Pipe slope calculator
Run length and slope: you get the total drop, slope per metre and angle.
Total drop
3.937 in
- Total drop3.937 in
- Slope per metre20 mm/m
- Angle1.15 °
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How it's calculated (visible formula)
drop = length × slope; ¼ in per ft ≈ 2%
- Net geometric measurement, no waste.
- Adjustable waste % for cuts and breakage.
- Purchase rounded up to sellable units (boxes, bags, buckets, boards).
How to calculate it, step by step
- Measure the horizontal run length.
- Pick slope for the drain type (⅛–¼ in per ft).
- Drop = length × slope.
- Check the outlet stays above the main.
- Lay out with a laser or level and string.
Worked example: 16 ft drain at ¼ in per foot (2%)
- 16 × ¼ in = 4 in
- 2% = 20 mm/m
→ 4 in (10 cm) of total fall.
Common mistakes
- Less than ⅛ in per foot — water ponds.
- Too steep on building drains — water outruns solids.
- Forgetting the fall when planning a dropped ceiling.
Pro tips
- Hang pipe often enough that it doesn't sag (per code spacing).
- Mark the drop on the wall before fixing the first hanger.
- Check slope with a level section by section.
- Plan drain fall before setting a dropped-ceiling height.
From calculation to quote
Add labour, overhead and your margin, and generate the professional PDF.
FAQs
- What slope for a drain pipe?
- ¼ in per foot for pipes up to 3 in; ⅛ in per foot is allowed for 3–6 in.
- How much is 2% slope?
- About ¼ in per foot (2 cm per metre).
- Can a drain be too steep?
- Over about ½ in per foot, liquids can leave solids behind.
- What slope does a toilet drain need?
- ¼ in per foot on the branch to the stack.
- Can a drain run flat?
- No — water ponds and solids settle.
- How do I check slope with a level?
- Set a 2 ft level on the pipe; ¼ in/ft means the bubble centers with a ½ in shim at one end.
Sources and standards
Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.