FabLayout

Pipe slope and grade calculator

Enter the horizontal run and the drop per foot. You get the total drop, the grade in percent, the angle and the length of pipe along the slope.

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Export from the Truemark app

The PDF cut sheet with this drawing and your marks is made in Truemark. It works offline on site and keeps your jobs.

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Total dropTotal drop221122″″RunRun120120″″
Scale drawing

Total drop

2 1/2in

Exact geometry

Grade
In the app
Angle
In the app
Pipe length
In the app

Every result is in Truemark

The free web version shows the main result and the drawing. Truemark gives you every mark and measurement, and exports a cut sheet. It is free to download.

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  • Grade is in percent. The drawing exaggerates the drop so it can be read.

How to lay it out

  1. Measure the horizontal run

    Measure the level distance the pipe has to cover, not the length along the pipe.

  2. Set the drop per foot

    Enter the fall you need for every foot of run, such as ¼″ per foot.

  3. Mark the low end

    Mark the low end of the run the total drop below the high end, with a level line or a laser.

  4. Cut the pipe

    Cut the pipe to the pipe length shown, which allows for the slope, and hang it between the two points.

Good to know

  • Grade is in percent. The drawing exaggerates the drop so it can be read.
  • FabLayout runs the same code as Truemark; the result is exact geometry.
  • Source: Plane trigonometry; same calculation as the Truemark app

Method and formula

Drain and sloped runs are specified as a drop per foot of horizontal run. Everything else follows from that and the length of the run.

  • Total drop = run ÷ 12 × drop per foot.
  • Grade = drop ÷ run × 100, in percent.
  • Angle = arctan(drop ÷ run).
  • Pipe length = √(run² + drop²), the length along the slope.

On normal drainage slopes the pipe length is barely longer than the run. The drop is the number to get right. The drawing on this page exaggerates the drop so it can be read.

Worked example

The page defaults: 120″ of run at ¼″ per foot. The total drop is 120 ÷ 12 × ¼ = 2½″. The grade is 2.5 ÷ 120 × 100 = 2.0833%, the angle is 1.1935°, and the pipe along the slope is 120.026″.

Questions

How do I convert ¼″ per foot to percent?

Divide the drop per foot by 12 and multiply by 100. ¼″ per foot is about 2.08%.

Should I measure the run along the pipe?

No. Measure the level, horizontal distance. On small slopes the difference is tiny, but the formula expects the horizontal run.

What drop per foot should I use?

Use the slope your local code or the drawings require for that pipe. This page calculates with whatever value you enter.

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