Interactive Calculator · 2026
Drain Pipe Capacity Calculator
Find the full-bore gravity flow of a drain. Enter the internal diameter, the fall as a 1:X slope and Manning's roughness — get the full-bore capacity in litres/second, the flow velocity and the capacity in m³/hr.
Full-bore capacity of this pipe as the fall steepens
Your drain run
Bore of the pipe — e.g. 110 mm soil, 75 mm waste, 160 mm external drain.
One unit of fall per X units of run. A gentler drain (larger X) carries less. Typical soil stacks fall around 1:40–1:80.
Pipe wall roughness — ~0.009–0.011 for smooth plastic (uPVC/HDPE), ~0.013 for concrete/salt-glazed, and higher for old or scaled pipe.
Capacity across a range of falls
Same Ø110 mm pipe and roughness n = 0.011 — steeper falls carry more.
These are full-bore figures — the flow if the pipe ran completely full. Real gravity drains are designed to run part-full (typically about half to three-quarters), so the usable capacity is less than shown, while a self-cleansing velocity of roughly 0.6–1.0 m/s is kept to stop solids settling.
Confirm the roughness n and the design fill against NBC 2016 Part 9 / CPHEEO before you rely on it.
How this is calculated
- Manning's equation for a full circular pipe: V = (1 / n) × R^(2/3) × √S, then Q = V × A.
- Area A = π × d² / 4 = π × 0.110² / 4 = 0.0095 m². Hydraulic radius R = d / 4 = 0.0275 m (full bore). Slope S = 1 / 60 = 0.0167.
- Velocity V = (1 / 0.011) × 0.0275^(2/3) × √0.0167 = 1.07 m/s.
- Capacity Q = V × A = 1.07 × 0.0095 = 0.0102 m³/s = 10.16 L/s = 36.58 m³/hr.
Indicative sizing for concept planning — this is the theoretical full-bore figure. Real drains run part-full, so allow a safe fill factor and self-cleansing velocity, and confirm with a qualified consultant and NBC 2016 Part 9 / CPHEEO before procurement.
Frequently asked questions
- How does the drainage pipe capacity calculator work?
- It uses Manning's equation for a circular pipe running full: velocity equals one divided by the roughness n, times the hydraulic radius to the power two-thirds, times the square root of the slope. For a full bore the hydraulic radius is the diameter divided by four. Capacity is then that velocity multiplied by the cross-sectional area, reported in litres per second, metres per second and cubic metres per hour.
- What inputs do I need and what values should I use?
- You enter three things. Internal diameter in millimetres, such as 110 for a soil pipe, 75 for waste or 160 for an external drain. The fall as a slope of 1 in X, where soil runs are often around 1:40 to 1:80. And Manning's roughness n, roughly 0.009 to 0.011 for smooth plastic like uPVC or HDPE, about 0.013 for concrete or salt-glazed, and higher for old or scaled pipe.
- Why is real drain capacity less than the full-bore figure shown?
- The result is the theoretical flow if the pipe ran completely full. Gravity drains are designed to run part-full, typically about half to three-quarters, so usable capacity is lower than the full-bore number. You also keep a self-cleansing velocity of roughly 0.6 to 1.0 metres per second so solids do not settle. Treat this as indicative sizing and confirm the design fill and roughness against NBC 2016 Part 9 or CPHEEO with a qualified consultant.
Related Guides — Deep-dive reading
Drainage Systems Guide for Indian Homes: DWV, Traps, Slopes & Stacks
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.
PlumbingConcrete (RCC) Pipes in India: Specifying NP2, NP3 and NP4 for Drainage and Stormwater
A professional's guide to reinforced cement concrete pipe under IS 458 — the NP2, NP3 and NP4 classes, spigot-and-socket rubber-gasket versus collar joints, large-diameter gravity drainage, bedding and strength, indicative rates, and where RCC still beats HDPE and DI for buried drains, culverts and external sewers.
PlumbingExternal 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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