Fall calculator

Convert a fall or slope between 1 in X, per cent, degrees and rise over run, work it out from a drop and a length, and get the fall over any run, for gutters, drains, paving and ramps.

Slope calculator

This calculator needs JavaScript, which is turned off or didn't load. You can still do it by hand: the formulas, the conversion chart and the worked examples below give the same answers.

Units

Millimetres Feet and inches

What do you want to work out?

Slope from rise and run Convert a slope Rise over a length Length for a rise

Rise (or drop)

The height difference from end to end.

Length (measured level)

The horizontal run, not the length along the slope.

Slope as a ratio
Percentage
Degrees
Rise ÷ run
Millimetres per metre
Inches per foot
Rise or drop
Length, measured level
Length along the slope

Rise or drop at this slope over

Length	Rise or drop
1 m
2 m
3 m
5 m
10 m
20 m

This is arithmetic, not a compliance ruling. The slope or fall a job needs comes from the code, the drawings and your council: see the common falls and their sources below.

Lengths in millimetres or feet and inches. The same maths as the Pitch calculator in the MeisterCalc app.

Published 6 November 2026 · By Sai Nath

Worked example: a 50 mm drop over 3 m of paving

Slope: 50 ÷ 3000 = 0.0167.
Per cent: 0.0167 × 100 = 1.67%.
Degrees: arctan(0.0167) = 0.95°.
1 in X: 3000 ÷ 50 = 1 in 60, which is 16.7 mm of fall per metre.

The other way round: an eaves gutter at the NCC minimum of 1:500 falls 6000 ÷ 500 = 12 mm over a 6 m run.

How it works

Slope = rise ÷ run, both in the same unit, with the run measured level.
Per cent = slope × 100. Degrees = arctan(slope). 1 in X = 1 ÷ slope.
Fall over a length = length ÷ X, or length × slope.

Common falls in the NCC Housing Provisions: eaves gutters not less than 1:500 (7.4.4), ground or paving next to a slab-on-ground house 50 mm over the first metre (3.3.3), and wet-area floors between 1:80 and 1:50 to the floor waste (10.2.12).

Pitch in MeisterCalc

Pitch in the MeisterCalc app

Open Home ▸ Pitch, pick Rise + Run and enter Rise 10 mm and Run 1000 mm. It shows 0.57°, a 1.00% grade, 1 in 100 and 10.0 mm of fall per metre. Run + Grade gives the fall over any run from a percentage.

On Home, tap Pitch
Pick Rise + Run: Rise 10 mm, Run 1000 mm
Read 0.57°, 1.00%

The app version, with every input, result and screen: Pitch in MeisterCalc.

Pitch is one of the app's paid calculators. Unlock it with a one-time purchase, no subscription required, or with a weekly subscription that starts with a 3-day free trial. Google Play shows the price in your currency. Every calculation runs on the phone, so it works with no signal on site. It runs on Android 7.0 or later. Get MeisterCalc on Google Play

Questions

Is 1 in 100 the same as 1:100 and 1%?

Yes. All three are 10 mm of fall for every metre of length.

What is a 2% slope in degrees?

arctan(0.02) = 1.15°. As a fall it is 1 in 50, or 20 mm per metre.

Which is steeper, 1 in 50 or 1 in 80?

1 in 50: 20 mm per metre against 12.5 mm. The smaller the X, the steeper the fall.

Sources

Building rules are set by the National Construction Code, the NZ Building Code or the Approved Documents and by your council, not by this page or the app.