Investment calculator
Project a pot forward year by year — and see what every assumption behind the answer is worth.
Calculator
Your projection, and the working behind it
What you would end up with
After 30 years, this projection ends at
£394,989.86
£188,308.53 in today’s money, at 2.50% inflation a year.
Adds a deflated column. The projection itself never changes — inflation is presentation, not arithmetic.
| Starting balance | £10,000.00 |
|---|---|
| Paid in over the term | £180,000.00 |
| Growth, after the fund’s ongoing charge | £217,279.97 |
| Charges deducted from the pot | −£12,290.11 |
| Final value | £394,989.86 |
Contributions are £500.00 a month, so the “paid in” line is what actually left your bank account, not what the pot is worth.
This is not a forecast. It is what would happen if a single rate held exactly, every period, for 30 years. Real returns arrive as a sequence, and the order they arrive in changes the answer. Nobody can tell you the rate — you typed it.
What would it take to reach a target?
Set this to £0 to hide the answer.
You would need £645.87 a month to reach £500,000.00 in 30 years, holding every other figure above fixed. That contribution finishes at £500,007.23.
Year by year
| Year | Opening | Paid in | Growth | Charges | Closing |
|---|---|---|---|---|---|
| 1 | £10,000.00 | £6,000.00 | £598.25 | £33.84 | £16,564.41 |
| 2 | £16,564.41 | £6,000.00 | £895.12 | £50.66 | £23,408.87 |
| 3 | £23,408.87 | £6,000.00 | £1,204.66 | £68.13 | £30,545.40 |
| 4 | £30,545.40 | £6,000.00 | £1,527.38 | £86.38 | £37,986.40 |
| 5 | £37,986.40 | £6,000.00 | £1,863.87 | £105.43 | £45,744.84 |
| 6 | £45,744.84 | £6,000.00 | £2,214.75 | £125.29 | £53,834.30 |
| 7 | £53,834.30 | £6,000.00 | £2,580.57 | £145.98 | £62,268.89 |
| 8 | £62,268.89 | £6,000.00 | £2,962.01 | £167.56 | £71,063.34 |
| 9 | £71,063.34 | £6,000.00 | £3,359.74 | £190.02 | £80,233.06 |
| 10 | £80,233.06 | £6,000.00 | £3,774.38 | £213.48 | £89,793.96 |
| 11 | £89,793.96 | £6,000.00 | £4,206.76 | £237.95 | £99,762.77 |
| 12 | £99,762.77 | £6,000.00 | £4,657.57 | £263.44 | £110,156.90 |
| 13 | £110,156.90 | £6,000.00 | £5,127.64 | £290.03 | £120,994.51 |
| 14 | £120,994.51 | £6,000.00 | £5,617.73 | £317.76 | £132,294.48 |
| 15 | £132,294.48 | £6,000.00 | £6,128.75 | £346.66 | £144,076.57 |
| 16 | £144,076.57 | £6,000.00 | £6,661.58 | £376.79 | £156,361.36 |
| 17 | £156,361.36 | £6,000.00 | £7,217.14 | £408.22 | £169,170.28 |
| 18 | £169,170.28 | £6,000.00 | £7,796.40 | £441.00 | £182,525.68 |
| 19 | £182,525.68 | £6,000.00 | £8,400.33 | £475.16 | £196,450.85 |
| 20 | £196,450.85 | £6,000.00 | £9,030.08 | £510.76 | £210,970.17 |
| 21 | £210,970.17 | £6,000.00 | £9,686.69 | £547.92 | £226,108.94 |
| 22 | £226,108.94 | £6,000.00 | £10,371.29 | £586.64 | £241,893.59 |
| 23 | £241,893.59 | £6,000.00 | £11,085.13 | £627.01 | £258,351.71 |
| 24 | £258,351.71 | £6,000.00 | £11,829.42 | £669.10 | £275,512.03 |
| 25 | £275,512.03 | £6,000.00 | £12,605.42 | £713.02 | £293,404.43 |
| 26 | £293,404.43 | £6,000.00 | £13,414.59 | £758.80 | £312,060.22 |
| 27 | £312,060.22 | £6,000.00 | £14,258.26 | £806.49 | £331,511.99 |
| 28 | £331,511.99 | £6,000.00 | £15,137.92 | £856.25 | £351,793.66 |
| 29 | £351,793.66 | £6,000.00 | £16,055.11 | £908.12 | £372,940.65 |
| 30 | £372,940.65 | £6,000.00 | £17,011.43 | £962.22 | £394,989.86 |
30 years, rolled up from monthly periods. The CSV export contains every period, not just the year ends.
