Drainage Field & Septic Tank Soakaway Calculator
Contact Us
Drainage Field Size Calculator
Size the drainage field (soakaway) that treats effluent from a septic tank or treatment plant, from the population and a percolation test — and work out Vp from your test times. Built on UK Building Regulations (Approved Document H2, paras 1.33–1.44).
Drainage field floor area
Approved Document H2 (1.44): At = p × Vp × 0.25 (m²), where p = population and Vp = the percolation value (seconds/mm) from a soil test.
Percolation value (Vp)
Enter the time, in seconds, for the water to fall from 75% to 25% full (a 150 mm drop) in each test. Vp = average time ÷ 150. Carry out at least three tests across two trial holes (AD H2 1.36–1.37).
Reference & rules
The method, the suitability limits and the siting/construction rules for a drainage field.
| Population p | Vp = 15 | Vp = 50 | Vp = 100 |
|---|
Percolation test. Dig a 300 mm square hole 300 mm below the pipe invert; pre-soak overnight; refill and time the fall from 75% to 25% (150 mm). Vp = time(s) ÷ 150. At least 3 tests over 2 holes; average them. Not in heavy rain, frost or drought.
Suitability. A drainage field is only suitable when average Vp is between 12 and 100. Below 12 the ground drains too fast (groundwater pollution risk); above 100 it is too slow to treat the effluent — an alternative is needed. The water table must not rise to within 1 m of the pipe invert.
Siting. At least 10 m from a watercourse or permeable drain, 50 m from a groundwater abstraction (and not in Zone 1), and 15 m from any building. No access roads, paving or other services within the disposal area.
Construction. Perforated pipe in trenches 300–900 mm wide on a 300 mm layer of 20–50 mm clean shingle, at a gradient no steeper than 1:200, laid as a continuous loop. Keep 2 m of undisturbed ground between parallel trenches.
This drainage field calculator sizes the drainage field — the septic tank soakaway — that disposes of the treated effluent from a septic tank or small sewage treatment plant. Enter the population (or the number of bedrooms) and the percolation value (Vp) from a soil test, and it returns the floor area the drainage field needs, following Approved Document H2 of the Building Regulations. It is for foul effluent only — if you need to soak away clean rainwater from roofs or paving, that is a surface-water soakaway and a different calculation (see below).
How to use the drainage field calculator
Enter the design population directly, or let the tool estimate it from the number of bedrooms (population = bedrooms + 2, minimum 5, per British Water Flows & Loads). Add the percolation value (Vp) from your soil test and, optionally, a trench width. The calculator returns the required drainage field floor area and an estimate of the total trench length.
Drainage field or surface-water soakaway? — read this first
The word “soakaway” covers two different things, and this tool only does one of them. A drainage field (also called a septic tank soakaway) disposes of treated foul effluent and is sized from a BS 6297 percolation test under Approved Document H2 — that is what this calculator does. A surface-water soakaway disposes of clean rainwater and is sized from a BRE 365 infiltration test instead. If you are dealing with rainwater, use the surface-water method, not this one.
How big does a drainage field need to be?
Approved Document H2 sizes a drainage field with the formula At = p × Vp × 0.25, where At is the floor area in square metres, p is the population, and Vp is the percolation value in seconds per millimetre. As a worked example, a five-person house on soil with a Vp of 20 needs a field of 5 × 20 × 0.25 = 25 m². The calculator runs this for you and can convert the area into a trench length.
The percolation test (Vp) explained
Vp is the average time, in seconds, for water to fall one millimetre in a test hole. You soak the hole, refill it, then time the drop from 75% to 25% full (a 150 mm fall) and divide by 150. Repeat across several trial holes and, ideally, seasons, then average them. A valid Vp for a drainage field is between 12 and 100; faster than 12 risks untreated effluent reaching groundwater, and slower than 100 means the ground will not soak it away.
When a drainage field will not work
If the Vp falls outside 12–100, the water table is high, or the site sits in a groundwater Source Protection Zone, a conventional drainage field may not be permitted and you will need an alternative such as a treatment plant discharging to a watercourse. Discharges to ground must also meet the Environment Agency’s general binding rules — including separation distances and a 2 m³/day limit. Our explainer on whether you need infiltration testing covers when a test is required for planning.
Further reading: BS 6297 infiltration testing for foul drainage fields · BRE 365 infiltration testing for surface-water soakaways · general binding rules for small sewage discharges.
Frequently asked questions
Use the Approved Document H2 formula At = p × Vp × 0.25. Multiply the population by the percolation value (Vp) and by 0.25 to get the floor area in square metres. A Vp of 20 for five people gives 25 m².
Yes — “septic tank soakaway” is the everyday name for a drainage field: the network of perforated pipes in trenches that disperses treated effluent into the ground. It is different from a surface-water soakaway, which takes clean rainwater.
A drainage field needs a Vp between 12 and 100 seconds per millimetre. Around 15–40 is typical workable ground; below 12 drains too fast for safe treatment and above 100 is too slow to soak away.
Under the general binding rules a drainage field must generally be at least 15 m from a building, 10 m from a watercourse, and 50 m from a well, spring or borehole used for drinking water.
One of our experienced Flood Risk Consultants will get back to you within 60 minutes
