How do I calculate drain pipe fall?

Convert the slope to a decimal and multiply it by the horizontal run length. For example, a 2% slope is 0.02, so a 10 m run has a calculated fall of 0.2 m.

What does a 2 percent pipe slope mean?

A 2% slope means the elevation changes by 0.02 units vertically for each 1 unit of horizontal run.

Can I enter a ratio such as 1:50?

Yes. A fall ratio of 1:50 corresponds to a decimal slope of 1 divided by 50, or 2%.

Drainage fall geometry

Drain Pipe Slope Drop Calculator — Find the Fall Over Any Run

Calculate how much a drainage pipe drops over a horizontal run from a percent slope, decimal slope or fall ratio. The tool also shows fall per metre so the result is easy to check against drawings.

Drainage slope inputs

Horizontal run

Slope format

Slope value

Run / vertical unit

For a ratio input, enter the denominator N in 1:N. For example, 1:50 is entered as 50 and equals a 2% slope.

Enter run length and slope.

Drainage drop result

Total calculated fall

—

—Slope

—Fall

—Fall per metre

—Run

—

Pipe fall formula

Decimal slope = slope % ÷ 100
Fall = horizontal run × decimal slope

Reading a ratio

A fall of 1 in N means one unit of vertical drop for every N horizontal units. Converting it to a decimal gives 1 ÷ N.

Why horizontal run matters

The calculated fall increases proportionally with run length when the slope remains constant. Double the run and the calculated fall doubles.

Worked example

A 12 m drain run at 2% slope:

Decimal slope = 2 ÷ 100 = 0.02
Fall = 12 × 0.02 = 0.24 m = 240 mm

Frequently Asked Questions

What is a 1:50 slope?

It corresponds to 2%, because 1 ÷ 50 = 0.02.

Can the run be entered in millimetres?

Yes.

Does this size a drainage pipe?

No. It only calculates geometric fall from the selected slope.