Sizes a foul drainage discharge stack and its branches to BS EN 12056-2 (System III). Add each appliance, tag it to a branch, and the calculator checks every branch and the combined stack against
Table 11/12, applying whichever is larger of the calculated flowrate or a single appliance's own discharge rate.
Tender-stage only. Verify final design to project spec and a qualified drainage engineer.
For design guidance only. Always verify with a qualified engineer.
About this foul drainage stack sizing calculator
This free foul drainage stack and branch sizing calculator sizes a soil and waste discharge stack and its branches using the discharge-unit method of BS EN 12056-2 (System III). Add each appliance, tag it to a branch, and the tool checks every branch and the combined stack against the standard’s Table 11 or Table 12 capacities, recommending a nominal bore for each. It is aimed at UK M&E and public health engineers, plumbers and contractors sizing above-ground drainage at tender or design stage. Everything runs in your browser. Nothing is uploaded.
How foul drainage stack sizing is calculated
Each appliance carries a discharge unit (DU) weighting from BS EN 12056-2 Table 2. The wastewater flow is Qww = K × √(ΣDU), where K is the frequency factor from Table 3 (0.5 intermittent, 0.7 frequent, 1.0 congested, 1.2 special use). The required capacity for a pipe is the larger of Qww or the single largest appliance’s own DU flowrate (rule 6.3.4); at stack level any continuous and pumped flows are added as Qtot = Qww + Qc + Qp (rule 6.3.3). The tool then picks the smallest diameter whose tabulated maximum flow (by square or swept entry) meets that capacity, from Table 11 for a primary-ventilated stack or Table 12 for a secondary-ventilated stack: floored at DN100 wherever a WC is connected. A stack offset below the top branch is flagged per the UK National Annex (ND.3.5.4) but does not change the recommendation.
Frequently asked questions
What are discharge units?
A discharge unit (DU) is a dimensionless weighting BS EN 12056-2 assigns to each appliance to represent its typical flow contribution, for example 0.3 for a wash basin, 1.3 for a bath or kitchen sink, and up to 2.0 for a 9-litre WC. Summing the DUs and taking the square root, scaled by the frequency factor K, converts a mix of appliances into a design flowrate in litres per second.
Primary or secondary ventilated: which table?
A primary-ventilated stack (the stack itself is the only vent) is sized against Table 11; a secondary-ventilated stack, which has a separate parallel vent stack, uses Table 12 and carries a higher capacity per diameter. Turn the switch on for a secondary-ventilated design.
Why is the stack forced to DN100 when a WC is present?
A WC branch and any stack serving one must be at least DN100 regardless of the calculated flow, so the tool floors the recommendation at DN100 whenever a WC is connected. Smaller calculated sizes are only valid for stacks with no WC.