The allowance is an adjustment, not the whole water bill
A large bill can contain ordinary household consumption, leaked water, fixed charges and a previous balance. Your repair invoice is another cost again. Before estimating an allowance, separate these items. Otherwise, even a correct adjustment for excess water can appear wrong because it was compared with an unrelated total.
Thames Water's household allowance guidance describes comparing use during the leak with a similar earlier period, then crediting the difference in cost if approved. Where earlier usage is unavailable, it describes a comparison based on a similar-sized property. Your own spreadsheet can check arithmetic and identify questions; it cannot determine eligibility or guarantee the credited amount.
This guide is for a household metered account. Do not import a business retailer's calculation or wholesale policy simply because it also uses the words leak allowance. Begin with the account type, repair evidence and decision applying to your property.
The most useful outcome is a calculation you can explain in one paragraph: these are the dates, these are the readings, this is the normal-use assumption, and these are the charges being compared. If one of those ingredients is uncertain, label it rather than concealing the uncertainty inside an apparently precise figure.
Use your bill, dated readings, repair record and allowance decision. The bill shows the charged period and rates; readings show consumption between specific points; the repair record helps establish when the fault was resolved; the decision shows what the company accepted. These dates may not all align.
| Input | What to record | Common mistake |
|---|
| Meter identity | Meter reference linked to the account and any replacement date | Combining readings from different meters |
| Reading intervals | Start and end figures, dates, times and actual/estimated status | Comparing unequal periods as though they were equal |
| Normal-use baseline | Period used and relevant household circumstances | Assuming all consumption during the leak was leakage |
| Charge basis | Applicable water/wastewater rates and separately listed fixed charges | Multiplying volume by a monthly direct debit |
Photographs should remain attached to the raw readings. Put calculations in a separate column so anyone reviewing the file can reproduce them. If the bill's reading differs from your photograph, resolve that discrepancy before building a long estimate on top of it.
Never change an observed figure to make it fit an expected allowance. If a digit was copied incorrectly, retain the photograph, correct the transcription and explain the correction. A transparent small mistake is easier to resolve than two incompatible versions of the same reading.
Convert meter movement into daily use
For readings recorded in cubic metres, subtract the earlier reading from the later one. Divide that volume by the elapsed days to compare periods of different lengths. Multiply cubic metres by 1,000 to express the same volume in litres. Keep units beside every figure so a decimal place does not turn a modest usage change into an enormous apparent leak.
For example, two hypothetical readings of 510.0 and 514.2 cubic metres, taken exactly 14 days apart, represent 4.2 cubic metres. That is 4,200 litres over the interval, or 300 litres per day. This is an arithmetic example, not a benchmark for what your household should consume.
If you only recorded whole cubic metres, do not later invent decimal precision. A short interval based on rounded readings can give a rough daily figure. Use the original display photographs and explain the resolution available. Longer, properly dated intervals can be easier to interpret than a collection of undated close-up images.
The published allowance form requests two post-repair readings at least 14 days apart. Record those as requested rather than replacing them with a one-minute meter observation. A short observation can raise a leak question; it serves a different purpose from an interval used to assess consumption.
A worked example with unequal comparison periods
Suppose a household's hypothetical pre-leak record shows 12 cubic metres over 40 days. The average is 0.3 cubic metres per day. During a later 30-day interval affected by a confirmed leak, the meter records 31.5 cubic metres. These raw totals cannot be subtracted directly because the periods differ.
Using the earlier daily average as an illustrative baseline, expected ordinary use over 30 days would be 9 cubic metres: 0.3 multiplied by 30. The difference between the recorded 31.5 and that illustrative 9 is 22.5 cubic metres. That is the estimated excess volume under this particular assumption.
| Step | Calculation | Result |
|---|
| Earlier daily average | 12 m³ divided by 40 days | 0.3 m³/day |
| Expected use over the affected interval | 0.3 m³/day multiplied by 30 days | 9 m³ |
| Illustrative excess | 31.5 m³ recorded minus 9 m³ expected | 22.5 m³ |
This calculation assumes the earlier period fairly represents ordinary use during the later period and that the accepted leak interval matches the 30 days. If occupancy, watering or other use changed, that assumption needs examination. Thames Water may use different evidence or an adjusted baseline; ask for its reasoning rather than treating this example as a prescribed formula.
Convert volume into money using the correct rates
Thames Water's metered household charges page lists water at £2.7346 per cubic metre and wastewater at £1.4721 per cubic metre from 1 April 2026, with fixed charges shown separately. Check the actual period and tariff on your bill: a leak spanning a rate change should not automatically be priced entirely at the newest rate.
For the hypothetical 22.5 cubic metres above, multiplying by the stated 2026 water rate gives £61.5285, or approximately £61.53. If that same volume were eligible for an adjustment at the stated wastewater rate, its wastewater component would be approximately £33.12. The illustrative combined amount would be about £94.65.
The word “if” matters. This is a transparent unit-rate illustration, not a promise that both components apply to your circumstances. Ask how wastewater was treated in the actual allowance and which policy or charging provisions were used. Do not quietly double an expected credit by assuming every charge follows the same treatment.
Small differences can also arise from how calculations are split across periods and rounded. Compare the unrounded components where available before describing a few pence as a substantive error. A material difference in days, volume or rate is a more useful starting point for review.
