Drainage Pipe Slope Calculator
Standard slope by diameter, fall over the run and velocity check.
Quick answer
With the default inputs: Nominal diameter DN110 · Pipe length 20 · Actual slope 2.0%Fall over the run 0.400 m
Edit the inputs below to recalculate.Inputs
Result
Fall over the run
0.400m
| Diameter | 110mm |
| Actual slope | 2.00% |
| Standard slope | 2.0% |
| Slope check | Adequate |
| Fall per metre | 20.0mm |
| Full-bore velocity | 0.99m/s |
| Self-cleansing | Adequate |
| Level difference | 0.400m |
| Fall at standard slope | 0.400m |
Velocity uses Manning, v = (1/n)·R^(2/3)·i^(1/2), with n = 0.013 for plastic and full-bore flow. Self-cleansing needs at least 0.6 m/s or solids settle; above 5 m/s the pipe erodes. Steeper is not better — fast water outruns the solids and leaves them behind, which is especially marked on branch runs.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| Fall | length × slope |
| Slope | fall ÷ length as a decimal |
| Velocity | (1/n) × R^(2/3) × i^(1/2) |
| Hydraulic radius | d ÷ 4 for a full circular pipe |
| Self-cleansing | v ≥ 0.6 m/s |
| Standard slopes | 2% at DN110, 1% at DN160 |
DN110 over 20 m at 2%: a 0.400 m fall, 20 mm per metre and 0.99 m/s full-bore — self-cleansing. Drop to 1% and velocity falls to 0.70 m/s, where solids settle over time.
Standard and minimum drainage slopes
| DN | Standard slope | Minimum slope | Fall per 10 m at standard | Fall per 10 m at minimum |
|---|---|---|---|---|
| DN50 | 3.5% | 2.5% | 350 | 250 |
| DN75 | 2.5% | 1.5% | 250 | 150 |
| DN110 | 2.0% | 1.2% | 200 | 120 |
| DN160 | 1.0% | 0.7% | 100 | 70 |
| DN200 | 0.8% | 0.5% | 80 | 50 |
| DN315 | 0.5% | 0.3% | 50 | 30 |
How to work it out
- Look up the standard slope for the bore (GB 50015-2019): 3.5% at DN50, 2.5% at DN75, 2.0% at DN110, 1.0% at DN160, 0.8% at DN200, 0.5% at DN315. Larger pipes tolerate flatter falls.
- Work out the fall h = length × slope. A 20 m DN110 run at 2% needs 0.4 m, which has to be found in the ceiling void before installation, not discovered during it.
- Check velocity: v = (1/n) × R^(2/3) × i^(1/2), with n about 0.009 for plastic and hydraulic radius R = d÷4 for a full pipe.
- Self-cleansing requires v ≥ 0.6 m/s, below which solids settle and block. Excess velocity scours the wall and is noisy, so there is an upper limit too.
- If the fall is unavailable there are two routes: raise the start point (costing ceiling or floor level) or use a larger bore, which permits a flatter slope. Changing the pipe size is usually cheaper than changing levels.
Where it goes wrong
- Assuming steeper is always better. On an over-steep run the liquid outruns the solids, which are left behind to dry and set, making blockages more likely. The standard slope is both a minimum and a sensible working value.
- Confusing the floor screed fall with the pipe fall. The 1-1.5% on a bathroom floor is for the tiles; the drainage pipe takes its own slope by diameter. They are different things.
- Leaving long horizontal runs without a rodding eye. DN110 runs beyond roughly 15 m need a cleaning eye at the head end, otherwise a blockage means breaking tiles.
- Forgetting that horizontal drainage is designed as partially full. Fill depth is capped (0.5 for DN110), so the full-bore velocity here is only a screening check; formal hydraulic design uses partial flow.
Material and cost
PVC-U drainage pipe runs 12-20 CNY/m at DN110 and 25-40 at DN160, fittings extra. Insufficient fall means raising the whole bathroom floor or lowering the ceiling, which costs many times the price of the pipe — the most expensive mistake in drainage.
On site
- Use two 45° bends instead of one 90° at the base of a stack. It noticeably reduces impact noise and the hydraulic jump height.
- Same-floor drainage (recessed slab or wall-hung) is where fall is hardest to guarantee. Wall-hung outlets need a purpose-made trap, not a gravity-bent pipe, or the seal will not hold.
- Trap seals must be at least 50 mm deep. Floor drains in unoccupied property need periodic priming, otherwise the seal evaporates and drain smells back — the commonest complaint in empty buildings.
- Never omit the vent. An unvented stack siphons the traps on the horizontal branches, producing gurgling and lost seals.
Basis
- GB 50015-2019 Standard for design of building water supply and drainage, drainage and hydraulic calculation
- GB 50242-2002 Code for acceptance of construction quality of building water supply, drainage and heating works, indoor drainage
- CJJ/T 29-2010 Technical specification for building drainage plastic piping engineering
FAQ
Why is a steeper drain worse?
Because fast water outruns the solids. On a steep run the flow is swift but shallow, so solids roll along the invert and stall wherever the gradient eases or a joint interrupts them, building a blockage. Codes cap branch slopes at about 15%, beyond which you use a backdrop or a larger pipe. It is the opposite of supply pipe logic, which catches people out.
What matters with floor drains and traps?
Use a trapped gully with a seal at least 50 mm deep, or sewer gas comes back — the commonest cause of bathroom smells. Traps need the same 50 mm seal, and rarely used drains must be topped up. Branch runs beyond about 4-6 m need a rodding eye, or a blockage cannot be cleared.
Is rainwater pipe sloped like drainage?
No. Rainwater carries high flow and grit but little organic matter, so slopes can be gentler — 1% at DN100 — while the upper velocity limit is stricter, 5 m/s, to prevent erosion. The base of a roof downpipe needs an energy dissipator, since water falling ten metres or more hits hard. Storm and foul must be kept separate; cross-connection fails environmental inspection.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Drainage Pipe Slope Calculator. 2026-10-25. https://jianzhujisuan.com/en/drain-slope
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.