Gutter Size Calculator
To size an eaves gutter, work out the roof's effective area (its plan area plus half its vertical projection), multiply by the design rainfall intensity for your location to get a flow rate in litres per second, then pick the smallest gutter that carries that flow. A typical UK semi-detached roof slope of 32m² at 35° produces about 0.95 l/s and needs a 115mm half-round gutter with a 63mm outlet.
This calculator uses the effective area method from BS EN 12056-3 and the rainfall intensities and gutter sizes published in Approved Document H of the Building Regulations. It shows every step of the working so you can check it.
Your roof
Any pitch works here. Approved Document H only tabulates 30°, 45° and 60°, but those are just three points on the same curve.
We default to the highest intensity on the map, so this calculator will never undersize your gutter. If you know your local figure from Approved Document H Diagram 1, select it to get a more precise result. How we get this number.
Your result
Enter your roof dimensions to see a recommendation.
Where this calculator applies, and where it does not
- Eaves gutters only. Approved Document H paragraph 1.2 requires valley gutters, parapet gutters, siphonic systems and flat roof drainage to be designed to BS EN 12056 in full. This tool does not cover them.
- England and Wales. The Approved Document H rainfall map stops at the Scottish border and does not cover Northern Ireland. Scotland works to the Scottish Technical Handbooks and Northern Ireland to its Technical Booklets.
- Half-round, laid level, outlet at one end. Those are the conditions Approved Document H Table 2 is published under. Deeper profiles carry more, and a centre outlet roughly doubles capacity.
- Gutter runs. Table 2 assumes the stop end sits no more than 50 times the water depth from the outlet. On long runs, capacity falls.
- This is a specification aid, not a substitute for design by a competent person.
Why roof pitch increases gutter load
A pitched roof catches more rain than its footprint suggests, because rain does not fall straight down. BS EN 12056-3 assumes rain is driven at 26.6° from vertical, which works out as a simple rule: the effective area is the plan area plus half the roof's vertical projection.
That gives an effective area multiplier of 1 + ½ × tan(pitch). It reproduces the three factors printed in Approved Document H Table 1 exactly:
| Pitch | Approved Document H Table 1 | 1 + ½·tan(pitch) |
|---|---|---|
| Flat | plan area | 1.000 |
| 30° | plan area × 1.29 | 1.289 |
| 45° | plan area × 1.50 | 1.500 |
| 60° | plan area × 1.87 | 1.866 |
| Over 70°, or a wall | elevational area × 0.5 | n/a |
Because the two agree, this calculator uses the continuous formula and accepts any pitch. Most UK roofs are not 30°, 45° or 60°, so rounding to the nearest tabulated value throws away accuracy for no reason.
Rainfall intensity is not the same everywhere in the UK
Design rainfall intensity for gutters varies across England and Wales from 0.012 to 0.022 litres per second per square metre, which is a range of nearly two to one. Approved Document H Diagram 1 maps it.
The geography surprises people. The highest intensities are not in the rainy west: they are in East Anglia, the Midlands and the South East, where short, violent summer thunderstorms dominate. Gutters are sized for a brief downpour, not for annual rainfall totals, so a wet hillside in Snowdonia has a lower design intensity than Norwich.
A number you will see elsewhere that you should not use
Many merchant sites still quote 75 mm/hour for gutter sizing. That figure comes from BS 6367, which has been withdrawn. HR Wallingford's SR 620, the official UK companion to BS EN 12056-3, states it should no longer be used here.
You will also see 0.014 l/s/m² presented as an all-purpose default. It is a real number from Approved Document H, but it belongs to paragraph 3.8, surface water drainage, and it points to Diagram 2, which is a different map with a lower range. Applying it to gutters would undersize them by around a third across the Midlands and the South East. Paragraph 1.1 is explicit that gutters take their intensity from Diagram 1, and it offers no default at all.
Approved Document H Table 2: gutter sizes
These are the sizes Approved Document H publishes for nominal half-round eaves gutters laid level, with a sharp-edged outlet at one end.
| Max effective roof area (m²) | Gutter size (mm) | Outlet size (mm) | Flow capacity (l/s) |
|---|---|---|---|
| 6.0 | None specified | None specified | Not stated |
| 18.0 | 75mm | 50mm | 0.38 |
| 37.0 | 100mm | 63mm | 0.78 |
| 53.0 | 115mm | 63mm | 1.11 |
| 65.0 | 125mm | 75mm | 1.37 |
| 103.0 | 150mm | 89mm | 2.16 |
Those roof areas already assume the worst-case rainfall intensity. Divide any row's flow capacity by its area and you get roughly 0.021 l/s/m², near the top of Diagram 1's range. That is why this calculator selects on flow rather than on area: if you know your local intensity is lower, you should get the benefit of it.
Frequently asked questions
What size gutter do I need for my house?
Most UK semi-detached and terraced houses fall between 112mm and 115mm half-round per roof slope with a 63mm downpipe. Larger detached houses, steeper roofs and long unbroken runs often need 125mm, a deeper profile, or a second outlet. Measure your slope and use the calculator above rather than assuming.
Is 112mm the same as 115mm?
They are different nominal sizes from different manufacturers and they are generally not interchangeable. FloPlast Half Round is 112mm; Brett Martin Roundstyle is 112mm; Approved Document H tabulates 115mm. Brackets, unions and stop ends are profile-specific, so mixing brands within a run usually will not seal even when the nominal diameter matches.
Does a downpipe in the middle of the run help?
Yes, substantially. A centre outlet roughly doubles a gutter's capacity compared with a single outlet at one end, because each half of the run drains a shorter distance. If your calculated flow is marginally over a gutter's capacity, moving or adding an outlet is usually cheaper than moving to a bigger profile.
Can I use this for a valley or parapet gutter?
No. Approved Document H paragraph 1.2 requires valley gutters, parapet gutters, siphonic systems and flat roof drainage to be designed to BS EN 12056 in full, because overtopping in those positions puts water inside the building rather than over the edge. Use a competent designer.
Does this work in Scotland or Northern Ireland?
The rainfall map in Approved Document H covers England and Wales only. Scotland works to the Scottish Technical Handbooks and Northern Ireland to its Technical Booklets. The effective area method is the same everywhere, but you will need a local design rainfall intensity, which BS EN 12056-3's National Annex provides.
Sources. Approved Document H (Drainage and Waste Disposal), Section H3 Diagram 1, Table 1 and Table 2. Contains public sector information licensed under the Open Government Licence v3.0. BS EN 12056-3 method as described in HR Wallingford Report SR 620, the UK companion guide.
Approved Document H Diagram 1 is a contour map. We cannot reliably resolve a postcode to a contour band from it, so we default to the highest intensity shown rather than guess a lower one. Read the full methodology.