What size gutters do I need for my roof?

The correct gutter size depends on the effective roof area draining into the gutter. Half-round 112mm gutters handle up to 0.6 l/s (suitable for roof areas up to about 29m²). Square 114mm handles 0.9 l/s (~43m²). Deep flow 170mm handles 1.5 l/s (~72m²). For larger roofs or areas with heavy rainfall, opt for deep flow or commercial-grade guttering. If you only know plan dimensions, use our Roof Area Calculator to find the effective roof area.

How many gutter brackets do I need?

Gutter brackets should be spaced at a maximum of 1 metre apart along the gutter run, with additional brackets within 150mm of any joint or fitting. For a 10m gutter run, you would need approximately 11 brackets. Some plastic guttering systems specify even closer spacing — check the manufacturer's instructions.

How many downpipes do I need?

As a rule of thumb, you need one downpipe for every 50m² of effective roof area, with a minimum of one downpipe per gutter run. For long runs over 10m, it is better to have a downpipe at each end with the gutter sloping towards both. This improves flow and reduces the risk of overflow in heavy rain.

What rainfall rate should I use for the UK?

The standard moderate UK rainfall intensity used for gutter sizing is 75mm per hour, as specified in BS EN 12056-3. However, this can vary by region — western and upland areas may experience higher intensities. For critical applications, check local rainfall data from the Met Office or your local Building Control authority.

Should I use round or square guttering?

Square (or ogee) guttering has a higher flow capacity than half-round of the same nominal size and looks more modern. Half-round is more traditional and suits period properties. Deep flow guttering is the best choice for large roofs. uPVC is the most common material — it is affordable, easy to install and low-maintenance. Cast iron or aluminium suit listed buildings and conservation areas.

Gutter Sizing Calculator — What Size Gutters and Downpipes Do I Need?

Gutter Sizing Calculator — Capacity, Lengths & Fittings

Calculate gutter capacity, number of lengths, downpipes, brackets and fittings for UK roofs based on rainfall rates.

Roof Area Draining to Gutter

Effective area of roof draining to this gutter

Gutter Run Length

Total length of gutter along eaves

Gutter Profile

Gutter Length Size

Include wastage allowance

Adds 5% extra for cuts

Wastage Percentage

5% is typical for guttering

Price Per Gutter Length (optional)

Enter your supplier price for a cost estimate

How We Calculate This

Our gutter sizing calculator determines the correct gutter size for your roof by comparing the rainwater runoff rate with the capacity of your chosen gutter profile. It also calculates the number of lengths, downpipes, brackets and fittings needed.

Sizing formula

Runoff rate (l/s) = Effective roof area (m²) × Rainfall intensity (mm/hr) ÷ 3600

We use the UK moderate rainfall intensity of 75mm/hr (BS EN 12056-3). If the gutter capacity exceeds the runoff rate, the size is adequate.

Gutter capacities

Half-Round 112mm: 0.6 litres/second
Square 114mm: 0.9 litres/second
Deep Flow 170mm: 1.5 litres/second
Ogee 130mm: 1.0 litres/second

Fittings calculation

Downpipes: 1 per 50m² of roof area (minimum 1)
Brackets: 1 per metre of gutter run, plus 1
Unions: 1 per joint between gutter lengths
Stop ends: 2 per gutter run (1 left, 1 right)

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Gutter Sizing Calculator — Capacity, Lengths & Fittings

How We Calculate This

Sizing formula

Gutter capacities

Fittings calculation

Frequently Asked Questions

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