Drop formula?
run (ft) × slope (in/ft).
Use Mode A (slope → fall) when you know the required slope and need total vertical drop over a run. Use Mode B (fall → slope) when you measured available elevation change and need the resulting slope.
Enter slope in in/ft, % grade, 1:X ratio (horizontal run per 1" rise), or mm/m. The calculator converts between all formats.
An unofficial 1/4"/ft planning reference for many ≤3" drains is shown separately — it is not a permit or code approval.
Concise answers for common searches — definitions, steps, and comparisons.
run (ft) × slope (in/ft).
About 2.08% grade (1:48 ratio, ~20.8 mm/m).
Drop = run × in/ft slope. Percent = (in/ft ÷ 12) × 100. Ratio 1:X where X = 12/inPerFt. mm/m = in/ft × 25.4 ÷ 0.3048.
Formula
totalDropIn = runFt × slopeInPerFt; slopeInPerFt = totalDropIn ÷ runFt; percentGrade = (slopeInPerFt ÷ 12) × 100; mmPerM = slopeInPerFt × 25.4 ÷ 0.3048.Slope → fall when you know required slope; fall → slope when you know available drop.
Type the drain run length in feet (typical range about 1–500 ft).
Mode A: enter slope in in/ft, %, ratio, or mm/m. Mode B: enter total vertical drop in inches.
Use slope, fall, percent, ratio, and mm/m rows. Check planning reference separately from calculations.
Input
run 50 · slope 0.25 in/ftOutput
12.5" drop · 2.08% · 1:48 · 20.83 mm/m50×0.25 = 12.5; (0.25/12)×100 ≈ 2.08%.
Input
run 40 · fall 10 in (fall→slope mode)Output
0.25 in/ft · 2.08% grade10 ÷ 40 = 0.25 in/ft.
Common real-world scenarios where this tool saves time.
50 × 0.25 = 12.5" total drop (~2.08% grade, ~20.8 mm/m).
10 ÷ 40 = 0.25 in/ft (1/4"/ft, ~2.08% grade).
Converts to 0.25 in/ft and shows ratio 1:48.
Step-by-step chains that connect related tools for common tasks.
Estimate drain diameter, then calculate required fall over the run.
| Slope | Total drop | mm/m |
|---|---|---|
| 1/8" / ft | 6.25" | 10.42 |
| 1/4" / ft | 12.5" | 20.83 |
| 1/2" / ft | 25" | 41.67 |
Use developed horizontal length including offsets, not straight-line room distance.
In fall→slope mode, confirm the computed slope fits between start and outlet.
The 1/4"/ft note is unofficial guidance. Local code and inspection control compliance.
Slope sets fall. Diameter comes from DFU/fixture design (Pipe Sizing Calculator).
1:X here means X inches of horizontal run per 1 inch of rise — 1:48 = 1/4"/ft.
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Multiply horizontal run (ft) by slope (inches per foot).
Divide total vertical drop (inches) by horizontal run (feet) to get slope in in/ft.
in/ft, % grade, 1:X ratio (horizontal run per 1" rise), or mm/m.
percentGrade = (inches per foot ÷ 12) × 100.
If slope is at least 0.25 in/ft, the tool notes a common 1/4"/ft planning reference for many ≤3" pipes. It is not a formal code approval.
No. It only computes fall and slope conversions.
Drain slope inputs remain local — EverydayTools does not collect drain-slope calculator entries.
Estimates only — not professional plumbing advice or code certification. Verify with a licensed plumber and local code.
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Reviewed on 2026-06-28.
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Inches of fall per foot of horizontal run — e.g. 0.25 for 1/4"/ft