Every fitting has an i beside it naming the exact figure off the spec sheet. The same guidance is built into the tool under ? Help.
Before you start
You need two things: the sanitaryware schedule with manufacturer references, and the planning decision notice — because that is what tells you whether the target is 125 or 110 litres per person per day. Everything else follows.
125 or 110?
125 l/p/d is the standard requirement. 110 l/p/d is the optional requirement, and it bites only where the planning authority has imposed it as a condition. This is the single most common thing to get wrong on an otherwise correct calculation, in both directions: designing to 110 when nobody asked wastes money, and missing a 110 condition means going back round the loop at discharge stage.
A live proposal would cut these to 105 and 100 l/p/d during 2026 with a six-month transition. It is a proposal only — today's standards remain 125 and 110 — but the tool has a stress-test button that shows you whether your specification would still pass at 105, which is worth a click on anything being built out over the next couple of years.
Table 1 — where each figure comes from
You only ever fill the capacity / flow column. The use factor and the fixed allowance are set by the DCLG methodology and cannot be changed by any calculator.
| Fitting | What to enter | Where it is |
|---|---|---|
| WC | Flush volume(s) in litres | The WC spec sheet — a dual flush is quoted as, say, "6/4 litre" |
| Basin taps | Flow rate, l/min | "Flow rate at 3 bar" on the technical sheet. Bath taps are excluded |
| Bath | Capacity in litres to the overflow | The bath's technical drawing — not the brim figure |
| Shower | Flow rate, l/min at 3 bar | "Max flow rate" on the spec sheet, or the flow regulator's rating |
| Kitchen tap | Flow rate, l/min at 3 bar | The tap manufacturer's spec sheet |
| Washing machine | Litres per kg dry load | Energy label: water per cycle ÷ rated load in kg |
| Dishwasher | Litres per place setting | Energy label: water per cycle ÷ place settings |
| Waste disposal | Present or not | The kitchen specification — it carries a fixed allowance |
| Water softener | Litres/person/day | Worked out in the Table 3 panel from the manufacturer's data |
Leave the washing machine and dishwasher defaults in place if the appliances are not chosen yet — they are the methodology's own figures and Building Control expects to see them where nothing is specified.
Rows 5 to 11 — the Regulation 36 total
None of this is editable. It is arithmetic on the fittings above.
- (5) total calculated use from Table 1.
- (6) and (7) less any greywater and rainwater contribution.
- (8) the normalisation factor, × 0.91.
- (9) total water consumption = [(5) − (6) − (7)] × 0.91.
- (10) external water use, a flat + 5.0 l/p/d.
- (11) the Regulation 36 total, and the number that must meet your target.
The verdict is judged on the unrounded row 11, so a figure displaying as exactly 125.0 can still fail on the true decimal.
Greywater, rainwater and softeners
Greywater recycles water from baths, showers and basin taps to flush WCs or feed a washing machine. The credit is the smaller of what the system supplies and what those uses demand — a large system feeding one WC saves only what that WC uses.
Rainwater follows BS 8515. The intermediate method works the yield out from roof area, a yield coefficient and annual rainfall; direct entry takes a daily litres-per-person figure straight off a system specification, and should only be used where the supplier has done the BS 8515 calculation. If greywater already meets part of the same demand, the tool caps the rainwater saving so the two cannot be counted twice.
Water softeners count only for water consumed beyond the 4% good-practice level. If the result is zero or negative, zero is used.
When it fails
The shower is almost always the largest single item, with the WC second. The tool looks for the single least-drastic change that would pass — a lower shower flow rate, a smaller flush — and tells you the value it would need, floored at realistic minimums. Where no single change is enough it says so, and the honest next moves are combining changes or adding recycling.
One report, several dwelling types
A dwelling type is one set of sanitaryware, not one plot. Group every plot that shares a specification into a single type and list them in the plots box: one fee covers all of them. Additional types in the same report are £15 +VAT each against £25 +VAT for the first, and every paid report carries an ID Building Control can verify in seconds.
What this tool is, and is not
It implements The Water Efficiency Calculator for New Dwellings (DCLG, 2009), as required by Approved Document G and Building Regulation 36, with every row shown so it can be checked. The calculated figure is a compliance metric, not a prediction of actual use.
Frequently asked questions
Is my target 125 or 110 litres per person per day?
125 l/p/d is the standard requirement under Regulation 36 and Approved Document G. 110 l/p/d is the optional requirement and applies only where the local planning authority has imposed it as a condition — so check the decision notice rather than assuming. Designing to 110 when it was not required costs the client money; missing a 110 condition costs a discharge of condition.
Where do the flow rates and flush volumes come from?
Manufacturer product data, off the spec sheets your sanitaryware schedule already cites. Taps and showers are quoted at 3 bar for a high-pressure system, or 0.1–0.5 bar for a gravity system; if a flow regulator is fitted, use the regulator's rating. Bath capacity is measured to the overflow, not the brim, and is on the technical drawing. Washing machines and dishwashers come off the energy label: water per cycle divided by rated load in kg, or by the number of place settings.
Can I change the use factors?
No, and no calculator can. The use factors are set by The Water Efficiency Calculator for New Dwellings (DCLG, 2009) — they are the regulation's assumptions about how often a fitting is used, not a prediction about your occupants. You only ever enter the capacity or flow rate column.
What if the same fitting is specified more than one way?
Use the multiple-fittings panel on that row. The official rule takes the greater of the average and 70% of the highest value, so specifying one ultra-low fitting cannot drag the average down. For WCs the rule is a plain average of the effective flush volume, where a dual flush is weighted as roughly one full flush in three.
Why does my result say FAIL when the number looks like it is exactly on target?
The verdict is judged on the unrounded total, not the figure displayed to one decimal place. A specification that reads exactly 125.0 can still be over on the true decimal. It is worth leaving a little headroom anyway — under 2 l/p/d is tight, and installed spec has a way of drifting from what was drawn.
How many dwelling types do I need?
One per distinct set of sanitaryware. If plots 1–4 and 7 share the same shower, WC, bath and taps, that is one type covering five plots — and one fee. Group properly and list the plots in the box so the report says which homes it applies to.
What does it cost?
Building the calculation on screen is free, for as long as you like. The planning-ready PDF is £25 +VAT for the first dwelling type and £15 +VAT for each additional type in the same report.
Where the method comes from
The documents behind every figure in the tool.
- The Water Efficiency Calculator for New Dwellings (DCLG, 2009) The methodology itself — Table 1 fittings and use factors, Table 2 multiple fittings, Table 3 softeners, Tables 4 to 5 for greywater and rainwater, the 0.91 normalisation and the 5 l/p/d external allowance.
- Approved Document G — sanitation, hot water safety and water efficiency The requirement itself, including the 125 l/p/d standard and the 110 l/p/d optional requirement.