one square metre is one litre. Compare that flow with the capacity of your gutter profile or outlet; positive headroom means the stated capacity exceeds the modelled storm, while negative headroom flags a bottleneck. Local design storms vary — use an intensity your building guidance recommends. Roof geometry helpers include roof pitch and covering counts in roof tile.
Worked example
80 m² × 75 mm/h ÷ 3600 ≈ 1.67 L/s. A 1.5 L/s outlet is slightly over capacity in this sketch.
Limits and assumptions
Uniform intensity and fully contributing catchment. Leaf guards, slopes and multiple outlets need detailed drainage design.
Frequently asked questions
Why divide by 3600?
Rainfall is in mm per hour; flow is in litres per second. There are 3,600 seconds in an hour, and 1 mm on 1 m² equals 1 L.
What rainfall intensity should I enter?
Use a design storm from local guidance or manufacturer tables — not yesterday’s light drizzle.
Is capacity for the gutter or the downpipe?
Enter whichever bottleneck you are checking; the tighter of the two governs.
Multiple roof faces?
Sum catchment areas that drain to the same gutter run.