How it works
pound.one answers three questions about British money — what it was worth, what you keep, and how long it lasts — using only official data, and it shows its working.
Addresses are the interface
/1990/100 is £100 in 1990, today. /1990/100/2005 compares two years. ?series=cpi swaps RPI for CPI. /income/40000 is a salary; /income/40000/net is a take-home target. /nhs/1000000 is a sum spent on a budget. Every page has a /card.png and an /api/… twin, so a link is the whole product.
Inflation
The default measure is the Retail Prices Index because it is the only official series that runs continuously from 1948. It tends to read a little higher than CPI and is no longer a National Statistic, which is why CPI is offered from 1988. Values before 1987 are chained from the ONS's annual RPI changes.
The "what it buys" lines use the ONS average shop prices behind the RPI — a pint of lager, a pint of milk, a loaf, a litre of petrol, a packet of cigarettes and so on. The ONS stopped publishing them in January 2025, so today's prices are that last reading carried forward with inflation. They are labelled as approximate wherever that applies. First-class stamps are hand-entered from Royal Mail's price history.
House prices are Nationwide's UK average, taken as the mean of each year's quarters. Wages are the ONS average weekly earnings for the whole economy, times 52. Both are means rather than medians, so read the house-to-pay multiple as a trend, not a mortgage quote.
Income
Tax rules are 2026/27 for England, Wales and Northern Ireland; Scotland has its own bands. It assumes an employee on a standard tax code with no pension contributions, student loan, benefits in kind or other income. Employer's National Insurance is shown at 15% above £5,000, ignoring the Employment Allowance. "Working for the Treasury until" is your effective rate as a share of the year — a simpler idea than the Adam Smith Institute's Tax Freedom Day, which counts every tax there is.
Where you rank uses HMRC's Survey of Personal Incomes, 2023-24. It covers the total income of everyone who pays income tax, so it includes pensioners and part-time workers and reads lower than full-time salary tables. Incomes have risen since, so a rank today is a shade flattering.
Spending
Each budget is a single published annual figure with its source beside it. Departmental figures are day-to-day (resource) budgets; capital spending is separate. Spending is treated as spread evenly through the year.
Compound
The lump sum goes in at the start of the year you name, a monthly contribution is paid at the start of every month from then until the month before the end, and charges come off monthly. The headline is in today's money: what you put in is brought forward with RPI too, so the multiple you see is the one that matters — what you actually gained, not what inflation added.
The S&P 500 series is Robert Shiller's price and dividend data, total return with dividends reinvested, converted to sterling at the Bank of England's daily $/£ rate. A UK investor's return in dollars is not their return in pounds, and over long runs the exchange rate does a lot of the work. Shiller's dataset has not been updated since 2024, so that is where the series stops; the page and the share card both say so rather than pretending it reaches today. Cash is the Bank of England's official Bank Rate compounded monthly, which no real savings account pays — treat it as a ceiling. Houses are the Nationwide average price, with no rent, no mortgage, no upkeep and no buying costs; it is a price, not a return.
The 60/40 and 80/20 mixes are backtests, rebalanced monthly, built from the two series above. Cash stands in for bonds, because there is no free, rebuildable UK gilt total-return series — that understates the long fall in yields, so the bond-heavy mixes read low. Vanguard LifeStrategy names resolve to the matching mix, but the funds only launched in 2011; before that the line is a model wearing the fund's name, which is why the caveat is printed on the share card as well as the page. Charges are taken off at 0.22% a year for the mixes and nothing for the raw indices. Tax is never taken off. None of this is advice, and past returns are a record, not a forecast.
Mortgage
The overpayment side is arithmetic and nothing else: a level monthly payment on the balance you give, interest charged monthly on what is left, and the term recomputed once you pay more. The months saved and the interest saved involve no forecast and no data. Some lenders charge interest daily, which makes overpaying slightly better than shown.
The break-even return is the rate at which overpaying and investing end level, comparing the two at the same date so the cash flows match: overpay and you are mortgage-free early, then invest the whole payment for the rest of the term. It comes out near your mortgage rate and a little above it at higher rates, because clearing the debt early frees a bigger payment to invest sooner. Plenty of calculators set interest saved against an investment pot instead, which are not the same kind of number.
The default assumed return is 9.3% a year — the S&P 500's total return from 1871 to 2024, in dollars, before charges and tax. It is not a forecast and you can change it. It is worth knowing what it hides: a sterling investor's return also depends on the exchange rate; a tracker costs about 0.1 to 0.2% a year and tax takes more outside an ISA; and our own sterling series reads about 13.5% a year only because it begins in 1975, close to a market bottom. Default mortgage rates are the Bank of England's averages of quoted household rates, refreshed with the rest of the data. None of this is advice, and the certain number and the guessed one are deliberately kept in separate halves of the page.
Privacy
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