Gutter & Downpipe Size Calculator
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Gutter & Downpipe Size Calculator
Size half-round eaves gutters, outlets and rainwater downpipes from your roof dimensions and pitch — built on UK Building Regulations (Approved Document H, Section 1, Tables 1 & 2) and the standard 75 mm/hr design rainfall.
Gutter, outlet & downpipe size
Works out the effective design area (Approved Document H Table 1), the peak roof run-off, and the smallest level half-round gutter, outlet and downpipe that will carry it (Table 2).
Effective roof area
Convert a roof’s plan (or elevational) area into the effective design area used for gutter sizing, applying the Approved Document H Table 1 pitch factor.
Effective area is what you carry into the gutter sizer and the reference tables. For a duo-pitch roof draining to a gutter on each side, size each gutter for one slope only.
Reference tables (Approved Document H)
The two tables behind this calculator. Table 2 capacities assume a level half-round gutter with a sharp-edged outlet at one end, at the standard 0.021 l/s/m² (75 mm/hr) design rainfall.
| Type of surface | Effective design area |
|---|---|
| Flat roof | plan area × 1.0 |
| Pitched roof at 30° | plan area × 1.29 |
| Pitched roof at 45° | plan area × 1.50 |
| Pitched roof at 60° | plan area × 1.87 |
| Pitched over 70° or any wall | elevational area × 0.5 |
| Max. effective area (m²) | Gutter ø (mm) | Outlet ø (mm) | Flow (l/s) |
|---|---|---|---|
| 6.0 | – | – | – |
| 18.0 | 75 | 50 | 0.38 |
| 37.0 | 100 | 63 | 0.78 |
| 53.0 | 115 | 63 | 1.11 |
| 65.0 | 125 | 75 | 1.37 |
| 103.0 | 150 | 89 | 2.16 |
A gutter laid to a fall, a deeper section, or a round-edged outlet can allow a smaller size (AD H 1.6). A rainwater pipe should be at least the size of the gutter outlet (1.10). Where a roof >25 m² discharges through one downpipe onto a lower roof, fit a distributor pipe (1.9). Valley, parapet, flat-roof, siphonic or adoptable schemes should be designed to BS EN 12056-3.
This gutter size calculator works out the smallest half-round eaves gutter, outlet and downpipe that will carry the rainwater from your roof, from the roof dimensions and pitch. It follows Approved Document H, Section 1 (Tables 1 and 2) and the standard 75 mm/hr design rainfall. For the full Building Regulations guidance, see Approved Document H.
How the calculator sizes a gutter
Enter the roof either as length x width or as a known plan area, with the surface type and pitch. The tool first works out the effective design area (Approved Document H Table 1, which adds an allowance for pitch), then the peak run-off, then the smallest level half-round gutter, outlet and downpipe that will carry it from Table 2.
Effective roof area and pitch
A pitched roof sheds more water to the gutter than its plan footprint suggests, so Approved Document H applies a factor based on the pitch. The calculator handles this for you — you enter the plan dimensions and the pitch, not the sloping area.
Why outlet position matters
A gutter's capacity depends on where the outlet is. An outlet at one end carries less than the same gutter with a central outlet feeding two ways. The calculator assumes a level gutter and standard outlet, which is the conservative case for most domestic roofs.
Further reading: foul, surface and combined sewers explained · keeping drains clear to prevent surface water flooding · Approved Document H.
Frequently asked questions
It depends on the effective roof area and the rainfall intensity. For a typical house a 112 mm half-round gutter with a 68 mm downpipe is common, but the calculator confirms the size from your actual roof.
Approved Document H uses 75 mm/hour for domestic roof drainage in England, which the calculator applies by default.
Enough that each gutter length stays within the capacity for its outlet arrangement. If one downpipe is undersized for the area, add another or increase the gutter size.
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