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Plumbing & Piping

Drain Pipe Slope Calculator — Fall per Foot

Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.

Compute drain grade from horizontal run and vertical drop. Planning estimates only — verify measurements and local codes on site.

Inputs

1200
0.524

Smart analysis

Recommendations

Tailored to your live inputs and result—guidance only, not professional advice.

  • Interpret

    Treat this as a planning estimate

    Primary result: Slope = 2.08%. Re-run with optimistic and pessimistic inputs to understand the range—not just a single point.

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Plain-English guide

Terms & how it works

Confused by a field? Read the short definitions here while you use the tool.

  • Horizontal run (ft)

    The “Horizontal run (ft)” value used in this tool’s formula. Adjust it to see results update live.

  • Vertical drop (in)

    The “Vertical drop (in)” value used in this tool’s formula. Adjust it to see results update live.

How this calculator works

Compute drain grade from horizontal run and vertical drop. Planning estimates only — verify measurements and local codes on site. Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run. Example: 20 ft run with 5 in drop → 2.08% slope, 0.25 in/ft fall.

Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.

Example: 20 ft run with 5 in drop → 2.08% slope, 0.25 in/ft fall.

When to use this Drain Slope

Common use cases

  • Compute drain grade from horizontal run and vertical drop.
  • Sanity-check with this case: 20 ft run with 5 in drop → 2.08% slope, 0.25 in/ft fall.
  • The relationship is: Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.

Localized examples

  • Plumbing & Piping: the example on this page uses the same formula as the widget.

What you get

  • Fields: Horizontal run (ft), Vertical drop (in)
  • Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.
  • Example on the page: 20 ft run with 5 in drop → 2.08% slope, 0.25 in/ft fall.

How to Calculate Drain Slope Step-by-Step

  1. 1Enter Horizontal run (ft), Vertical drop (in). Use the values you have, not a catalog average, unless that is all you know.
  2. 2Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.
  3. 3Worked case: 20 ft run with 5 in drop → 2.08% slope, 0.25 in/ft fall.
  4. 4Buy materials after you walk the job. This is a first-pass quantity, not a purchase order.

Governing formula

Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.

20 ft run with 5 in drop → 2.08% slope, 0.25 in/ft fall.

Example: set Horizontal run (ft) = 20, Vertical drop (in) = 5, then read the result.

Variable definitions

  • runLengthFt

    Horizontal run (ft)

    The “Horizontal run (ft)” field. Default 20; typical range 1–200.

  • dropIn

    Vertical drop (in)

    The “Vertical drop (in)” field. Default 5; typical range 0.5–24.

  • The takeoff (Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.) ignores waste, code, and what you find when you open the wall. Measure twice on site.
  • Nothing here is uploaded for the calculation itself.

Frequently Asked Questions

Slope % = (drop ÷ run) × 100; fall per foot = drop ÷ run.

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Reviews & Ratings

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4.8

out of 5.0

4 reviews

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Your rating

Aisha K.

Mar 12, 2026

Clean layout and the results update instantly. Exactly what I needed for a quick planning check.

Daniel R.

Feb 28, 2026

Very usable on mobile. Would love a save/export option later, but the math feels solid.

Priya S.

Feb 3, 2026

The FAQ section answered my questions before I even had to search. Smooth dark mode too.

Marcus L.

Jan 19, 2026

Simple inputs, clear outputs. This replaced three bookmarks I used to juggle.

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