Drainage Fall Calculator

Enter pipe size, material, design flow, and drainage type to calculate hydraulic fall or full-flow capacity and screen eligible foul-drain conditions against Approved Document H Table 6.

For educational purposes only. Not a substitute for professional engineering judgment.

Calculator Inputs

Calculation Mode

Choose whether to calculate required gradient or flow capacity

Mode

Drainage Type

Table 6 screening applies to foul drains only; surface-water results are hydraulic

Drainage Type

Table 6 Nominal Pipe Size

Mapped from supported product diameters; confirm the pairing against product data

Nominal size for Part H

Connected WCs

Required because the flatter Table 6 rows are conditional on WC count

Number of connected WCs

Pipe Diameter

Select pipe size in millimetres

Diameter (mm)

Pipe Material & Condition

Select material to determine Manning's roughness coefficient

Material

Manning's n Override (Optional)

Leave blank to use default for selected material

n value

Peak Design Flow Rate

Used for the Manning calculation, capacity check and Table 6 flow band

Peak flow (l/s)

Drainage Fall Overview

Drainage fall (also called gradient or slope) is the vertical drop of a pipe over its horizontal length — what makes a gravity drain flow. Choose a mode above:

Calculate Required Gradient — enter a design flow (l/s) to find the gradient needed to convey it, using the Manning equation S = (n Q / (A R^2/3))².
Calculate Flow Capacity — enter a gradient (mm/m or 1:X ratio) to find the full-bore flow the pipe can carry and the corresponding full-bore capacity velocity.

For foul drains, Table 6 is assessed only when the peak-flow, nominal pipe-size and WC conditions match a published row. Surface-water and unmatched cases remain hydraulic results without a Part H compliance claim.

Approved Document H Table 6 — foul drains

Pipe	Peak flow	Typical minimum gradient	Condition / capacity
75mm	<1 l/s	1:40	4.1 l/s maximum capacity
100mm	<1 l/s	1:40	9.2 l/s maximum capacity
75mm	>1 l/s	1:80	2.8 l/s maximum capacity
100mm	>1 l/s	1:80	Minimum 1 WC; 6.3 l/s at 1:80
150mm	>1 l/s	1:150	Minimum 5 WCs; 15.0 l/s capacity

Exact rows and conditions from Approved Document H Table 6 for foul drains. The 100mm >1 l/s row is 1:80 (not 1:100). Table 6 does not provide a blanket surface-water minimum. Confirm nominal product size and the current document with building control.