Whose inflation assumption the real-terms figures use
Whose inflation assumption?
The “today’s money” figures above need an inflation assumption, and nobody knows the right one. You can use your own, or the Office for Budget Responsibility’s. Your figure is kept either way — choosing the OBR does not overwrite the box, and switching back restores what you typed.
Your own is the default and always will be. An official forecast arriving as a default would be this page choosing an assumption for you.
In use: 2.50% a year — the figure you entered. Your own figure is 2.50%.
| Fiscal year | OBR forecast |
|---|---|
| 2023-24 | 5.67% |
| 2024-25 | 2.35% |
| 2025-26 | 3.44% |
| 2026-27 | 2.01% |
| 2027-28 | 1.95% |
| 2028-29 | 2.04% |
| Long run, 2075-76 | 2.00% |
The OBR publishes a path and this page takes one rate. The figure offered is their long-run assumption, which is where the forecast settles rather than what it says about the next few years — and the near-term figures above are visibly different. Averaging the path instead would be a number this page computed wearing the OBR’s name.
The OBR does not forecast a return on investments, so the growth rate above is yours and nothing here can fill it in. There is no equity return, fund return or portfolio growth in their work at all. The closest-looking thing they publish is economic growth, which says nothing about what any investment does — it is deliberately not held by this site, for that reason: “Real GDP growth” — Not an investment return. The closest-looking row and the most dangerous. “Nominal GDP growth” — Same, with inflation in it.
A forecast is not a fact. This is what one body expected on one day; the OBR revises at every round and has been substantially wrong before. It is an assumption you are choosing, not an expected outcome for you, and a projection built on it is still arithmetic on an assumption rather than a prediction.
Contains Office for Budget Responsibility data licensed under the Open Government Licence v3.0. Note: Medium term forecast from the March 2026 Economic and fiscal outlook., published 28 May 2026. The OBR’s data.
The conventions this answer used, and what each one is worth
| Convention | What you have set | What it is worth on your figures |
|---|---|---|
| Contribution timing | Start of each month | Start: £394,989.86. End: £393,738.84. Difference: £1,251.02. |
| Rate basis | Effective (AER) | Effective: £394,989.86. Nominal: £401,945.12. Difference: £6,955.26. |
| Fund ongoing charge (OCF) | 0.45% a year, inside the fund | Costs £35,960.23 over the term — and appears nowhere in the charges column. |
| Platform fee | 0.25% a year on the value | Costs £19,470.42 over the term. |
| Fixed annual fee | £0.00 a year | Costs £0.00 over the term. |
| All three charges together | — | Cost £57,578.53 over the term. |
The three charge lines do not add up to the total, and that is arithmetic rather than an error: money a charge does not take stays invested and is then charged by the others. Removing one charge leaves a bigger pot for the remaining two to work on, so removing all three is worth more than the three removals added together.
The charges column totals £12,290.11 — that is not what investing cost you. It is what was deducted from the pot. The ongoing charge is taken inside the fund before the unit price is struck, so it reaches you as £35,960.23 of growth that never happened.
Why timing is worth what it is. Paying at the start of a period means every contribution is invested one period longer, so the gap is one period’s growth on the contributions — currently 0.3697% a month, after the ongoing charge. It is not a fixed percentage of the pot, and on annual contributions it is roughly twelve times larger than on monthly ones.
Where the pennies go. Every period is rounded to whole pence and the schedule is the truth: the final value is the last row’s closing balance and nothing else. That costs a pound or two of drift against the textbook formula over forty years, and it buys a table that adds up.
One period governs everything. Your monthly setting drives contributions, compounding and charges together. Monthly contributions into annually-compounding growth is not something this calculator can express, and it says so rather than quietly picking one.
