Foul Drainage Flow-Rate Calculator
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Foul Drainage Flow-Rate Calculator
Estimate the peak foul flow for a building — from the number of dwellings or by counting appliances (discharge units) — then get the recommended minimum drain size and gradient. Built on UK Building Regulations (Approved Document H, Tables 5 & 6) and BS EN 12056-2.
Peak flow from number of dwellings
The quick method for housing. Uses Approved Document H1 Table 5 — the peak flow from a typical household group (1 WC, 1 bath, 1–2 washbasins, 1 sink, 1 washing machine) — interpolated for your number of dwellings.
Peak flow by discharge units
The appliance-based method (BS EN 12056-2). Count the appliances, pick how the building is used, and the tool returns Qww = K√∑DU, the design flow, and the recommended drain.
Reference tables
The data behind this calculator: Approved Document H Tables 5 & 6, plus the BS EN 12056-2 discharge units and frequency factors.
| Number of dwellings | Peak flow (l/s) |
|---|---|
| 1 | 2.5 |
| 5 | 3.5 |
| 10 | 4.1 |
| 15 | 4.6 |
| 20 | 5.1 |
| 25 | 5.4 |
| 30 | 5.8 |
| Peak flow | Pipe (mm) | Min. gradient | Max. capacity (l/s) |
|---|---|---|---|
| < 1 l/s | 75 | 1:40 | 4.1 |
| < 1 l/s | 100 | 1:40 | 9.2 |
| > 1 l/s | 75 | 1:80 | 2.8 |
| > 1 l/s | 100 | 1:80 * | 6.3 |
| > 1 l/s | 150 | 1:150 † | 15.0 |
* 100 mm at 1:80 needs at least 1 WC connected. † 150 mm at 1:150 needs at least 5 WCs; with fewer, lay at 1:100. A foul drain must be ≥75 mm; a drain carrying a WC or trade effluent must be ≥100 mm (AD H 2.33).
| Appliance | DU (l/s) |
|---|
Frequency factor K: 0.5 intermittent (dwelling/office) · 0.7 frequent (hospital/school/hotel) · 1.0 congested (public toilets) · 1.2 special (laboratory). Qww = K√∑DU; the design flow is never taken as less than the largest single discharge unit connected.
This foul drainage flow rate calculator estimates the peak foul flow for a building from the number of dwellings or by counting appliances (discharge units), then gives the recommended minimum drain size and gradient. It follows Approved Document H (Tables 5 and 6) and BS EN 12056-2. For the underlying guidance, see Approved Document H.
Peak flow from the number of dwellings
The quick method for housing uses Approved Document H Table 5, taking the peak flow from a typical household group and interpolating it for your number of dwellings. Enter the dwellings served by the drain or sewer and the calculator returns the estimated peak foul flow and the recommended drain.
The discharge-unit method
For mixed or non-domestic buildings the appliance-based method (BS EN 12056-2) is more accurate. Count the appliances, choose how the building is used, and the calculator returns the design flow (Qww) and the recommended drain size and gradient.
From flow to drain size and gradient
The peak flow sets the minimum drain diameter and the gradient it must be laid at to stay self-cleansing — the same Approved Document H thresholds used in our fall calculator. Undersizing the drain or laying it too shallow both lead to blockages and, eventually, sewer flooding.
Further reading: foul, surface and combined sewers explained · what is sewer flooding · Approved Document H.
Frequently asked questions
Either interpolate from the number of dwellings (Approved Document H Table 5) or sum the discharge units of the appliances (BS EN 12056-2). The calculator does both.
A 100 mm drain suits most housing; larger buildings or longer runs may need 150 mm. The calculator recommends the size from the peak flow.
A weighting given to each appliance to reflect how much and how often it discharges. Summed and converted, they give the design flow for the drain.
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