UK · metric

Gutter Fall Calculator UK

Set the fall for a UK eaves gutter in millimetres per metre and 1:X ratios — find the fall, check one you have, or lay out outlets on a long run.

UK gutter fall finder

Live

Enter the gutter run and a fall rate — the calculator returns the total fall to build in.

Gutter run to outlet i

Fall rate i

Live preview — the gutter tilts to your fall

Gutter (sloped) Water flow Level reference

Enter a run and a fall rate — the fall appears here.

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About this tool

What is the Gutter Fall Calculator UK?

The Gutter Fall Calculator UK exists for British plumbers and roofers who specify and install eaves gutters to BS EN 12056-3 and Building Regulations Part H. UK rainwater work is done entirely in metric, and fall is quoted as a 1:X ratio or millimetres per metre — conventions that don't appear in most general-purpose slope tools. This page computes the fall in millimetres for any UK eaves gutter run, checks an installed fall against recommended practice, finds the maximum run a given fall supports, and plans outlet positions for long walls — working from the metric inputs and ratio notation used on British jobs.

The maths

Gutter Fall Calculator UK Formula

UK gutter fall is calculated from the run and a 1:X ratio:

Fall (mm) = Run (m) × (1000 ÷ X)

The term 1000 ÷ X converts the ratio to mm per metre. For 1:350: 1000 ÷ 350 ≈ 2.857 mm/m. A 6 m eaves gutter at 1:350 therefore needs 6 × 2.857 ≈ 17 mm of fall. Reversing to find the ratio from a measured drop: X = Run (mm) ÷ Drop (mm). A 12 mm drop over 6 000 mm run gives X = 6000 ÷ 12 = 500, i.e. a 1:500 ratio.

This UK gutter fall calculator works in metric by default — run in metres, fall in millimetres — and quotes the fall as a 1:X ratio and mm per metre, the forms used on British jobs. UK eaves gutters are usually laid to a gentle continuous fall of around 1:350 to 1:600 toward the outlet, in line with the rainwater design principles of BS EN 12056-3. Enter your run and a ratio to get the fall in millimetres, or check a fall you have already set.

UK reference

Common UK falls

Ratio	mm per m	Fall over 6 m	Use
1:600	1.7 mm/m	10 mm	Gentle / short runs
1:500	2.0 mm/m	12 mm	General purpose
1:350	2.9 mm/m	17 mm	Common choice
1:300	3.3 mm/m	20 mm	Long / heavy rain
1:200	5.0 mm/m	30 mm	Steep / valley

Interactive — pick a fall ratio, drag the run

Sample run 24 ft

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Falls shown are common UK practice for guidance only — confirm the requirement for your system against BS EN 12056-3, Building Regulations Part H, and the gutter manufacturer's fixing instructions.

Prefer the word "slope" or need both unit systems? Use the UK gutter slope calculator.

Frequently asked

Questions answered

What fall should a UK gutter have?

UK eaves gutters are typically laid to a gentle fall of around 1:350 to 1:600 — roughly 1.7 to 3 mm for every metre of run — toward the outlet. BS EN 12056-3 covers rainwater design; many manufacturers recommend a slight continuous fall rather than a level gutter.

How do I calculate gutter fall in mm per metre?

Multiply the run in metres by the fall rate in mm/m. A 6 m run at 3 mm/m falls 18 mm end to end. Enter the run and pick a ratio above and the calculator gives the fall in millimetres, plus the equivalent in/ft, percentage and degrees.

What does a 1:600 fall mean?

A 1:600 fall means the gutter drops 1 mm for every 600 mm of length — about 1.7 mm per metre. It is a very gentle but effective fall used on many UK half-round and square-line gutters.

Can a UK gutter be fitted level?

Some installers fit gutters level on short runs, relying on the depth of the channel, but a small continuous fall toward the outlet drains far better and reduces standing water, silt and overflow. A fall is strongly preferred on longer runs.

How far apart should UK gutter outlets be?

Keep each outlet serving a sensible length — often up to about 10 m — and on long runs fall the gutter both ways from a high point to an outlet at each end. The Outlets mode plans this from your total length.

15 focused calculators

The complete gutter tool suite

Every gutter question has its own purpose-built calculator — fall, slope, pitch, angle, flow and outlet spread, for the US, UK, Australia and New Zealand.

Fall Per Foot Pitch Angle Flow Spread Rain Fall (UK) Slope (UK) Fall (AU) Box Fall (AU) Fall (NZ) Seamless Roof Fall (AU)

Need the full gutter slope toolkit?

Switch between drop, slope check, max run and downspout planning on the main calculator — with the same live diagram.

Open the main calculator