Rainwater drainage

Gutter Fall Calculator

Work out the fall a gutter needs, check a fall you already have, or find how far one fall will carry — with a live cross-section that tilts as you type.

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?

The Gutter Fall Calculator exists to answer the single most common drainage installation question: how much lower does the outlet end need to be? Without the right fall, rainwater sits in the gutter channel, breeds mosquitoes, accelerates corrosion, and overflows in any significant storm. This tool removes the guesswork by computing the exact vertical drop for any run length and fall rate, and it works in all four directions — from run to drop, from measured drop back to a true fall rate, from a fixed drop to the longest permissible run, and as an outlet planner for long or double-draining gutters. Results come in inches, millimetres, grade percent, a 1:X ratio and degrees.

The maths

Gutter Fall Calculator Formula

All four modes of this calculator share one core relationship:

Fall = Run × Rate

Where fall is the total vertical drop (in inches or mm), run is the horizontal gutter length, and rate is the fall per unit of run. Rearranged, the same formula gives the other quantities:

Rate = Fall ÷ Run · Run = Fall ÷ Rate

For example, a 24 ft run at the 1:480 minimum (rate = 0.25 in / 10 ft = 0.025 in/ft) needs 24 × 0.025 = 0.6 in of fall. To find the rate from a 15 mm drop over an 8 m run: 15 ÷ 8000 = 0.001875, which equals a 1:533 ratio or 1.875 mm/m.

The gutter fall calculator solves one simple relationship — fall = run × rate — in whichever direction you need. Enter a run and a fall rate to get the vertical drop to build in; switch to Check fall to turn a measured drop into a true fall in inches per foot, millimetres per metre, a 1:X ratio and degrees; use Max run to see how far a given fall will carry; or use Outlets to break a long gutter into evenly draining sections.

Worked example

A 24 ft run at 1/4 in per 10 ft

A 24 ft gutter draining to one outlet at the minimum fall needs 24 ÷ 10 × 0.25 = 0.6 in of fall — about 15 mm, a 1:480 ratio, or 0.12°. Mark the high end roughly 5/8 in above the outlet end and run a string line so the whole length drops evenly. Load it in the calculator above and the diagram tilts to match.

Interactive — step through the 24 ft example

Reference

Common fall rates compared

Fall rate	Ratio	mm per m	Per 10 ft	Use
1:600	1:600	1.7 mm/m	0.20 in	Gentle / short runs
1:480	1:480	2.1 mm/m	0.25 in	Common minimum
1:350	1:350	2.9 mm/m	0.34 in	Long / heavy rain
1:240	1:240	4.2 mm/m	0.50 in	Fast drainage
1:200	1:200	5.0 mm/m	0.60 in	Box / valley gutters

Interactive — pick a fall rate, drag the run

Sample run 24 ft

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Fall rates are planning figures. Always confirm the minimum against your local plumbing or building code before you set out a run — see the UK, Australian and New Zealand versions for region-specific guidance.

Frequently asked

Questions answered

What is gutter fall?

Gutter fall is the deliberate downward slope built into a gutter so rainwater runs to the outlet instead of standing in the channel. It is the vertical drop divided by the horizontal run, and it can be written as inches per foot, millimetres per metre, a 1:X ratio, a percentage grade, or an angle in degrees.

How much fall should a gutter have?

A common rule is at least 1/4 inch of fall for every 10 feet of run, which is roughly 1:480 or 2 mm per metre. Many installers prefer a touch more — around 1:350 to 1:500 — for faster drainage on long or heavy-rain runs. Avoid going so steep that the gutter looks visibly crooked against the fascia.

How do I calculate the fall for a gutter?

Multiply the run length by the chosen fall rate. For example, a 24 ft run at 1/4 in per 10 ft needs 24 ÷ 10 × 0.25 = 0.6 in of fall. The calculator does this in any unit and also works backwards from a measured drop or an available drop.

Which way should a gutter fall?

A gutter should fall toward its downspout outlet. On a long wall you can fall the whole run to one end, or crown it high in the middle and fall both ways to an outlet at each end. The outlet planner mode works out how many outlets a length needs.

Can too much fall be a problem?

Yes. Excessive fall makes water race past the outlet, can cause splashing, and looks slanted next to a level roofline. It also leaves the high end shallow, which can overflow in a downpour. Staying within a typical band keeps drainage clean and the line straight.

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