Every input has an i beside it. Click it and it tells you what the number is and where to get it. The same guidance is built into the tool under ? Help.
Before you start: three things worth having to hand
- The LPA's nutrient-neutrality advice note. It names your catchment and often the apportionment and occupancy the authority expects.
- The name of the treatment works serving the site, if you are on mains drainage. Your sewerage undertaker will confirm it. Pick it from the list in the tool and the permit figures fill themselves.
- The official Natural England calculator workbook for your catchment, for the land-use export coefficients in Table 2. Everything else in the tool can be answered from your own drawings.
A quicker way to see how it all fits together: open the tool and press Example. It loads a complete, working project so you can see what a finished assessment looks like before you type anything, then press New and do your own.
What the calculator is actually doing
Natural England's methodology compares the nutrient your development will discharge against the nutrient the land already exports, and then adds a precautionary margin:
budget = ( waste water − current land use + future land use − SuDS ) × 1.2
The waste-water term dominates for new housing: occupancy × dwellings × water use (l/p/d, including Natural England's +10 allowance) × 365 × nutrient concentration ÷ 1,000,000. The ×1.2 is Natural England's 20% precautionary buffer, and it is not optional. One credit offsets 1 kg of nutrient per year, so the budget in kg/yr is the number of credits you need.
Step 1 — Catchment and nutrient
This is the single choice everything else follows from. It sets whether you are assessed for nitrogen, phosphorus or both, which treatment works appear in step 3, whether a 2030 permit step applies, and what the indicative credit price is.
Where to get it: the LPA's advice note, or our catchment map. Poole Harbour and the Solent are nitrogen; the River Avon and the Somerset Levels are phosphorus; Stodmarsh and The Broads are both, and give you two independent budgets, each mitigated with its own credits.
Share of the discharge reaching the protected site is 100 for most sites. Only change it where the authority has told you a treatment works sends just part of its effluent into the protected catchment — it can be very small indeed, and Canterbury uses 0.13% for May Street at Herne Bay. Where a catchment has sub-areas, picking yours fills the figure in for you.
Credit price is optional and never appears on the report. It is there so you can gauge what the shortfall will cost. Check the unit: Natural England and the Solent partnership quote per kg, while Stour and Norfolk quote per 0.1 kg — choosing the right unit is how you avoid a ten-fold error in your own costing.
Step 2 — Project details
Report header only. None of it changes a figure, so you can leave it until last — but fill it in before you export, so the PDF reaches the case officer with the site, applicant, authority and reference already on it.
Step 3 — Waste water from the new dwellings
This is where most of your budget comes from, and the step worth slowing down on.
| Input | What to enter | Where it comes from |
|---|---|---|
| Number of dwellings | The new dwellings the application creates | Your drawings. For a replacement dwelling or a conversion, ask the LPA how they want the existing load treated — practice varies |
| Occupancy | 2.4 people per dwelling unless told otherwise | Natural England's default. Canterbury accepts 1.6 for flats at Stodmarsh; the Cumbria catchments set their own from 1 May 2026 |
| Date of first occupancy | When the first home will be lived in | Your programme. It selects which permit tier applies — before 1 April 2025 uses the "current" column, on or after uses post-2025 |
| Water-efficiency standard | 110 or 125 l/p/d | Your planning conditions, or your Part G water calculation. Natural England adds its +10 l/p/d allowance on top automatically |
| Drainage / treatment | Mains sewer, septic tank or package treatment plant | Your drainage strategy. Off mains, Natural England publishes the concentrations, so there is nothing to look up |
| Treatment works | Pick it from the list | Your sewerage undertaker confirms which works serves the site |
| Permits, now and after 2030 | mg TN/l and mg TP/l | Filled automatically when you pick the works — otherwise the Stage 1 WwTW tab of Natural England's catchment workbook |
Why the permit boxes start empty
On mains drainage the treatment works' consent sets most of the budget. NutrientMonkey deliberately opens those boxes empty and refuses to produce a figure until it has a real consent, rather than starting on a generic default that would quietly make your budget wrong by about a third while looking entirely finished. Pick your works and the waiting stops — the tool carries Natural England's own consents for 675 works across 22 catchments.
If you are off mains
Natural England publishes the concentrations for you: 96.3 mg/l TN and 11.6 mg/l TP for a septic tank, 72.9 and 9.7 for a package treatment plant. Choose the "NE default" option and there is nothing to look up. Use "manufacturer value" only where you hold a certificate for a better-performing plant.
Steps 4 and 5 — Land use, before and after
These two tables describe the same piece of ground twice: how it is used today, and what will cover it once the scheme is built. Include only the land whose use is actually changing — not the whole holding, and not the parts of the red line that stay as they are. The areas in step 5 should total the same as the areas in step 4.
Areas are in hectares (1 ha = 10,000 m²), so a single house plot is usually 0.03 to 0.08 ha. Measure them off the site plan; most CAD and GIS packages report a closed polygon's area directly.
After development, the usual split is residential urban land for the built footprint and gardens, open urban land for roads, drives and hardstanding, and greenspace, woodland or water for public open space, planting and attenuation ponds.
The export coefficients
An export coefficient is how much nutrient a land use leaches, in kilograms per hectare per year — and it is the one thing a calculator cannot know for you, because the real values depend on catchment, farm type, NVZ status, rainfall band and soil drainage. NutrientMonkey loads indicative figures so you can see a working model straight away, and tells you on screen for as long as any of them are still defaults. Take the real ones from Table 2, the land-use lookup, of the official Natural England calculator for your catchment.
