BuildFlo Design Guide · Drainage · Building Regulations Part H
Drainage gradient and fall calculator
Calculate foul or surface water pipe fall over length or invert levels. Free UK drainage gradient calculator.
What this calculator does
Drain gradient helps UK building services engineers with foul / surface-water fall, invert levels, or design 1:n sizing. The calculation follows Building Regulations Part H methods used in everyday MEP design — enter your project inputs below for a first-pass result.
Whether you searched for "drain gradient", "drain gradient calculator", or "how to drain gradient", 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: 1:N / 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 drain gradient under UK standards.
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Frequently asked questions
- What gradient is required for foul drainage?
- Typical minimum falls: 1:40 for 100 mm pipe, 1:80 for 150 mm — confirm against Building Regulations Part H and the pipe diameter.
- How do I calculate invert levels from gradient?
- Invert drop = length / gradient ratio. Enter upstream invert, pipe length, and gradient to find downstream invert.
- What is the difference between foul and surface water gradients?
- Surface water drains may allow flatter gradients where self-cleansing velocity is maintained. Always check the applicable Approved Document.