MachineCalcs

Pipe Slope Calculator

Calculate pipe fall, end invert elevation, slope percent, inches per foot, 1:n pitch ratio and angle from run length and slope or from two invert elevations.

Calculator

Inputs

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Use slope mode when the required pitch is known. Use invert mode when start and end elevations are measured.

Horizontal run length along the pipe layout, not the sloped pipe centerline length.

m

Slope as fall divided by horizontal run times 100. 2.083% equals 1/4 in per foot.

%

Gravity drains usually fall from start to end. Use rise only for an elevation comparison or non-gravity layout.

Invert elevation at the upstream or starting point. The datum can be arbitrary if only the relative change matters.

m

Results

Default result
Edit inputs
Fall over run(dE)
0.127m
Pass

falls from start to end

Also computed

End invert elevation(E2)30.353m

Slope percent(S)Pass2.083%

falls from start to end

Slope per foot(pitch)Pass0.25in/ft

falls from start to end

Pitch ratio(1:n)481:n

1:48.0

Pipe angle(theta)1.193°

Run length used(L)6.096m

Pipe profile run fall 0.25 in/ft 2.08% slope, 1:48.0 ratio
Method notes 3 notes
  • Geometry: fall = run x slope percent / 100, and slope percent = fall / run x 100.
  • Useful conversions: 1/8 in/ft = 1.04%, 1/4 in/ft = 2.08%, and 1/2 in/ft = 4.17%.
  • This is a layout calculator only. Final drainage and plumbing work still needs adopted code, local amendments, fixture loads, pipe material, cleanouts, venting, traps, site elevations and inspection requirements.

Pipe slope is the fall divided by the horizontal run. This calculator converts between pipe fall, end invert elevation, percent slope, inches per foot, 1:n pitch ratio and angle, using fall = run x slope/100 and slope = fall/run x 100. It is a geometry and layout screen only; adopted plumbing code, pipe size, fixture loads, venting and site conditions still control final drainage work.

Continue workflow

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How to use this calculator

  1. Choose the mode. Use slope mode for a known target pitch, or invert mode when both elevations are measured.
  2. Enter the run length. Use the horizontal run between the two points.
  3. Enter slope or elevations. Use percent slope in slope mode, or start and end invert elevations in invert mode.
  4. Read fall and pitch. Review fall, end invert, percent slope, inches per foot, 1:n ratio and angle.

How it works

Pipe slope is simple geometry: fall = run x slope / 100 slope % = fall / run x 100 The end invert is the start invert minus the fall: E2 = E1 - fall

The calculator also reports the same pitch as inches per foot and as a 1:n ratio. Common layout conversions are 1/8 in/ft = 1.04%, 1/4 in/ft = 2.08% and 1/2 in/ft = 4.17%.

For pressure and pump checks on water-like systems, use the pipe flow pressure drop calculator, pipe size by flow velocity calculator and pump NPSH calculator.

Worked example

Verified against the live calculator

A 20 ft drain run at 1/4 in/ft pitch has 5 in of fall. If the start invert is 100.00 ft, the end invert is 99.583 ft. The same pitch is 2.083%, or a 1:48 ratio.

Frequently asked questions

How do you calculate pipe slope?

Pipe slope is fall divided by horizontal run. As a percent, slope = fall / run x 100. As inches per foot, slope = fall inches / run feet.

How much fall is 1/4 inch per foot?

One quarter inch per foot is 0.25 in/ft, which is 2.083% slope. Over 20 ft, that gives 5 inches of fall.

What is an invert elevation?

Invert elevation is the elevation of the inside bottom of the pipe. This calculator can solve an end invert from a known start invert and slope, or back-solve slope from two invert elevations.

Does this tell me the required plumbing code slope?

No. It performs the geometry conversion only. Required slope depends on adopted code, pipe size, fixture load, pipe material, use, inspection requirements and local amendments.

Method & assumptions

  • Run length is horizontal layout distance, not the sloped pipe centerline length.
  • Positive fall means the end invert is lower than the start invert.
  • The result is geometry only, not a drainage-code, fixture-unit, venting or inspection decision.
  • Final plumbing and site work still needs adopted code, local amendments, pipe material, cleanouts, traps, venting, supports, bedding, frost/site conditions and measured field elevations.
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