Drainage Fall Calculator — UK Building Regs Gradients

Our drainage fall calculator uses the minimum gradients specified in UK Building Regulations Approved Document H to calculate the required fall for your drainage pipe run.

The formula

Fall per metre (mm) = 1000 ÷ Gradient ratio

Total fall (mm) = Pipe length (m) × Fall per metre (mm)

Foul-drain minimum gradients (AD H Table 6)

Approved Document H, Section 2 (H1 Foul drainage), Table 6 (“Recommended minimum gradients for foul drains”, para 2.34) lists these foul rows, keyed to pipe size and peak flow:

• 100/110mm foul, peak flow <1 l/s: 1:40 minimum (25mm per metre) — maximum capacity 9.2 l/s
• 100/110mm foul, peak flow >1 l/s: 1:80 minimum (12.5mm per metre) — maximum capacity 6.3 l/s, requires at least 1 WC
• 150mm foul, peak flow >1 l/s: 1:150 minimum (6.7mm per metre) — maximum capacity 15.0 l/s, requires at least 5 WCs

For 160mm foul this calculator defaults to 1:60— that is a conservative trade convention, not an Approved Document figure (it is steeper than AD H’s 1:150, so it over-orders fall rather than under-orders). On larger shared systems that meet the ≥5-WC / >1 l/s threshold, use the Custom Gradient field to apply AD H’s 1:150.

Surface-water (rainwater) minimum gradients (AD H para 3.15)

Approved Document H, Section 3 (H3 Rainwater drainage), para 3.15 sets these minimum gradients for buried rainwater drains directly: “75mm and 100mm rainwater drains should be laid at not less than 1:100. 150mm drains and sewers should be laid at gradients not less than 1:150 and 225mm drains should be laid at gradients not less than 1:225.” For pipes larger than 225mm, AD H refers to BS EN 752-4:

• 75/100/110mm surface water: 1:100 minimum (10mm per metre)
• 150/160mm surface water: 1:150 minimum (6.7mm per metre)
• 225mm surface water: 1:225 minimum (4.4mm per metre)

Hydraulic capacity at a given gradient is read from AD H Diagram 3; above-ground rainwater systems (gutters and downpipes) are designed to BS EN 12056-3.

Important notes

• These are minimum falls — steeper gradients within reason are acceptable
• For a 100/110mm foul drain on a single dwelling (peak flow under 1 l/s), 1:40 is the AD H minimum
• Drains must be laid on a firm, even bedding (pea gravel or granular material)
• All drainage work requires Building Regulations approval