How to use this UK FIRE and compound interest calculator
A compound interest calculator shows what a starting pot plus a monthly contribution can become if returns are reinvested. This one adds a FIRE target — financial independence, retire early — so you can see years-to-goal beside a growth chart. The math uses monthly compounding and pounds. Inside a Stocks & Shares ISA that growth is tax-free; a SIPP is taxed on the way out. The engine is the same either way.
- Enter the starting pot. Cash or investments already earmarked for the goal, not your entire net worth unless that is the plan.
- Set the monthly addition. Stay inside the annual ISA allowance if you are modelling an ISA; pension annual allowance if you are modelling a SIPP. The calculator does not cap contributions for you.
- Choose an annual return. A long-run diversified mix is often modelled around 5%–8%. That is an assumption, not a forecast. Markets do not compound in a straight line.
- Pick a projection window and a FIRE number. A common UK shorthand is 25 times annual spending (the “4% rule”). Adjust for State Pension, a paid-off mortgage, and the 25% tax-free pension lump sum if those apply.
- Read value, contributions, and growth. Growth is projected value minus what you put in. If years to FIRE reads “Never,” the contribution or rate is too low for that target.
- Save the path. Compare “£500 a month at 7%” with “£750 a month at 6%” without losing the first run.
Inflation, sequence-of-returns risk, platform fees, and tax on withdrawals outside an ISA are outside this model. Treat the chart as a sketch of compounding.
Why a growth chart beats a single future-value number
Future value is a lump sum. Most people need to see the curve: slow in the early years, then steeper if contributions keep landing. That shape is why starting this decade matters more than finding a perfect rate.
- Contributions dominate early; compounding dominates later. In year three, most of the line is money you deposited. In year twenty-five, a large share can be growth — if you stayed invested.
- Wrappers change tax, not the engine. ISA, SIPP, and a general investment account use the same compound formula. Tax treatment is why many UK FIRE plans fill the ISA first for flexibility and the pension for the allowance.
- The FIRE target makes the curve honest. Six hundred thousand pounds is abstract until you see whether your savings rate gets there in 18 years or 40.
- It is a motivation tool, not a trading tool. There is no ticker and no advice to buy a product.
Compound interest, PEPs, ISAs, and FIRE
Compound interest is old bookkeeping. The modern future-value formula sat in actuarial tables long before personal computers. PEPs in 1987 and ISAs from 1999 gave UK savers a tax-free wrapper around that formula. Personal pensions and then SIPPs did the same with income-tax relief on the way in.
The FIRE movement of the 2010s asked a slightly different question: not “what will I have in 30 years?” but “when does the pot cover my spending?” That is years-to-target, which is future value solved for n. UK readers still use a 4%-style withdrawal rule as a sketch, then layer on the State Pension and the 25% tax-free lump sum.
Drawing the path in SVG keeps the page fast. Use it to understand compounding. Do not confuse a smooth line with a promise from the market, and do not treat it as regulated advice.