Quick answer: a BRE 365 spreadsheet holds four blocks — the soakage test (f = Vp75–25 ÷ (ap50 × tp75–25)), the design rainfall depths for each storm duration with a climate-change uplift, the storage rows (S = A × R − as50 × f × D, largest row governs) and the half-drain check (ts50 = 0.5 S ÷ (as50 × f) ≤ 24 h). The example below gives f = 5.85e-5 m/s, a governing 1 h storm and 3.22 m³ of storage.
Free to size. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Percolation test in, sized soakaway out, every row on this page shown in the report.
Block 1 — the soakage test
The first rows hold the trial pit and its three fills. The digest wants a pit 0.3–1 m wide and 1–3 m long, dug to the depth of the intended soakaway, filled three times to at least 75% of its effective depth, and the time for the water to fall from 75% to 25% recorded each time. The slowest fill is used. The infiltration rate is the volume between the 75% and 25% levels divided by the pit's internal surface area at 50% effective depth — base excluded, because it silts — divided by the time.
| Row | Input / formula | Example |
|---|---|---|
| 1 | Pit length L (m) | 1.5 |
| 2 | Pit width W (m) | 0.6 |
| 3 | Effective depth De (m) — base to inlet level | 1.2 |
| 4 | Fill times 75%→25%, three fills (min) | 38 / 52 / 61 |
| 5 | Vp75–25 = L × W × 0.5 De (m³) | 0.54 |
| 6 | ap50 = 2 (L + W) × 0.5 De (m²) | 2.52 |
| 7 | tp75–25 = slowest fill (s) | 3660 |
| 8 | f = row 5 ÷ (row 6 × row 7) (m/s) | 5.85e-5 |
Block 2 — the design rainfall
This is the block a downloaded sheet gets wrong most often, because it is site-specific. The digest designs for a 1-in-10-year return period at every duration from a few minutes to a day. Depths come from the Flood Studies Report method: the 5-year 60-minute depth M5-60 and the ratio r are read for the site from the national maps and converted to 10-year depths for each duration with the digest's growth factors. A climate-change uplift is then applied — the Environment Agency publishes the allowances, and 40% is what most authorities now ask for on new residential drainage. The example uses illustrative depths for a southern-England site; the ones for your site are different.
| Row | Duration D | M10 depth (mm) | × uplift 1.4 (mm) | R (m) |
|---|---|---|---|---|
| 9 | 15 min | 20 | 28.0 | 0.0280 |
| 10 | 30 min | 26 | 36.4 | 0.0364 |
| 11 | 1 h | 32 | 44.8 | 0.0448 |
| 12 | 2 h | 40 | 56.0 | 0.0560 |
| 13 | 4 h | 50 | 70.0 | 0.0700 |
| 14 | 6 h | 56 | 78.4 | 0.0784 |
Block 3 — the storage rows
For each duration the sheet works out what arrives and what soaks away during the storm; the difference is what the soakaway must hold. Inflow is the impermeable area drained times the rainfall depth. Outflow is the soakaway's own internal surface area to 50% of its effective depth, as50 (base excluded again), times the infiltration rate, times the duration in seconds. Because as50 depends on the soakaway's dimensions, the block is iterative: guess a size, run every row, enlarge until the largest storage fits. The example is a 2 m × 2 m plan, 1.5 m effective depth, so as50 = 4 × 2 × 0.75 = 6.00 m², draining 100 m² of roof and drive.
| Row | Duration | Inflow A × R (m³) | Outflow as50 × f × D (m³) | Storage S (m³) |
|---|---|---|---|---|
| 15 | 15 min | 2.80 | 0.32 | 2.48 |
| 16 | 30 min | 3.64 | 0.63 | 3.01 |
| 17 | 1 h | 4.48 | 1.26 | 3.22 |
| 18 | 2 h | 5.60 | 2.53 | 3.07 |
| 19 | 4 h | 7.00 | 5.06 | 1.94 |
| 20 | 6 h | 7.84 | 7.59 | 0.25 |
Row 17 governs: the 1 h storm needs 3.22 m³. Shorter storms bring less water; longer ones give the ground time to keep up. That storage is what the construction has to provide as void — about 10.7 m³ of rubble-filled excavation at a 30% void ratio, or 3.4 m³ of crates at 95%; the crate calculator turns it into a number of units.
