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Drainage Fall Calculator — UK Building Regs Gradients
Calculate the correct gradient and fall for drainage pipes based on UK Building Regulations Approved Document H.
Total horizontal length of pipe run
Invert level at start of run, if known
Leave 0 for the default. Enter e.g. 150 for AD H Table 6's 1:150 (150mm foul, ≥5 WCs) or 80 for the 1:80 row (100mm foul, >1 l/s)
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How We Calculate This
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
Frequently Asked Questions
For foul drainage (toilets, sinks, baths) at a peak flow under 1 l/s — the usual single-dwelling case — a 110mm pipe needs a minimum fall of 1:40 (25mm per metre); this is the value in UK Building Regulations Approved Document H, Section 2 (H1 Foul drainage), Table 6 (para 2.34). For surface water (rainwater), the minimum fall for a 75mm or 100mm rainwater drain is 1:100 (10mm per metre) — that figure comes directly from AD H, Section 3 (H3 Rainwater drainage), para 3.15, which also sets 1:150 for 150mm drains and 1:225 for 225mm drains (pipes larger than 225mm follow BS EN 752-4). This drainage gradient calculator works out the total fall for your pipe run length.
A fall (also called gradient, slope or drop) of 1:40 means the pipe drops 1 metre for every 40 metres of horizontal run, or equivalently 25mm per metre. This is the standard minimum gradient for 110mm foul drainage pipes. A steeper gradient (e.g. 1:20) is fine — it is the minimum that must be met.
Approved Document H sets no maximum gradient for short domestic foul runs, so there is no regulatory upper limit to meet. As a design rule of thumb, though, a very steep run can let the liquid outrun the solids — drainage design aims for a self-cleansing velocity of about 0.75 m/s (so solids stay in suspension) up to roughly 3 m/s (above which the flow can leave solids behind and erode the pipe), the velocity basis used in BS EN 752. In practice most groundworkers keep foul drains within a few degrees of the AD H minimum (around 1:40 to 1:80 for 110mm) rather than laying them excessively steep. This is best practice, not a Building Regulations requirement.
Foul drainage must connect to the public foul sewer. Surface water can discharge to a soakaway — use our Soakaway Sizing Calculator to size one — or to a surface water sewer. A French drain can handle localised ground water — see our French Drain Calculator. Never connect surface water to the foul sewer.
Yes — any new drainage work or modification to existing drainage requires Building Regulations approval under Part H. This includes new connections to the public sewer, changes to pipe runs, and installation of new soakaways. Contact your local council Building Control department before starting work. Ensure patios and paved areas also drain correctly — see our Paving Slab Calculator.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.