Drainage Flow
Calculator
Calculate pipe flow capacity and select the right underground drainage pipe size. Based on the Colebrook-White equation for PVC-U pipes per Approved Document H.
How to use
- 1 Select Pipe Size: Choose between 110mm (standard domestic) or 160mm (high capacity).
- 2 Set Gradient: Choose a standard gradient or enter a custom value. Steeper = more capacity.
- 3 Select Type: Foul, surface water, or combined — each has different minimum gradient requirements.
Building Regs Reference
- Foul drainage (110mm): Min 1:80 with 1 WC, 1:40 otherwise
- Surface water: Min gradient 1:100
- Self-cleansing velocity: Min 0.75 m/s to prevent silting
- Reference: Approved Document H, Building Regulations
*Calculations based on Colebrook-White equation for PVC-U pipes (roughness kb = 0.03mm, Manning's n = 0.011). Always verify with a drainage engineer for complex layouts.
Pipe Flow Calculator
Configure your drainage parameters below.
Optional — for sizing recommendation.
Flow Capacity
Flow Velocity
Recommended Pipe
110mm PVC-U is suitable
Adequate capacity and velocity for this configuration.
Gradient Warning
The selected gradient may not meet minimum requirements for this drainage type.
Drainage Installation Tips
Getting the Gradient Right
Use a spirit level and straight edge or laser level. For a 1:40 fall, that's 25mm drop per metre of pipe run. Mark the invert levels at each end before digging.
Pipe Bedding
Lay pipes on a 100mm bed of 10mm pea gravel. Surround with gravel to 150mm above the pipe crown. This protects the pipe and maintains gradient.
Access Points
Install inspection chambers at every change of direction, gradient, or pipe size. Maximum distance between access points is 45m for 110mm pipe.
Building Control
Drainage work connecting to public sewers needs Building Control approval. Contact your local authority before starting work. A Section 106 agreement may be required.
Kalsi Underground Drainage Products
We supply a full range of PVC-U underground drainage pipes, fittings, and inspection chambers in both 110mm and 160mm. All manufactured to BS EN 1401 standards.