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Designing a sewer network? Build the longitudinal profile and a CAD-ready DXF.
Paste your coordinates — Altivo computes slopes and elevations, and the DXF export drops straight into AutoCAD. No install, no dongle, in the browser.
Professional calculator for determining the slope of sewer pipes based on terrain elevations and section length. The tool operates in two modes: calculating slope from given elevations (H1, H2, L) or determining the end elevation from a given slope and length. An additional module calculates the elevation at any intermediate point along the sewer route. Designed for sewer network designers, surveyors and construction managers.
Enter the starting elevation H1 [m above sea level] and ending elevation H2 [m above sea level] of the sewer, along with the horizontal section length L [m]. The calculator will automatically compute the slope in percent [%] and per mille [‰].
Alternatively: enter the starting elevation H1, the required slope [%] and length L — the calculator will determine the ending elevation H2. Useful when designing a sewer with a predetermined minimum slope.
Use the intermediate point module: enter the distance from the start of the sewer, and the calculator will compute the pipe invert elevation at that point. You can use the slider for smooth distance adjustment.
The sewer slope is calculated using the formula: S = (H1 − H2) / L × 100%, where H1 is the starting elevation [m above sea level], H2 is the ending elevation [m above sea level], and L is the horizontal section length [m]. The result is expressed in percent [%] or per mille [‰] (1% = 10‰).
Minimum slopes for gravity sewers follow the 1/DN rule — the minimum slope is the inverse of the pipe diameter. For DN 200 it is 0.5% (5‰), for DN 300 — 0.33% (3.3‰), for DN 400 — 0.25% (2.5‰). Slopes below these values risk sediment accumulation and pipe blockage.
Maximum slopes are limited due to pipe erosion and the risk of sewage phase separation. For PVC/PP pipes, the maximum slope is 15% (DN ≤ 200) to 10% (DN > 200). For vitrified clay pipes — 10% and 8% respectively.
Proper slope should ensure self-cleaning velocity: min. 0.7 m/s for sanitary sewers and min. 0.5 m/s for storm sewers. At the same time, velocity should not exceed 5 m/s (sanitary) or 7 m/s (storm) — excessive velocity causes pipe material erosion.
Practical advice when designing sewer slopes:
The minimum slope of a gravity sewer follows the 1/DN rule — the minimum gradient equals the reciprocal of the nominal diameter in millimetres. For DN 160 that gives 0.625% (6.25‰), for DN 200 — 0.5% (5‰), for DN 300 — 0.33% (3.3‰), for DN 400 — 0.25% (2.5‰). The overriding criterion is hydraulic: the slope must ensure a self-cleansing velocity of at least 0.7 m/s in sanitary sewers and 0.5 m/s in storm sewers.
For a DN 160 house connection the minimum slope is 1.5% (15‰), with 2% (20‰) as the typical design value. It is higher than the 1/DN rule alone would suggest because a house connection carries small, intermittent flows — at a lower gradient the sewage never reaches self-cleansing velocity and the run silts up. For DN 110 (branches and single-family connections) use 2–3%.
1% = 10‰, so divide per mille by 10 to get percent: 5‰ = 0.5%, 15‰ = 1.5%, 25‰ = 2.5%. Multiply by 10 for the reverse. In practice 1% means 1 cm of fall per metre of pipe, and 1‰ means 1 mm per metre. The calculator shows both notations at once, so no manual conversion is needed.
Use S = (H1 − H2) / L × 100%, where H1 is the invert level at the start of the run [m a.s.l.], H2 the invert level at the end [m a.s.l.], and L the horizontal length of the run [m]. Example: H1 = 124.50 m, H2 = 124.20 m, L = 60 m gives 0.30 / 60 × 100% = 0.5% (5‰). Note that the formula uses pipe invert levels, not ground levels or manhole cover levels.
The maximum slope is limited by pipe wall erosion and separation of the sewage phases. For PVC and PP pipes the accepted limits are 15% for DN ≤ 200 and 10% for DN > 200; for vitrified clay, 10% and 8% respectively. The upper hydraulic criterion is velocity: no more than 5 m/s in sanitary sewers and 7 m/s in storm sewers. Where the terrain drops sharply, use drop manholes instead of forcing the gradient.
Storm sewers allow lower minimum slopes because the required self-cleansing velocity is lower (0.5 m/s instead of 0.7 m/s) and flows are periodically very high. For DN 300 a storm sewer needs at least 0.2% (2‰) against 0.33% (3.3‰) for a sanitary sewer. In exchange, storm sewers tolerate higher maximum velocities — up to 7 m/s.
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