A worked example you can check
These are the figures the calculator above loads with, so you can check every number in this section against it without typing anything. £10,000.00 to start, £500.00 a month paid at the start of each month, a growth assumption of 5% a year as an effective rate, a 0.45% fund ongoing charge, a 0.25% platform fee and no cap, over 30 years.
| Starting balance | £10,000.00 |
|---|---|
| Paid in over 30 years | £180,000.00 |
| Growth, after the fund’s ongoing charge | £217,279.97 |
| Charges deducted from the pot | −£12,290.11 |
| Final value after 30 years | £394,989.86 |
| The same pot in today’s money, at 2.50% inflation | £188,308.53 |
Three things in that table are worth pausing on, and all three are conventions rather than arithmetic anybody could argue with.
- Paying at the start of each month rather than the end is worth £1,251.02 here. That is one month’s growth on the contributions — 0.3697% a month at these settings — and not the ~5% figure that gets quoted, which is the annual-contribution case and overstates the monthly one about twelvefold.
- Treating the 5% as a nominal rate — dividing it by twelve and compounding twelve times — would add £6,955.26 to the answer, because twelve twelfths of 5% compound to more than 5%. A fund’s quoted return and a savings account’s AER are effective rates, so that would be inflating an assumption the reader never made.
- The charges column totals only £12,290.11, but the charges cost £57,578.53. The 0.45% ongoing charge is taken inside the fund, so it never appears as a deduction; it reaches you as £35,960.23 of growth that did not happen.
Methodology: exactly what this calculator does
What the calculator does, period by period
There is one loop and it is the only place money moves. For each period, in this order: open with the previous closing balance; add a start-of-period contribution if that is the timing you chose; apply the period’s growth; deduct the platform fee and the fixed fee’s share of the year; add an end-of-period contribution if that is the timing you chose; close. Every figure is a whole number of pence and every row satisfies opening + contribution + growth − fees = closing exactly. The final value is the last row’s closing balance and nothing else — no closed-form formula is evaluated alongside it, so the headline cannot disagree with the table.
The rate
Your annual rate is converted to a periodic one exactly once. On the effective basis the periodic rate is (1 + r) ^ (1 / n) − 1, so n of them compound back to precisely the rate you typed. On the nominal basis it is r / n, which compounds to more — that is the mortgage and loan convention, and it is offered as an explicit choice rather than applied quietly.
The fund’s ongoing charge reduces the rate multiplicatively: net = (1 + r) × (1 − ocf) − 1, not r − ocf. The two differ by the r × ocf term — the charge levied on the year’s own growth — and over decades the subtractive shortcut overstates a pot by around 2%. The multiplicative form is also what a fund’s published net return is: the unit price is struck after the charge.
The three kinds of charge
The ongoing charge is a drag on the return and never appears as a deduction. The platform fee is charged on the value of the pot and is deducted from it; it divides nominally, because “0.25% a year charged monthly” means one twelfth of 0.25% a month as a matter of the platform’s tariff — a fee schedule is not a compounding return. The fixed fee is flat and spread across the year’s periods. Neither deducted charge can take money that is not there: the balance floors at zero rather than going negative, so a pot exhausted by charges shows zero from that period on.
Rounding
Rounded to whole pence at every period boundary. The alternative — carrying fractions and rounding only for display — gives a table whose rows visibly do not add up. The cost is a drift of a pound or two against the textbook formula over a forty-year monthly projection, which is a much smaller problem than a table a reader can catch out, on a projection whose growth rate is an assumption anyway.
Inflation
The inflation assumption in force never moves the projection. The schedule is nominal, every row of it is nominal, and today’s-money figures are produced by deflating a finished figure: real = nominal / (1 + i) ^ years. Ticking “today’s money” is a change of units, not a change of plan.
Whose inflation assumption, and the only official figure on this page
You can use your own inflation assumption or the Office for Budget Responsibility’s long-run one. Yours is the default and always will be — an official forecast arriving as a default would be this page choosing an assumption on your behalf and calling it yours. Choosing the OBR’s does not overwrite your figure either: it stays exactly as you left it, so switching back restores it.
The OBR publishes a year-by-year path and this page takes a single rate, so what is offered is their long-run assumption — the last year they publish, which for CPI is the inflation target the whole forecast converges on. The near-term path is shown beside it so the size of that substitution is visible rather than hidden, and no average of the path is taken, because an average would be a figure this page computed wearing the OBR’s name. Every figure carries the forecast round it came from, because the OBR revises at every round and has been substantially wrong before: it is an assumption you are choosing, not an expected outcome.
