Calculate the required fall (drop) of a pipe over a given length for a target slope percentage.
How to Use the Pipe Slope Calculator
Enter the "Pipe Length (m)" for the pipe run you are installing.
Enter the "Required Slope (%)" you want to achieve (defaults to 1%).
Click "Calculate" to see the required fall in meters and millimeters over the pipe run.
Worked Example: Kitchen Drain to an Inspection Chamber
A kitchen waste pipe runs 12 m from the sink outlet to the nearest inspection chamber, and you want a slope of 1.5%.
Fall = 12 × 1.5 ÷ 100 = 0.180 m, so the calculator shows 0.180 m (180 mm). The pipe invert (the inside bottom of the pipe) at the chamber must be 180 mm lower than at the start of the run.
Work this out before laying any pipe. If the chamber inlet is only 150 mm below the start point, a 12 m run can manage just 0.15 ÷ 12 × 100 = 1.25%, and you would need to lower the chamber connection, raise the starting point, or shorten the route.
Slope Expressed Different Ways
Drawings, plumbers and codes describe the same slope in different forms. This table converts between them, with the fall for a 12 m run in the last column:
Slope (%)
Ratio (1 in N)
Fall per metre
Angle
Fall over 12 m
0.5%
1 in 200
5 mm
0.29°
60 mm
1%
1 in 100
10 mm
0.57°
120 mm
1.5%
1 in 66.7
15 mm
0.86°
180 mm
2%
1 in 50
20 mm
1.15°
240 mm
2.5%
1 in 40
25 mm
1.43°
300 mm
3%
1 in 33.3
30 mm
1.72°
360 mm
To convert a ratio to a percentage, divide 100 by N, so 1 in 80 is 1.25%. Do not confuse percent with degrees: a 2% slope is only about 1.15°, so a spirit-level reading in degrees needs converting first.
Setting the Slope on Site
Fix the levels at both ends first, then stretch a string line between them. Measuring down from the string at regular intervals keeps the bed at an even gradient instead of dipping in the middle.
A water-filled level tube or a laser level is more reliable than a short spirit level over long runs, because small errors add up along the length.
Support buried pipes on a firm, even bed so they do not sag after backfilling. A sag creates a low point where solids collect.
Slope matters for gravity lines such as soil, waste, rainwater and AC condensate pipes. Pressurised water-supply pipes do not need a fall to work, although a slight fall toward a drain-down point helps when emptying them.
Minimum gradients depend on pipe diameter and pipe use. Follow the requirements of your local building bye-laws or the plumbing designer for the project.
Frequently Asked Questions
Fall = Pipe Length × (Slope % ÷ 100). For example, a 20m pipe run at 1% slope needs 0.20m (200mm) of fall from one end to the other.
A common minimum is 1-2% (roughly 1/8 to 1/4 inch of fall per foot), though exact requirements depend on your local plumbing code and pipe diameter — see the Drainage Pipe Slope Calculator for diameter-based recommendations.
Too flat a slope can cause solids to settle and clog the pipe; too steep a slope can cause liquids to drain faster than solids, also leading to blockages. Staying within the recommended range avoids both issues.
Divide the fall by the length and multiply by 100, keeping both in the same unit. A drop of 0.16 m over 8 m is 0.16 ÷ 8 × 100 = 2%.
Strictly, slope is fall divided by horizontal distance. At drainage gradients of a few percent the two lengths differ by a tiny fraction of a millimetre per metre, so a tape measured along the pipe is accurate enough.