Interactive Calculator · 2026
Sewer Flow Velocity Calculator
Check whether a sewer or drain runs fast enough to stay clean. Enter the internal diameter, the slope (1:X) and Manning's roughness — get the full-bore velocity in m/s, whether it clears the self-cleansing threshold and the pipe capacity in litres per second.
Full-bore velocity vs the ~0.6 m/s self-cleansing threshold
Your pipe run
Bore of the drain or sewer — common gravity lines are 100, 150, 200 or 300 mm.
One unit of fall for every 100 units of length. A flatter run (larger X) means slower flow.
Pipe roughness — ~0.010 for smooth uPVC/HDPE, ~0.013 for stoneware and new concrete, higher when aged.
Velocity vs the self-cleansing threshold
A gravity sewer needs to run fast enough to keep solids in suspension so it scours itself clean.
At 1:100 this line flows full-bore at 0.86 m/s, which is above the ~0.6 m/s self-cleansing minimum — solids stay moving. If it is too slow, steepen the fall (smaller X) or drop the pipe size.
Self-cleansing is normally assessed at design (part-full) flow, not just full bore — treat this as an indicative full-bore check.
How this is calculated
- Hydraulic radius (full bore) = diameter ÷ 4 = 150 mm ÷ 4 = 0.04 m.
- Slope = 1 ÷ X = 1 ÷ 100 = 0.0100.
- Velocity (Manning) = (1 ÷ n) × R^(2/3) × √S = (1 ÷ 0.013) × 0.11 × 0.1 = 0.86 m/s.
- Capacity = velocity × area × 1000 = 0.86 × 0.02 m² × 1000 = 15.23 L/s.
Indicative check for concept planning. Self-cleansing velocity is usually assessed at design (part-full) flow, and some codes target ~0.75–0.9 m/s. Confirm against NBC 2016 Part 9 / CPHEEO and a qualified consultant before construction.
Frequently asked questions
- How does the sewer flow velocity calculator work?
- It uses Manning's equation to find the velocity a pipe reaches when flowing full. Velocity equals one divided by the roughness n, times the hydraulic radius raised to the two-thirds power, times the square root of the slope. For a full circular pipe the hydraulic radius is the diameter divided by four, and the slope is one divided by X for a 1:X fall. It then multiplies velocity by the pipe area to give capacity in litres per second.
- What inputs do I need and what are sensible values?
- You need the internal diameter in millimetres, the slope as 1:X, and Manning's roughness n. Common gravity lines are 100, 150, 200 or 300 mm. For roughness, use about 0.010 for smooth uPVC or HDPE, and roughly 0.013 for stoneware or new concrete, with higher values as pipes age and pick up deposits. Steeper falls, meaning a smaller X, give faster flow.
- What is a safe self-cleansing velocity for a sewer?
- A gravity sewer must run fast enough to keep grit and solids in suspension so it scours itself clean. A common indicative minimum is around 0.6 m/s, and many codes target roughly 0.75 to 0.9 m/s at design flow. If flow is too slow, solids settle and the line silts up and blocks. This tool checks full-bore velocity, so treat it as an early planning guide only.
Related Guides — Deep-dive reading
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