Gutter Sizing Calculator — Capacity, Lengths & Fittings

The correct gutter size depends on the effective roof area draining into the gutter, and on how the gutter is installed. The figures below are the manufacturer's BS EN 607 tested capacities for the conservative level, single end-outlet layout: half-round 112mm handles 0.82 l/s (FloPlast, ~40m²), square 114mm 1.53 l/s (~73m²), ogee 130mm 1.83 l/s (Polypipe Extra Capacity, ~88m²), and deep flow 170mm 3.87 l/s (FloPlast XtraFlo, ~185m²). Moving the outlet to the centre of the run roughly doubles these areas, and a 1:350 fall adds about 25-30%. For larger roofs or heavy-rainfall areas, 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.

Maximum bracket spacing depends on the profile. Half-round, square and ogee gutters take a bracket every 1 metre (Polypipe's Ogee Extra Capacity bracket is rated for 1.0m centres, and FloPlast half-round/square the same). The heavy 170mm Deep Flow / XtraFlo industrial profile holds far more water, so its brackets go no more than 600mm apart. A bracket should also be fitted within 150mm of every joint, union, angle, stop-end and outlet. For a 10m run that is about 11 brackets in half-round, square or ogee, or about 18 in 170mm deep-flow. This calculator sets the spacing automatically from the profile you choose. Always check the manufacturer's instructions, as heavy snow-load areas specify closer spacing (down to around 400mm). Use our Gutter Bracket Calculator for a detailed bracket and fitting breakdown.

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.

The design rainfall intensity for category-1 lowland England, Wales and lowland Scotland is 75mm per hour, taken from the National Annex to BS EN 12056-3. Exposed, upland and western areas — west Scotland, the Lake District/Cumbria, Snowdonia, west Wales and Cornwall — use higher intensities of around 100 to 150mm per hour, so raise the Rainfall Intensity field in those regions or you risk under-sizing. For critical or higher-risk buildings, check local rainfall data from the Met Office or your Building Control authority.

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.