Step 6 — SuDS removal
If ponds, wetlands, swales or basins will take nutrient back out of the surface-water load, enter the kilograms per year and the budget drops. If there are none, or the drainage design is not fixed yet, leave it at zero — that is the conservative answer and nobody will argue with it. A claimed removal with no drainage assessment behind it is exactly the kind of thing that comes back from the authority's ecologist. The figures come from your drainage engineer, using the CIRIA C815 (nitrogen) or C808 (phosphorus) removal rates.
Step 7 — Reading the answer
- NEUTRAL — the budget is zero or below. The scheme adds no net load and needs no mitigation.
- MITIGATION REQ — the normal outcome for a new dwelling in a sensitive catchment. Not a failure: it is the number of credits you need to buy.
- CHECK INPUTS — something is missing or contradictory, and the panel says what.
- SELECT WORKS — the tool is waiting for a real treatment-works permit before it will commit to a figure.
Where a works' consent tightens at April 2030 you get two budgets, before and after. Buying permanent credits only, you need the greater of the two. Or you can split it: cheaper temporary credits (valid to 1 April 2030) for the pre-2030 excess, plus permanent credits for the enduring baseline underneath. The tool shows all three figures, and the split is often the cheaper route where a scheme offers temporary credits at all. Once you have your budget, the nutrient credits calculator turns it into published scheme prices by catchment and provider.
Five mistakes worth not making
- The neighbouring catchment. Nitrogen and phosphorus catchments sit side by side in places. The wrong one gives a confident answer for the wrong nutrient.
- Leaving the export coefficients on the defaults. They are indicative starting points, not your catchment's values. The tool flags it; the flag is worth reading.
- Counting the whole site in step 4. Only the land whose use is changing belongs there.
- Dropping occupancy without evidence. 2.4 is the default for a reason. A lower figure needs the authority's published rate behind it.
- Claiming SuDS removal before the drainage design exists. Zero is safe; an unevidenced number is not.
Saving, sharing and working offline
- Recent — every project autosaves in your own browser. Nothing is uploaded.
- Save / Load — a project file you can keep with the job or email to a colleague.
- Share — a private link that reopens the exact model on any device.
- Install app — NutrientMonkey installs on a phone or desktop and then works with no connection at all. You only need one to export the paid PDF.
- ? Help — the same guidance as this page, built into the tool. The ? key opens it.
What you get when you export
Working on screen is free, and Copy budget summary gives you the figures as plain text for an email to your case officer, also free. The paid export (£79 +VAT) is a planning-ready PDF: catchment, waste-water load, current and future land use, SuDS, the pre- and post-2030 budgets, the credits required and the full methodology notes, with a unique report ID your case officer can verify in seconds. Dual-nutrient catchments get both assessments in one report, and corrections re-export free for 24 hours.
What NutrientMonkey is, and is not
It implements Natural England's published nutrient-budget methodology (NECR459 and the catchment calculators) across all 27 catchments, with the arithmetic shown line by line so an ecologist can check every step. The official Natural England catchment calculator remains the statutory tool — your figures should reproduce in it. It does not replace ecological, drainage or engineering advice.
Stuck on something the tool cannot answer? We are a real consultancy, not just a website — get in touch or call 01202 623236.
Frequently asked questions
Where do I find out which catchment my site is in?
The local planning authority's nutrient-neutrality advice note names it — that is usually the document that told you an assessment was needed. Our catchment map covers all 27. Take care near boundaries: the phosphorus-led River Avon runs right alongside nitrogen-led Poole Harbour, and picking the neighbour gives you a confident answer for the wrong nutrient.
Where do the treatment works permit figures come from?
Natural England's own catchment calculator workbook, on the Stage 1 WwTW tab. NutrientMonkey carries those consents for every catchment that has them — 675 works across 22 catchments — so in most cases you pick your works from a list and all four boxes fill themselves. Your sewerage undertaker will confirm which works serves the site.
Why are the permit boxes empty when I open the tool?
Deliberately. On mains drainage the works' consent sets most of the budget, and Natural England's standard consent at first occupancy is commonly 27 mg/l. A tool that opened on a plausible-looking generic default would hand you a budget roughly a third too low while looking entirely finished. So NutrientMonkey waits until it has a real consent — pick your works and the waiting stops.
What occupancy should I use?
Natural England's default of 2.4 people per dwelling, unless your authority has published something else. Two known exceptions: Canterbury accepts 1.6 for flats in the Stodmarsh catchment, and the four Cumbria catchments set their own rates from 1 May 2026. An unevidenced lower occupancy is the first thing an ecologist will challenge.
What are export coefficients, and why are the defaults only indicative?
An export coefficient is how much nutrient a land use leaches, in kilograms per hectare per year. The real values depend on your catchment, farm type, NVZ status, rainfall band and soil drainage, so no calculator can know them from your postcode alone. NutrientMonkey loads indicative figures so you can see a working model immediately, and says on screen for as long as any of them are still defaults. Replace them from Table 2 of the official Natural England calculator for your catchment before you rely on the budget.
My result says MITIGATION REQ. Have I done something wrong?
No — that is the normal outcome for a new dwelling in a sensitive catchment. It is not a failure message; it is the number of credits you need to buy. Needing mitigation is what nutrient neutrality usually looks like in practice.
How much of the tool is free?
All of it, on screen, for as long as you like — including the live budget, the credit requirement and the plain-text budget summary you can paste into an email to your case officer. You only pay to export the planning-ready PDF report (£79 +VAT).
Where the method comes from
The documents behind every default in the tool.
- Using the nutrient neutrality calculators (Natural England) The official guidance and the catchment calculator workbooks — the source of the Stage 1 WwTW permit lookup and the Table 2 land-use export coefficients.
- Nutrient pollution: reducing the impact on protected sites Why habitats sites in these catchments need a nutrient budget before planning permission can be granted.