Pipe Slope vs Fall vs Grade: What Is the Difference?

Slope and grade describe a rate

The underlying ratio is vertical change divided by horizontal distance. Multiply that ratio by 100 to express it as percent grade. In pipe work, the same rate may instead be written as millimeters per meter (mm/m) or inches per foot (in/ft). The unit label matters: 2% and 2 degrees are different measurements.

For a downhill pipe, “fall” refers to the magnitude of the drop. A drawing must still identify the high and low ends. A slope number alone does not tell you which way the pipe runs.

Worked example: 10 meters at 2% grade

Assume a straight run measured horizontally between the intended endpoints. The specified grade is 2%, and the pipe falls toward the downstream end.

1. Convert the rate: 2% ÷ 100 = 0.02.
2. Calculate fall: 10 m × 0.02 = 0.20 m.
3. Convert the result: 0.20 m × 1,000 mm/m = 200 mm.

The grade is 2%; the fall over this 10 m run is 200 mm. The equivalent rate is 20 mm/m because 2% of 1,000 mm is 20 mm. The 200 mm total is not a slope unit.

If the horizontal run were 5 m at the same 2% grade, the fall would be 5 m × 0.02 = 0.10 m, or 100 mm. Doubling the run doubles the total fall while the grade stays 2%.

How to read the terms on a plan

• “2% grade” or “20 mm/m slope” gives a rate. You still need the horizontal run to calculate the total fall.
• “200 mm fall” gives a total drop. You still need the horizontal run to work out the grade.
• “2% fall” mixes a rate with a total-change word. Check the drawing or specification to see whether it means 2% slope or a stated drop over a particular distance.

Use the same reference points for the run and elevation change. Measuring along the sloping pipe instead of horizontally changes the ratio. Check whether a plan states a nominal rate, a minimum rate, or a pair of required elevations before using a calculated drop.

Calculate the fall on iPhone

The free Pipe Slope / Fall tool in Plumbing Calculator & Tools accepts a positive run in feet or meters and a positive slope in percent, in/ft, or mm/m. For the example above, enter 10 meters and 2%. The calculated total fall is 200 millimeters. Check that the run, slope unit, and result match your hand calculation.

You can copy or share a valid slope result as text, or save it with an optional note in a project. Record which endpoint is high, because the calculator gives the amount of fall rather than the physical direction of the run.

What to record

• Horizontal run, units, and measurement endpoints.
• Original slope or grade value and its unit.
• Calculated total fall, output unit, and any rounding.
• High and low endpoints and the drawing or requirement used to verify the layout.

Related guides

For the full calculation from run and slope, see How to Calculate Pipe Fall from Length and Slope. To compare percent with in/ft and mm/m inputs, read the Pipe Slope Calculator Guide.