Calculate the correct slope and fall for drainage pipes. Enter pipe length and gradient ratio to get the required drop, or calculate gradient from known fall.
Calculate plot, lawn, and banked garden areas from dimensions
100mm foul drain: 1:40 minimum (1.5°). 150mm foul drain: 1:60 minimum. 225mm sewer: 1:90 minimum. Surface water 100mm: 1:80 minimum. Always check local building control and water authority requirements.
Drainage pipes need a minimum flow velocity of 0.75 m/s to prevent solids settling. At 1:40 on a 100mm pipe, flow velocity is approximately 1.0 m/s. Too steep (over 1:10) causes liquids to run away from solids.
Clay and PVC pipes have different friction coefficients. PVC is smoother and can achieve self-cleansing velocity at slightly shallower gradients. Always use the manufacturer's flow data for critical drainage design.
Building regulations require access points (rodding eyes, inspection chambers) at every change of direction, every 15m on straight runs for 100mm pipes, and every 30m for 150mm+ pipes. Plan access points before setting gradients.
fall = pipe_length × (1 / ratio)
ratio = 1 : (pipe_length / fall)
slope% = (fall / pipe_length) × 100
angle = arctan(fall / pipe_length)
Drainage design for new builds, connections to public sewers, and any drainage works within 3m of a building require building control approval. For complex drainage systems, shared drains, or sites with high water tables, consult a drainage engineer.
UK Building Regulations: 1:40 (2.5%) for 100mm foul drains, 1:80 (1.25%) for 100mm surface water. Check local water authority for specific requirements — some require steeper gradients.
Fall = 15 × (1/40) = 0.375m = 375mm. The inlet needs to be 375mm higher than the outlet. At 1:80 (surface water): 15 × (1/80) = 187.5mm fall.
If the gradient exceeds about 1:10 (10%), liquids flow too fast and leave solids behind, causing blockages. The ideal range is 1:40 to 1:80 for most domestic drains. For steep sites, use drop chambers to maintain proper gradient between sections.
Yes, but sewer connections must comply with local water authority specifications. Typical lateral gradients are 1:40 to 1:80 for 100-150mm pipes. Always get water company approval before connecting to a public sewer.