How we bring an older price to today's money
If you followed a small tag next to a price, this is what it means. The figure was published in an earlier year, and we have moved it to 2026 using a published index rather than leaving you to guess how old it is.
The important thing first, because it is the thing most likely to mislead you. Moving a price on an index does not go and look at what the job costs now. It takes one published number and multiplies it. If the original price was right for its date, the answer is about right for today. If the market for that one trade has moved differently from construction as a whole, it is not, and nothing about the arithmetic will tell you so.
What we actually do
A price in our cost register carries the date it was true. When we cite it against a figure in a note and that date is not the current year, we divide the index at the target date by the index at the original date and multiply the price by the result. That is the whole calculation. One division.
The index we use depends on what kind of price it is, because a wage, a bag of plaster and a fitted kitchen have not moved together. These are the defaults, and the footnote on the page always names the one actually used:
| What the price is | Index used | Measured error |
|---|---|---|
| A job price: labour, materials and the contractor on it | Construction Output Price Index, housing repair and maintenance (Great Britain) Office for National Statistics falls back to BCIS All-in Tender Price Index (1985 = 100 series, as published by the VOA) where the first does not reach | 9.2% over 38 pairs |
| A material price on its own | Construction material price index, repair and maintenance, United Kingdom Department for Business and Trade (statistics collected by ONS on DBT's behalf; weights from BCIS) falls back to Construction material price index, all work, United Kingdom where the first does not reach | 14.9% over 12 pairs |
| A wage or a day rate | Average Weekly Earnings, construction sector, total pay excluding arrears, seasonally adjusted (CDID K5CD) Office for National Statistics | 1% over 6 pairs |
| A service: a survey, a test, an inspection | Construction Output Price Index, housing repair and maintenance (Great Britain) Office for National Statistics falls back to BCIS All-in Tender Price Index (1985 = 100 series, as published by the VOA) where the first does not reach | not measured |
| Hire of plant or equipment | Construction Output Price Index, housing repair and maintenance (Great Britain) Office for National Statistics falls back to BCIS All-in Tender Price Index (1985 = 100 series, as published by the VOA) where the first does not reach | not measured |
8 groups of material get their own index instead of the general basket, because when one material runs away from the others an average cannot carry it:
- timber, joist, stud, batten, rafter, carcass, softwood, hardwood, decking, sawn — Producer Price Index, Imported sawn or planed wood (CDID GHLN)
- plywood, osb, sheet material — Producer Price Index, Imported plywood (CDID FTY2)
- rebar, reinforcement, mesh, reinforcing — Producer Price Index, Concrete reinforcing bars (steel) (CDID CRB)
- structural steel, steel beam, ub, uc, rsj, universal beam — Producer Price Index, Fabricated structural steel (CDID FSS)
- precast, concrete product, lintel — Producer Price Index, Pre-cast concrete products (CDID EW6K)
- paint, emulsion, eggshell, gloss — Producer Price Index, Paint (aqueous) (CDID HM2E)
- door, window, joinery, builders woodwork — Producer Price Index, Builders woodwork: doors and windows (CDID HJBG)
- aggregate, gravel, sand, ballast — Producer Price Index, Gravel, Sand, Clays and Kaolin - incl Aggregate Levy (CDID EVNU)
How wrong has it been?
We can answer this, because the register holds 56 items that were priced twice, years apart, by sources we trust. Escalating the earlier price and comparing it with the later one is a real test, and it is the only honest way to state an error bar.
Mean absolute error 9.5%. Median 8.6%.
That is the number to keep in your head. A price we have brought forward three years is a good guide and is not a quotation, and the error is not evenly spread:
- A job price: labour, materials and the contractor on it — 9.2% mean error. An installed rate moves with its own trade, not with the mean of all trades. Structural steel came out 15 per cent under and rooflights 17 per cent over on the same index in the same years.
- A material price on its own — 14.9% mean error. Worst of the three, and biased high. A producer price index measures what a manufacturer charges, not what a merchant shelf charges: a Screwfix socket went 1.68 to 1.65 across ten years while the index rose 60 per cent.
- A wage or a day rate — 1% mean error. Near exact. The wage agreement IS the index, so there is nothing to go wrong.
When we refuse to do it at all
Multiplication always produces a number, so the work is knowing when that number would mean nothing. These are the cases where the tool declines and the figure stays on its original date, which is why some prices in our notes carry no current-money version. A refusal is part of the method, not a gap in it.
- A fee is set, not indexed
- A fee, tax, grant or cap is set, not indexed. It steps when somebody changes it, and the size of the step has nothing to do with inflation. The householder planning fee went from 258 to 548 in one move. Use the published figure for the date you want.
- The tax changed underneath it
- Installing an energy saving material was zero rated from 1 May 2023 and reverts to 5 per cent on 1 April 2027, where it had been 20. An inclusive price from before that date and one from after are not the same measurement, and indexing between them reports a 17 per cent fall that is a tax change.
- A shop price is not a factory price
- A merchant list price does not follow the producer price index over a long period, and this is measured rather than suspected. A 2-gang socket at the same URL went 1.68 to 1.65 in ten and a half years while the index it would have been escalated on rose 60 per cent.
- Nothing sensible indexes it
- An escalator has to know whether it is moving a material, a wage or a whole installed rate, because those three have come apart.
- The index does not reach back that far
- No loaded series covers that date for this kind of figure.
- The index does not reach forward that far
- The series does not publish that date.
What this is not
- It is not a quotation, and it is not a survey of what anyone is charging this week.
- It does not know about your access, your postcode, your minimum charge or your VAT position.
- It does not re-observe the price. Nobody went back to the supplier. The original source is linked in the footnote and you can go and look at it, which is the point of citing it rather than asserting it.
- It is not applied to the author's own figures. The prices written in our notes are never rewritten to match the register. The register's job is to say where a comparable price came from and when it was true, and where the two disagree we record the disagreement rather than editing the page.
Index data current to 2026-09-25. The full set of 104 series we hold, and the cost index they support, is at the UK renovation cost index.