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Stormwater sizing

Gutter Flow Calculator

Turn roof area and a design rainfall into the flow your gutter must carry — then see how many outlets of each size that flow needs.

Gutter flow & outlet sizer
Live

Typical design storms run 75–150 mm/hr (3–6 in/hr). Use your local authority's figure where possible.

Outlets needed for the flow
roof area — L/s
Gutter + outlets Required flow
Required capacity
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Flow (L/s)
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Flow (US gpm)
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Outlets needed
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Area per outlet
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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.

Ballpark capacities

Outlet flow capacity

Outlet≈ CapacityEquivalent
2×3 in / 50×75 mm0.7 L/s≈ 11 gpm
3 in / 75 mm round1.5 L/s≈ 24 gpm
3×4 in / 75×100 mm1.7 L/s≈ 27 gpm
4 in / 100 mm round3.2 L/s≈ 51 gpm
5 in / 125 mm round5.5 L/s≈ 87 gpm
Interactive — drag area & rainfall, pick an outlet
Required flow — L/s Outlets needed
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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.
Frequently asked

Questions answered

How do I calculate gutter flow?
Multiply the roof plan area by the rainfall intensity and convert to a flow rate: Q (litres/second) = area (m²) × intensity (mm/hr) ÷ 3600. An 80 m² roof in a 100 mm/hr storm sheds 80 × 100 ÷ 3600 ≈ 2.2 L/s. The calculator does this and then sizes the outlets.
What rainfall intensity should I use?
Use your local design storm — often a 1-in-20-year, 5-minute intensity from a rainfall map or your building authority. Typical design figures run 75–150 mm/hr (3–6 in/hr). Higher intensities mean more flow and more or larger outlets.
Is roof area the sloped area or the footprint?
Use the plan (footprint) area — the flat area the roof covers when viewed from above — because that is what collects the rain. For very steep roofs some codes add a wind-driven allowance; check locally if your pitch is severe.
How many downspouts do I need for the flow?
Divide the required flow by the capacity of one outlet and round up. The tool uses ballpark per-outlet capacities so you can compare 75 mm round, 100 mm round and rectangular outlets, and shows the roof area each one ends up serving.
Does gutter slope affect flow capacity?
Yes — a steeper fall moves water to the outlets faster and slightly raises the gutter’s own carrying capacity, while the outlet itself sets how fast water can leave. Pair this flow result with the fall from the main slope calculator for a complete design.

Need the full gutter slope toolkit?

Switch between drop, slope check, max run and downspout planning on the main calculator — with the same live diagram.

Open the main calculator