Our assumptions, explained
Every projection rests on assumptions about the future: how fast prices rise, what investments earn, how long you live. Here are the main ones Hundred Summers starts from, in plain English: the figure, why we chose it, and which way it leans. Where a choice had to be made we mostly chose caution, so a plan that works here is less likely to have been flattered; investment growth is the exception, where we use central figures and let the stress test show the range.
Every figure on this page is read from the same code that runs the app, and you can change any of them in your own plan. Tax rates and thresholds are checked against their official pages every morning: the full list, with the changes coming, is on UK money rules we track. The full detail, with sources, is in the methodology.
Inflation
How fast prices rise. Your spending, your salary, and every figure you enter in today's money grow at this rate.
Why: The Bank of England aims for 2%. We assume a little more, so in every projection your costs rise slightly faster than the Bank's target. The stress test lets inflation move around this figure and, once it rises, stay high for several years, as it did in the 1970s and in 2022.
Investment growth
What your investments are assumed to earn each year, before inflation and before charges. A pension's growth follows the risk level you choose for it: Cautious 3.5%, Cautious-Moderate 4.3%, Moderate 5%, Moderately Adventurous 5.6%, Adventurous 6.5%. A savings account follows what it holds: cash 2%, bonds 4%, shares 6%, mixed / multi-asset 5%.
Why: Broad long-run figures, blending shares at about 6%, bonds at about 4% and cash at about 2%. After 2.5% inflation, shares' 6% is about 3.5% a year of real growth, before charges. We don't claim these are cautious: they are central starting points, not forecasts. You can set your own rate for any pension or account, and the stress test shows what happens when markets don't oblige.
Investment charges
Platform, fund and any adviser fees together, taken off the growth of everything invested (pensions and investment accounts, not cash).
Why: The government's Pension Charges Survey 2020 found an average charge of 0.48% for members of automatic-enrolment workplace pensions. A personal pension or a platform with actively managed funds often costs more: enter your own if you know it. Plans created before 27 September 2026 kept the 0% they were built with until changed.
Market ups and downs
The stress test runs your plan through 500 simulated markets. In a typical year shares move up or down by about 16% around their average, bonds by about 6% and property by about 8%.
Why: Very bad years happen more often than a textbook bell curve would suggest, so the simulation allows for them. Assets move together the way they have in the past: shares and property tend to fall together, and bonds suffer when inflation rises. The same simulated markets are used every time, so a change to your plan, not chance, is what moves the result. Separately, the historical backtest replays real market years from 1928 onwards (a US series, as a stand-in for developed markets).
Tax thresholds and rates
Income tax and National Insurance thresholds stay at today's levels until 5 April 2031, as the law now says, then rise 2.5% a year. Tax rates are today's, plus changes already announced in law, held from then on.
Why: Nothing is guessed: the rules are the ones in force, checked against the official pages every morning. Two choices lean cautious: the £100,000 point where the personal allowance starts to be withdrawn stays frozen for good, as it has since 2010, and when a tax treatment is unclear we assume the one that charges tax.
The State Pension
Your own amount comes from your forecast or your National Insurance record (the full new State Pension is £12,548 a year today). It rises 2.5% a year from now and starts at your State Pension age.
Why: The triple lock raises it each year by the highest of earnings growth, price inflation or 2.5%. We assume the 2.5% floor, no faster than our inflation figure, so it keeps its value but gains nothing in real terms. The triple lock is a government commitment, not a guarantee, and could change.
Care costs
If your plan includes later-life care, its cost grows at this rate rather than with general inflation.
Why: Care costs have historically risen faster than prices in general.
Property
Every property in the plan, your home included, grows at this rate.
Why: The same as our inflation figure, so property keeps its value but adds nothing in real terms. That leans cautious for what a home adds to your wealth; a higher value would also mean more inheritance tax.
How long the plan runs
A new plan runs to this age unless you change it; the money has to last every year to the end.
Why: The Office for National Statistics puts life expectancy at 65 at about 84.5 for men and 87.2 for women. 90 is beyond both, because money that lasts to 90 also lasts to 87, and not the other way round. Where the app estimates your own life expectancy and you'd rather not record your sex, it uses the longer women's figure.
Final-salary pension increases
How a final-salary (defined benefit) pension grows before it is paid and rises once it is being paid.
Why: In line with our inflation figure. Schemes have their own rules, often capped; set your scheme's own rate if you know it.
Your spending
Your everyday spending, in work and in retirement, keeps pace with inflation for the whole plan, unless you set it to change.
Why: Many people spend less in their eighties than in their sixties. We don't assume it: you can set spending to fall, or add extra for your early retirement years, but the default is that your money has to keep up the same living standard to the end.
No borrowing, no early pensions
If a year's income doesn't cover its spending, the gap is drawn from savings. If savings run out, the gap counts as unfunded and the plan fails that year.
Why: The plan never borrows to cover a gap and never draws a pension before the age it can be reached, however large it is. That is how the money works in real life, and it means a plan that looks fine on paper but runs dry in a particular year is shown as running dry.
These are starting points, not forecasts, and not advice. Small differences compound over decades, so it is worth trying lower growth and higher inflation in your own plan to see how much they matter to you.