BuildFlo Design Guide · Drainage · BS EN 12056
Drainage stack sizing calculator
Peak stack flow and minimum diameter from discharge units per BS EN 12056. Free drainage stack sizing calculator.
What this calculator does
Drainage stack helps UK building services engineers with peak stack flow and minimum diameter from discharge units (bs en 12056). The calculation follows BS EN 12056 methods used in everyday MEP design — enter your project inputs below for a first-pass result.
Whether you searched for "drainage stack", "drainage stack calculator", or "how to drainage stack", this page gives you a working tool plus the context to use it on a real job.
When to use it
Use this for early-stage sizing and sanity checks on UK building services work. It is not a substitute for issued calculations — verify against the project specification, drawings, and relevant standard before sign-off.
How to use it
1. Fill in the inputs for your scenario (defaults are starting points only). 2. Click Calculate — results include the method and assumptions where applicable. 3. Verify against the project specification and relevant standard before issuing.
Typical output unit: mm.
Tips
- Defaults are indicative — replace with project-specific values from the architect, structural, or MEP spec.
- For issued calculations, save results in your BuildFlo project and attach to the calculation folder.
- Need the regulation citation? Open BuildFlo chat in the app and ask about drainage stack under UK standards.
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Frequently asked questions
- How do I size a soil vent pipe?
- Sum discharge units from connected appliances per BS EN 12056, apply the frequency factor, and select the minimum stack diameter from the standard tables.
- What are discharge units?
- Each fixture type has a DU value representing its relative discharge frequency and volume. Summed DUs determine peak stack flow.
- When do I need a secondary vent?
- Where trap seal protection or branch length limits are exceeded. Check BS EN 12056 and the project's drainage strategy.