Work out the gutter run, the pitch-adjusted roof catchment feeding it, how many downpipes that catchment needs and how many brackets to fix along the run. Every sizing figure is editable, because gutter practice varies by region and rainfall.
Measuring the Eave, the Roof and the Pitch
The gutter run is the horizontal length of eave you're actually guttering. On a simple gable house with two straight eaves, that's two separate runs — calculate each one on its own. The calculator returns the straight run only: drops down to the downpipes, angles at corners and returns around dormers or bargeboards all add length to the final order, so tally those fittings from your elevation.
The roof width feeding the eave is a plan measurement — how far the roof reaches out from the eave you're guttering to the ridge (or the far side of a lean-to). Measure it on the ground from the fascia to the point directly below the ridge, including the overhang, because every bit of that plan sheds water your gutter has to carry. On a roof of two different spans meeting at a valley, treat each surface separately and give the valley its own downpipe consideration — valleys concentrate flow.
Pitch matters because a sloping plane presents more surface to the rain than its plan footprint. The calculator handles this with a multiplier equal to the sloping length divided by the plan length — the hypotenuse ratio of the roof triangle. At 6 in 12 the factor is 1.118, so 72 m² of plan behaves like 80.5 m² of catchment. It's geometry, not a code value, and it changes the answer only modestly — but on a large roof it's often the difference between one downpipe and two.
What the Results Mean
- Gutter Length: The straight run. Add drops, angles, unions and any returns when ordering.
- Effective Catchment: Plan area × pitch multiplier. This is the figure to compare against your area-per-downpipe rule.
- Downpipes: Catchment divided by your m²-per-downpipe figure, always at least one, rounded up.
- Brackets: Spacing run out along the eave with a bracket at each end. Corners and outlets typically want a bracket close by regardless of spacing.
The Area-per-Downpipe Figure
The default of 50 m² (about 540 sq ft) per downpipe is a typical figure for 100 mm (4 in) half-round gutters in moderate rainfall — it is not a universal rule. Larger gutters and downpipes carry more, so they allow more area each; regions with intense cloudbursts allow less. If your roof is big, your rainfall heavy, or your gutters smaller than 100 mm, lower the figure and let the calculator add downpipes. Manufacturer sizing charts for your region are the final authority; the gutter and downpipe guide walks through the reasoning.
Pro Tip
Set the fall before you fix a single bracket. Gutters need a slight slope toward each outlet — a few millimetres per metre is the usual practice — so snap a chalk line from the high point to the outlet and hang brackets to the line, not to the fascia's own edge, which is rarely level.
The Math Behind the Calculator
Catchment sizing is one multiplication and one division. The pitch multiplier is the hypotenuse of a right triangle with run 12 and your chosen rise:
pitch_multiplier = √(1 + (rise ÷ 12)²)
effective_catchment = eave_length × roof_plan_width × pitch_multiplier
downpipes = max(1, ceil(catchment ÷ area_per_downpipe))
brackets = floor(eave_length ÷ bracket_spacing) + 1
Pitch Multiplier Reference
These are pure geometry — the sloping length of the rafter line divided by its horizontal projection — not code values. Rainfall intensity, gutter profile and outlet size are handled through the editable area-per-downpipe figure instead.
Assumptions & Limitations
Every number above rests on these constants. If your project differs, change the matching input — or read the linked guide for the full reasoning.
- Gutter length equals the straight eave run — drops, angles, unions and returns are extra
- One eave calculated at a time; a full roof needs each run done separately
- 50 m² per downpipe is a typical figure for 100 mm half-round gutters in moderate rainfall — varies by gutter size, downpipe size and rainfall intensity
- Pitch multipliers are hypotenuse ratios (geometry), not regional code values
- Quantities only — this site deliberately does not price labour, delivery or materials
Worked Example: 12 m Eave on a 6/12 Roof
A single-storey wing with a 12 m (39 ft) eave, a roof plan width of 6 m (20 ft) draining to that eave, a 6 in 12 pitch, gutters sized at the typical 50 m² per downpipe, and brackets at 1 m (about 3 ft) centres. These are the calculator's default values, so the numbers below are what loads on the page:
- Catchment: 12 × 6 = 72 m² of plan.
- Pitch adjustment: 72 × 1.118 = 80.5 m² effective catchment.
- Downpipes: 80.5 ÷ 50 = 1.61, rounded up = 2 downpipes.
- Gutter length: the straight run, 12 m — plus two outlets and any corner angles when ordering.
- Brackets: floor(12 ÷ 1) + 1 = 13 brackets.
Now change one variable: the same house in a heavy-rainfall region where you allow only 35 m² per downpipe gives 80.5 ÷ 35 = 2.3 → 3 downpipes. That's the whole argument for the editable field — the roof didn't change, the weather assumption did.
Reminder
These are planning estimates for material quantities, not a drainage design. Outlets also need somewhere to go: downpipes must connect to drainage that can carry the flow away from the building, and local rules on stormwater discharge vary. For complex roofs or problem sites, a plumber or roofer's judgement beats any rule of thumb.