Interactive Calculator · 2026
Drain Slope Calculator
Work out the fall a drain pipe needs. Enter the run length, choose a 1:X slope ratio and the pipe diameter — get the fall per metre and the total fall in millimetres and metres so you can set your invert levels.
Total fall over the run at steeper (1:40) → gentler (1:100) slopes
Your drain run
Horizontal run of pipe from start to end.
Nominal bore — sets the suggested slope band.
Lower X = steeper fall (1:40). Higher X = gentler fall (1:200). Steeper isn't always better — too steep lets water outrun the solids, too flat lets the pipe silt up.
Total fall at common slope ratios
How far the drain drops over your 10 m run at each standard gradient.
A 1:40 fall drops the pipe fast (25 mm every metre); 1:100 barely drops it (10 mm every metre). For a 150 mm drain, a gentler fall of about 1:60 to 1:100 is typical.
The self-cleansing velocity — not just the fall — is what keeps a drain clear, so match the slope to the design flow, not the maximum you can build.
How this is calculated
- Fall per metre = 1000 ÷ X = 1000 ÷ 60 = 16.67 mm/m.
- Total fall = length × 1000 ÷ X = 10 × 1000 ÷ 60 = 166.67 mm.
- Total fall (m) = total fall ÷ 1000 = 166.67 ÷ 1000 = 0.17 m.
Indicative sizing for concept planning. Common minimum slopes are around 1:40 for a 100 mm soil pipe and gentler for larger pipes, but the governing figure is the self-cleansing velocity — confirm against NBC 2016 Part 9 and a qualified consultant before construction.
Frequently asked questions
- How does the drain slope calculator work?
- It converts a 1:X slope ratio into a fall. Fall per metre in millimetres is 1000 divided by X, so a 1:60 slope gives about 16.7 mm of drop per metre. Total fall is the drain length times 1000 divided by X, giving the vertical drop over the whole run in millimetres and metres. Use these figures to set your invert levels from the start to the end of the pipe.
- What slope should I use for my drain pipe?
- It depends on the pipe diameter and the flow. For a 100 mm soil or drain pipe a fall of about 1:40 to 1:60 is typical; for a 150 mm drain a gentler 1:60 to 1:100 is common, and larger pipes are usually gentler still. Too steep lets water outrun the solids and too flat lets the pipe silt up, so match the slope to the design flow rather than picking the maximum you can build.
- How accurate is this drain slope calculator?
- The maths is exact for a straight, uniform gradient, but the result is indicative for concept planning only. The governing figure for a real drain is the self-cleansing velocity, not just the fall, and it depends on flow, pipe material and bore. Confirm the minimum gradient and invert levels against NBC 2016 Part 9 and a qualified plumbing or drainage consultant before you build.
Related Guides — Deep-dive reading
External Underground Drainage in India: Layout, Inspection Chambers, Gradients and the Connection to Sewer or STP
The buried gravity network that carries soil and waste from the building to the boundary — how it runs from the stack to the property line, where inspection chambers and manholes sit, the fall each pipe size needs, the materials and bedding that hold it, and how it hands over to a municipal sewer, a septic tank or an on-site treatment plant.
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How a bathroom's waste and soil pipes actually work — the difference between waste lines (basin, shower) and the soil line (WC), the fall you need, pipe sizes in mm, floor slope to the floor trap, how it all joins the stack, and how traps and venting stop blockages and smells.
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The section pillar for household drainage — the two waste streams, gravity and gradients, traps and the water seal, the soil stack and branch layout, why venting matters, keeping foul and rain separate, and how it all connects to sewer, septic or an STP at the plot boundary.
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