What your result means
This planner answers two different questions depending on which mode you pick. In the investing modes it tells you what a pot is likely to be worth: how much of it is money you contributed, how much is growth, and what that total is genuinely worth once inflation has had two decades to work on it. In the withdrawal modes it answers the harder question — how long the money lasts if you keep taking an income from it.
The figure most people miss is today's purchasing power. A pot of £840,000 in twenty years' time sounds transformative, but at 3% inflation it buys what about £465,000 buys now. That is not a rounding error, it is nearly half the headline. Every result here shows both numbers so you are never planning against a figure that has quietly lost its meaning.
The annualised return row is a money-weighted return, not a simple growth rate. With regular contributions, later money has less time to compound, so dividing the final pot by the total invested badly overstates performance. This calculates the rate that actually reconciles every contribution and its timing with the final balance — the same method a fund factsheet uses.
Almost every calculator stops at the moment your plan gets interesting
There are hundreds of SIP calculators and a decent number of SWP calculators, and almost none of them talk to each other. But the two are the same plan viewed from opposite ends: you spend thirty years filling a pot precisely so you can spend the next thirty emptying it, and the only number that matters is whether the second half works.
The invest, then withdraw modes run both phases in one pass. Your contributions build a corpus, and that exact corpus — not a round number you guessed — becomes the opening balance for the withdrawal phase. That handover is where plans quietly fail, and it is the one thing a two-calculator approach can never show you.
How it works
The engine runs a month-by-month simulation rather than a single closed-form formula, because a real plan has too many moving parts for one equation: contributions that rise each year, charges taken from the balance, withdrawals that step up with inflation, and a switch from paying in to taking out.
Each month the balance grows at one twelfth of the compounded annual return, charges are deducted, and any contribution or withdrawal is applied. On each anniversary, step-up increases are applied and any tax on realised gains is calculated. Every year-end balance is recorded, which is what fills the year-by-year table and draws the chart.
In withdrawal modes only the gain portion of each withdrawal is treated as taxable, in proportion to how much of the remaining balance is growth rather than original capital — the same part-disposal logic HMRC and Indian fund houses apply. Returning your own capital to yourself is not a taxable event.
How to use this calculator
- Pick a goal: Invest, Withdraw, or Invest, then withdraw.
- Choose the mode within that group — SIP, step-up SIP, lump sum, SWP and so on.
- Fill in the three or four basic inputs. That is enough for a usable answer.
- Open Advanced options only if you need them: inflation, fund charges, tax, contribution timing and the withdrawal stress test.
- Read the headline, then open the year-by-year breakdown to see exactly where the money goes.
Formula
Ct = the cash flow in month t (a contribution, or a negative withdrawal), r = expected annual return, f = annual charge, n = total months, i = the net monthly rate. Real values divide the result by (1 + inflation)years.
Worked example
A £10,000 opening investment plus £500 a month, increased 10% each year, at a 10% return with 3% inflation over 20 years:
Investment gains £486,709
Final corpus £840,359
In today's money £465,286
Lost to inflation £375,072
Real return 6.80% a year
Gains exceed contributions, which is the whole point of a long horizon — but £375,072 of the headline is inflation, not wealth. Add a 1% annual charge and the pot falls to £755,382: you pay £47,300 in fees and lose £84,977 of final value, because every pound of charge also forfeits the growth it would have earned.
Frequently asked questions
What return should I assume?＋
Nobody knows the future, so use it as a range rather than a forecast. Long-run global equity returns have been roughly 5% to 7% a year above inflation, which at 3% inflation implies something like 8% to 10% nominal. Run your plan at a pessimistic rate as well as a hopeful one — if it only works at 12%, it is not a plan.
Does it handle both UK and Indian tax?＋
Yes. The UK preset applies the £3,000 annual exempt amount and then 18% or 24% depending on your income tax band, with an ISA option that removes tax entirely. The India preset applies 12.5% on equity fund gains above ₹1.25 lakh a year, or your slab rate for debt funds. There is also a flat-rate option for everywhere else.
Why is my annualised return lower than the return I entered?＋
Two reasons. Fund charges are deducted from the balance every month, so a 10% return with a 1% charge compounds at roughly 9%. And in withdrawal modes the stress test, if switched on, replaces the first five years with a negative return — which drags the whole-period average down sharply.
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Assumptions & limitations
This is a projection engine, not a forecast. The following simplifications apply to every mode:
- Returns are assumed constant every year. Real markets are not — they deliver the average through a sequence of good and bad years, and the order those arrive in changes the outcome.
- Growth is compounded monthly from the annual rate you set, so 10% a year becomes 0.797% a month rather than a flat 0.833%.
- Fund charges are deducted monthly from the balance. Platform fees, trading costs and bid-offer spreads are not modelled separately — fold them into the charge figure if you want the full picture.
- Inflation is applied at a single constant rate to produce the "today's money" figures. Your personal inflation rate depends on what you actually buy.
- Nothing here is financial advice, and no projection is a promise. Use it to compare scenarios against each other, not to predict a number.