Block 4 — the half-drain check
The last rows make sure the soakaway will be ready for the next storm. The digest requires it to empty from full to half full within 24 hours: ts50 = 0.5 S ÷ (as50 × f). For the example, 0.5 × 3.22 ÷ (6.00 × 5.85e-5) = 1.3 hours — a comfortable pass. Slow ground fails this row long before it fails the storage rows: below roughly 1 × 10⁻⁶ m/s the check cannot be met at any sensible size, and the design has to move to attenuation with a controlled outfall instead.
Where spreadsheet calculations go wrong
The same errors recur in home-made calculations: rainfall depths copied from a previous site; an uplift that has been superseded; the base of the pit or the soakaway included in the surface area (it should not be — it silts up and the digest excludes it); the fastest fill used instead of the slowest; and a construction void ratio that does not match what is built. None of these is a spreadsheet's fault exactly, but every one of them is invisible in a sheet that shows a green cell. The BRE 365 explainer walks the method with the reasoning behind each step.
What you get instead
SoakMonkey runs all four blocks free in the browser: enter the trial pit and three fills, the area drained and the site, choose a construction, and the storm table, governing duration, dimensions and half-drain time appear as you type. The Building Control-ready PDF — £25 +VAT — prints every row above for your site, with a report ID the inspector can verify online. Nothing to download, nothing to keep a version of.
Frequently asked questions
Is there a BRE 365 soakaway calculation spreadsheet?
Spreadsheet versions of the BRE Digest 365 method circulate, and many drainage engineers keep one. There is nothing secret in them: the digest defines the soakage test, the infiltration-rate formula, the storage equation and the half-drain check, and a sheet just holds those formulas in rows. What a sheet cannot hold is the design rainfall for your site — the Flood Studies Report depths have to be read for the location and uplifted for climate change — which is where most home-made sheets go wrong.
Where can I download a BRE Digest 365 spreadsheet?
We do not publish one, on purpose. A downloaded sheet is a copy that ages: a rainfall depth left over from another site, an uplift that was 20% when the sheet was built and is 40% now, a storage formula with the base area still in it. SoakMonkey runs the same calculation free in the browser from your own percolation test, with the rainfall looked up for the site, and the Building Control-ready PDF shows every row.
What does a BRE 365 spreadsheet need to calculate?
In order: the infiltration rate f from the trial-pit test; the soakaway's internal surface area to 50% of its effective depth, a_s50, with the base excluded; then for each storm duration D the inflow (area drained × rainfall depth), the outflow during the storm (a_s50 × f × D) and the storage S = inflow − outflow; the largest S is the design volume; and finally the time to half-empty, t_s50 = 0.5 S ÷ (a_s50 × f), which must be under 24 hours.
Can I do the BRE 365 calculation by hand?
Yes, and this page shows every step with numbers. The arithmetic is not hard; the judgement is in the inputs — a soakage test done to the digest (three fills, the slowest one), the right 10-year rainfall depths for the site, the climate-change uplift the local authority expects, and a construction whose void ratio matches what will be built. Get those right and a hand calculation, a spreadsheet and SoakMonkey give the same answer.
Is SoakMonkey free?
Yes. Entering the percolation test, the catchment and the construction and reading the size on screen is free with no sign-up. You pay £25 +VAT (£30 inc VAT) only when you export the Building Control-ready PDF, which carries the storm table, the infiltration analysis, the half-drain check and a report ID Building Control can verify online.
Related: how to do the percolation test · what size soakaway do I need? · the BRE 365 rules on the building regulations reference.