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Drainage Gradient Calculator — Fall, Invert Levels & Compliance
Check or calculate drainage gradients for foul and surface water pipes against the Building Regulations Part H minimum falls, with a too-steep trade-practice check.
Horizontal distance of pipe run
Invert level at start (upstream). Datum-relative — may be negative below Ordnance Datum or on a local benchmark
Invert level at end (downstream). Datum-relative — may be negative below Ordnance Datum or on a local benchmark
Number of level check points
Ground level at start for depth check
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How We Calculate This
This calculator checks drainage gradients against Part H minimum requirements or calculates the required end invert level for a compliant gradient.
The formula
Gradient = (Start Invert - End Invert) / Pipe Length
Fall per metre (mm/m) = Gradient × 1000
AD H 2015 minimum (flattest) gradients
Foul figures are from Table 6 (“Recommended minimum gradients for foul drains”); surface-water figures are from AD H Section H3, paragraph 3.15.
- 100mm foul (Table 6): 1:80 (12.5mm/m) with ≥1 WC; 1:40 only if peak flow <1 l/s
- 150mm foul (Table 6): 1:150 (6.7mm/m) with ≥5 WCs
- 75/100mm surface (para 3.15): 1:100 (10mm/m)
- 150mm surface (para 3.15): 1:150 (6.7mm/m)
Frequently Asked Questions
Per Approved Document H 2015 Table 6, the flattest permitted (minimum) gradient for a 100mm foul drain is 1:80 where peak flow exceeds 1 l/s with at least one WC connected (12.5mm fall per metre). A steeper 1:40 applies only where peak flow is below 1 l/s. Use our Foul Drainage Calculator for a full drainage design.
Surface water drains should have a minimum gradient of 1:100 for 100mm pipe and 1:150 for 150mm pipe. Unlike foul drains, surface water contains no solids so steeper gradients are less critical. As a design rule of thumb the pipe should still reach a self-cleansing velocity of about 0.7 m/s — the velocity that underpins the AD H / BS EN 752 minimum gradients. See our Surface Water Drainage Calculator for pipe sizing.
Approved Document H sets no maximum gradient — Table 6 lists only the flattest (minimum) permitted falls. There is, however, a long-standing trade concern: on a very steep foul drain the liquid can run faster than the solids and leave them stranded, which then builds into a blockage. The commonly cited onset of this is around 1:20 (50mm per metre). Note that 1:40 — Maguire’s Rule — is actually the recommended self-cleansing target, not a problem gradient. Where the ground forces a fall steeper than about 1:20, use a backdrop manhole or rest-bend to take the level down in a vertical step rather than a steep run. This is industry good practice, not a Building Regulations limit.
The invert level is the internal bottom of the pipe, measured from a datum (usually Ordnance Datum). To find the downstream invert: Downstream IL = Upstream IL - (Gradient × Distance). For example, 1:40 gradient over 10m means 0.25m fall, so if start IL is 99.50, end IL is 99.25.
The values in Approved Document H Table 6 are the flattest permitted. A 150mm foul drain may be laid as flat as 1:150 where at least 5 WCs are connected (sufficient flow for self-cleansing); a 100mm foul drain as flat as 1:80 with at least 1 WC. Going flatter than the Table 6 minimum requires building-control agreement based on a flow calculation.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.