Do not use the direct debit as a consumption rate
A monthly payment is not the price of a cubic metre. It can reflect expected future use, an account balance and the payment schedule. Dividing a leak volume by the number of monthly payments does not calculate the charge for that water.
Read the bill's consumption and charge lines separately from its payment section. An allowance may reduce the account balance while the payment arrangement is reviewed separately. If you expect a changed instalment, ask when and how it will be considered rather than assuming the next debit must equal the credit divided by twelve.
Likewise, a credit is not necessarily a bank refund. Check where the adjustment appears and the revised balance. Keep the original and revised bills together so you can see which lines changed. A screenshot of the final balance alone makes it difficult to reconstruct whether the intended adjustment was applied.
Choose a fair normal-use comparison
The baseline is often the most important assumption. A period when the property was empty will not usually describe the same household use as a period with several occupants. A summer interval with garden watering may differ from winter use. State relevant changes so the reviewer can assess comparability.
Make a short household timeline next to the meter record: move-in date, long absences, changes in occupants and known unusual use. Avoid trying to estimate every shower after the event. Clear known changes are more useful than a detailed but speculative reconstruction designed to produce a preferred number.
Consider another hypothetical comparison: normal use of 0.3 cubic metres per day over 30 days implies 9 cubic metres. A baseline of 0.4 would imply 12 cubic metres, changing the estimated excess by 3 cubic metres. This sensitivity check shows why a disputed baseline should be discussed explicitly instead of arguing only about the final pound amount.
Where the earlier period may already contain leakage, say so and explain the evidence. Using a leaking period as “normal” could distort a comparison, but an unusually high reading alone does not establish when the defect began. Ask how the start date was determined and what additional records could improve it.
When you have little or no previous meter history
A newly occupied home, replacement meter or recent switch to metered billing can leave gaps. Keep those circumstances distinct. A replacement meter might have an exchange reading, while a newly metered property may lack any comparable measured period.
Ask what normal-use basis Thames Water used and what household information it considered. The current household page describes a similar-property approach where historical records are unavailable. Do not substitute a wholesale policy's post-leak formula or an online average and present it as the household company's required method.
Post-repair readings still help demonstrate the current pattern, provided normal use and any continuing issues are recorded honestly. If the home remained empty after repair, make that clear. An exceptionally quiet interval should not be portrayed as a typical occupied month simply because it increases an estimated allowance.
Make the leak period and billing period explicit
The dates printed at the top of a bill do not necessarily mark the start and end of the leak. A six-month bill may include normal months, an uncertain onset and a known repair date. Ask which part of the interval the allowance covers and how its starting point was established.
Separate discovery from onset. You may know when you first saw a wet patch without knowing when the pipe began losing water. A notification, smart-meter change or engineer's findings might help, but each has limits. Record confirmed dates and estimates in different columns.
If there were two repairs, explain whether they addressed the same fault or separate sources of water loss. Do not combine two distinct leaks into one continuous period without evidence. Conversely, do not assume a first attendance ended the leak if the contractor only investigated and repaired it later.
Where a period spans a tariff change, request the allocation across rates. A rough calculation may spread excess evenly for illustration, but the actual profile could differ. Label any simplifying assumption and ask whether the company's evidence supports a more specific allocation.
Check the revised bill in a fixed order
First check the property and meter. Then compare the credited period, volume or usage basis, applicable rates and total adjustment. Finally check how that adjustment affects the account balance and future payment information. This order keeps an administrative mismatch from being mistaken for a complicated mathematical problem.
- Confirm the allowance reference belongs to this account and repaired leak.
- Find the explanation of the accepted period and normal-use comparison.
- Reproduce any stated volume calculation using the same units and dates.
- Check the relevant water and wastewater treatment separately.
- Identify fixed charges and unrelated balances that remain on the bill.
- Ask about any difference between the calculated credit and the amount posted.
If a calculation detail is absent, request it instead of guessing. A short question such as “Which daily-use baseline and number of days produced this adjustment?” can be more productive than sending a spreadsheet with ten alternative totals.
How to request a calculation review
State the specific line you believe needs correction. For example: “The decision uses a 20-day period, but the attached repair record and readings appear to cover 30 days. Please explain the period chosen and whether this evidence changes it.” Use your own dates and documents; this is a suggested structure, not a claim that every repair record proves the entire leakage period.
Attach a one-page calculation with the source of each input. Mark estimates and state any assumptions about occupancy or rates. Ask the company to identify where its method differs. That makes the disagreement answerable and avoids comparing two unexplained totals.
If the problem is refusal of eligibility rather than arithmetic, use our rejected-allowance guide. If you have not applied yet, the allowance-form guide helps organise the initial submission. Neither a calculation review nor an application should be assumed to suspend normal payment arrangements; ask the company about your account if payment is a concern.
If current usage still suggests a leak
A bill adjustment is not a test of the present plumbing. If readings remain unexpectedly high after repair, check known consumption and report the ongoing pattern. Keep that technical question separate from a historic calculation dispute so neither prevents the other from being addressed.
Send Leak Fix London the repair history and current water-loss evidence if a further leak investigation and repair may be needed. We can document work we perform; Thames Water decides the allowance and calculates any credit.
Our rate is £150 per hour. If we do not find the leak, you do not pay for time spent looking; see the pricing and scope. A disagreement over a completed bill calculation alone is not a reason to purchase another leak search. Use the evidence and review route first where the physical leak is already resolved.
