Gutter Slope Calculator Open Calculator
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.

Live preview — the gutter tilts to your fall
level reference drop run HIGH END LOW END →
Gutter (sloped) Water flow Level reference
Enter a run and a fall rate — the fall appears here.
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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
run drop water runs to the outlet
Reference

Common fall rates compared

Fall rateRatiomm per mPer 10 ftUse
1:6001:6001.7 mm/m0.20 inGentle / short runs
1:4801:4802.1 mm/m0.25 inCommon minimum
1:3501:3502.9 mm/m0.34 inLong / heavy rain
1:2401:2404.2 mm/m0.50 inFast drainage
1:2001:2005.0 mm/m0.60 inBox / valley gutters
Interactive — pick a fall rate, drag the run
level reference fall HIGH
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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.

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