Drainage Calculators
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Unda's drainage calculators are a free suite of UK tools for sizing and checking drainage — from pipe gradients and soakaways to SuDS flow controls and rainwater reuse. Each one shows its working against the standard behind it, so you can trust the number and cite the source. They are built for a fast, defensible first pass at a drainage calculation; for a scheme you need to submit, our engineers can take it from sketch to an issued design. Pick a calculator below.
Browse the calculators by category
The tools are grouped the way a drainage scheme is actually built — managing and treating surface water first, then controlling its discharge, then the buried pipework, then foul drainage, and finally water reuse. Choose a category to find the calculator you need.
SuDS attenuation, treatment & design allowances
Source control and the inputs that feed every other calc.
Surface-water conveyance & capacity
How much an open channel, swale or gutter can carry.
Flow controls & discharge restriction
Sizing the device that throttles your outflow to a permitted rate.
Pipe network design & buried details
The geometry and construction of the pipe run itself.
Foul drainage — mains connected
Flows and pumping where there’s a public sewer to reach.
Off-mains foul treatment & ground disposal
Sizing tanks and disposal where there’s no public sewer.
Water reuse & non-potable supply
Capturing and reusing water to cut mains demand and runoff.
One of our experienced Flood Risk Consultants will get back to you within 60 minutes
What can you work out with these drainage calculators?
The set covers the full surface-water-to-foul journey. For source control and SuDS, you can size green and blue roof storage, check a treatment train with the SuDS water quality calculator, and apply the climate change and urban creep allowances that feed every other figure. For conveyance and control, there are tools for swale and channel capacity, gutters, and the vortex, orifice and weir controls that restrict discharge to a consented rate.
For the pipe network you can work out the drainage fall and gradient, check pipe cover depth, take off bedding, and size manholes and inspection chambers. For foul drainage there are peak-flow and pumping-station calculators; for off-mains sites, septic tank, cesspool, drainage field and land-drain sizing. And for water reuse, the rainwater harvesting and grey water calculators size collection, demand and storage.
What standards are the calculators based on?
Every result is tied to a recognised UK source. Pipe sizing, gradients, manholes, septic tanks and cesspools follow Approved Document H of the Building Regulations; gutters follow Approved Document H Section 1. SuDS conveyance, flow controls, water quality and roof storage follow The SuDS Manual (CIRIA C753) and the national standards for sustainable drainage systems. Foul flows use BS EN 12056, off-mains drainage fields use BS 6297, water reuse uses BS EN 16941 and BS 8515, and climate change uplifts come from the Environment Agency's peak-rainfall allowances. The calculators are a quick first pass — they do not replace a designed, coordinated drainage strategy.
Who are the drainage calculators for?
They are built for the people who size drainage day to day: civil and drainage engineers checking a quick figure, architects and self-builders scoping a scheme, contractors taking off materials, and planners sense-checking a submission. Anyone can use them free, with no sign-up.
From a quick calculation to a design you can submit
A calculator gives you a number; a planning authority wants a coordinated strategy behind it. When you need that, Unda prepares sustainable drainage strategies and SuDS design, preliminary drainage design for planning, and the detailed design needed to discharge drainage planning conditions.
Frequently asked questions
Yes. Every calculator on this page is free, runs in your browser, and needs no sign-up or download.
Across the set: Approved Document H, The SuDS Manual (CIRIA C753), the national standards for SuDS, British Water Flows & Loads, BS 6297, BS EN 752, BS EN 12056, BS EN 16941, BS 8515, and the Environment Agency's climate change allowances. Each calculator states the one it applies.
They are designed for quick sizing and checking. A scheme submitted for approval should be designed and signed off by a competent drainage engineer, which is where our services come in.
Start from the category that matches your task: surface water and SuDS, flow controls, the pipe network, foul drainage, off-mains treatment, or water reuse. Each card describes what its calculator does.
Yes. Unda provides drainage strategies, SuDS design, foul and surface water design and flood risk assessments across the UK.