The OBR does not forecast a return on investments, so the growth rate stays yours and nothing on this page can fill it in. There is no equity return, fund return or portfolio growth anywhere in their work. The closest-looking thing they publish is economic growth, which says nothing about what an investment does — and this site deliberately does not hold that series at all, rather than holding it unused one click from a growth field.
Goal seek
“What must I pay in to reach £X” is solved by bisection on the contribution, down to the penny, holding every other input fixed. If no contribution inside the search range reaches the target, the answer is that it is out of reach and by how much — the solver never hands back the top of its own search range as though it were the answer.
Sources, and why there is no rates table
Every other calculator on this site cites gov.uk, because it implements statutory rates, bands and thresholds that can be checked against a published source. This one has no statutory figures at all. The growth rate, the charges, the term and the inflation assumption are all yours. There is nothing here that an authority could confirm, because a growth assumption is not the sort of thing that can be correct.
So this page carries no “verified against HMRC guidance” stamp and it will not borrow one. The claim it makes is arithmetic only: given these inputs and the conventions stated above, the schedule is right, every row reconciles exactly, and every headline figure is read off it.
What this is not
It is not a forecast. A single path at a constant rate badly understates the spread of real outcomes: returns arrive as a sequence, and the order they arrive in changes the answer. It does not model tax, ISA or pension allowances, variable rates, or drawdown. And it is information, not advice — it cannot know your circumstances and it is not a recommendation to invest in anything.
Nothing you type here is transmitted or stored — there is no application server and no database. A share link is the exception: it carries your figures in the URL. What that means.
Frequently asked questions
- How much will £500 a month be worth in 30 years?
On this calculator's default assumptions — a £10,000.00 starting balance, 5% a year as an effective rate, a 0.45% fund ongoing charge, a 0.25% platform fee and contributions paid at the start of each month — the pot reaches £394,989.86 after 30 years, of which £180,000.00 is money you paid in. Change any assumption above and the figure changes; it is arithmetic on your inputs, not a prediction.
- Does it matter whether I invest at the start or the end of the month?
Yes, but by less than is usually claimed. Paying at the start means every contribution is invested one period longer, so the difference is one period's growth on the contributions — £1,251.02 on the default figures above. The often-quoted "about 5%" is the annual-contribution case, where one period is a whole year; for monthly contributions it overstates the effect roughly twelvefold.
- What is the difference between an effective and a nominal growth rate?
An effective annual rate compounds to exactly itself over a year: the periodic rate is (1 + r) ^ (1/n) − 1. A nominal rate is simply divided by the number of periods, so compounding it back gives more than the rate you started with. A fund's quoted return and a savings account's AER are both effective; mortgage and loan APRs are nominal. On the default figures above, choosing nominal instead of effective adds £6,955.26.
- Why isn't the fund charge in the charges column?
Because that is not where you meet it. A fund's ongoing charge is taken inside the fund, before the unit price is struck, so it reaches you as a smaller return rather than as a deduction from your account. The charges column is what was taken out of the pot; it is not the cost of investing. On the default figures the charges column totals £12,290.11 while the charges actually cost £57,578.53.
- What growth rate should I use?
There is no correct answer and this calculator will not pretend otherwise — the rate is your assumption, not a figure it can look up. The useful approach is to try several and look at the spread rather than to pick one and trust it: the difference between the answers tells you more than any single answer does.
- Does this calculator account for inflation?
It restates the answer in today's money, and it does not touch the projection to do it. The schedule is nominal throughout; the inflation assumption is used only to deflate finished figures, so ticking "today's money" changes the units on the screen and nothing about the plan.
- Is this a forecast of what I will actually get?
No. It computes what would happen if one rate held exactly, every single period, for the whole term. Real returns arrive as a sequence rather than as an average, and the order they arrive in changes the outcome — two portfolios with identical average returns can end up a long way apart. Treat the number as arithmetic on an assumption, not as an expectation.
- Is anything I type here sent anywhere?
Not by typing it. The calculator runs entirely in your browser, there is no application server and no database, and the share link and CSV export are both assembled in the tab you are reading this in — creating a link makes no request at all. The one exception is worth knowing: a share link puts your figures in the URL, so if you send one and somebody opens it, their browser requests that address and the figures travel with it, the way any web address does. Nothing is stored either way.