See exactly how your money grows over time. Enter a starting balance, choose your compounding frequency, and add regular deposits or withdrawals to model any savings or investment scenario — from a Cash ISA to a 401(k).
How is compound interest calculated?
The standard formula for compound interest is one of the most powerful equations in personal finance. It shows why starting early matters so much more than the amount you invest.
When you add regular contributions — monthly savings into a Cash ISA or weekly deposits into a brokerage account — the formula extends to include the future value of an annuity. The calculator handles this automatically by simulating each month of your chosen duration, applying the effective periodic rate, then adding or subtracting your contributions.
Step-by-step walkthrough
Suppose you invest the equivalent of 10,000 in your local currency at a 5% annual rate, compounded monthly, over 10 years. The effective monthly rate is 5% ÷ 12 = 0.4167%. After month one, your balance is 10,000 × 1.004167 = 10,041.67. After month two, 10,041.67 × 1.004167 = 10,083.51. Each month, the interest amount is slightly larger — you are earning interest on interest. After 120 months, the balance reaches 16,470 without a single extra deposit. Nearly 6,500 of that is pure interest growth, generated entirely by compounding.
Add a regular monthly contribution of 200 at the same rate and duration, and the final balance jumps to around 47,500 — with roughly 23,500 contributed and 7,500 earned in interest on top of the original lump sum. This illustrates why a savings goal calculator that ignores compounding understates your real potential by a significant margin.
Named example: Alex's 10-year savings plan
Alex is 30 years old and starts a Stocks and Shares ISA with an equivalent of £8,000/$10,000, making monthly contributions of £150/$190. Their diversified index fund averages a 7% annual return, compounded monthly. Applying the formula: after 10 years, Alex's balance is approximately £42,000/$53,000. Of this total, roughly £25,000/$32,000 is money Alex actually deposited, and £17,000/$21,000 is pure compound growth — nearly 40% of the final balance generated without additional effort.
Now consider what happens if Alex learns how to calculate compound interest on savings and starts just five years earlier, at age 25, with the same plan. By age 65, Alex accumulates around £550,000/$700,000 instead of £370,000/$470,000 — those five extra years at the start add more than £180,000/$230,000 to the final balance, demonstrating what investors call "the power of the early years."
The rate type input in this calculator matters for the calculation. If your account quotes a monthly rate (common in some US savings products), entering it as "% per month" converts it to the equivalent annual figure before running the formula. Always use the AER (Annual Equivalent Rate) in the UK or APY (Annual Percentage Yield) in the US for like-for-like comparisons — these are the true annual rates accounting for compounding frequency, as standardised by the FCA and the FDIC respectively.
What does my compound interest result mean?
Your final balance tells you the total value of your pot at the end of the chosen period — but the most revealing number is the interest share percentage. If compound growth accounts for less than 20% of your final balance, your time horizon is relatively short or your rate is low. Over 30 years at typical equity market returns of 6–8%, compound growth typically accounts for 60–75% of the final figure. That means the majority of long-term wealth comes from growth, not from your own contributions.
If your result is lower than expected, the most common levers are: a longer time horizon, a higher interest rate (or switching to an asset class with better historical returns), increasing your regular contributions, or starting earlier. The "doubles in" figure — shown as the Rule of 72 result — tells you how long it takes your money to double at the current rate. If doubling takes more than 20 years, the inflation-adjusted return is likely very low and you may want to explore higher-growth options.
How do UK savers maximise compound growth?
The most powerful tool for UK savers is the Individual Savings Account (ISA). The annual ISA allowance of £20,000 (correct as of the 2026–27 tax year, per HMRC guidance) allows interest, dividends, and capital gains to compound entirely free of income tax and capital gains tax — permanently, not just deferred. Over 30 years, the tax drag on a non-ISA account at a basic rate (20%) or higher rate (40%) is substantial. A basic-rate taxpayer earning 5% outside an ISA effectively earns 4% after tax; inside an ISA, the full 5% compounds. The difference over three decades is tens of thousands of pounds on a modest pot.
For longer time horizons, a Stocks and Shares ISA invested in low-cost index trackers has historically significantly outperformed Cash ISAs after inflation. The compound interest comparison between a Cash ISA at 4.5% and a globally diversified equity fund averaging 7% real return is dramatic over 20+ years. NS&I Premium Bonds offer a different structure — prize-based returns with 100% capital protection — and are worth comparing if capital safety is a priority. The effective "interest rate equivalent" varies monthly.
What about US investors?
For US savers, the equivalent tax-advantaged wrappers are the Roth IRA and 401(k). A Roth IRA compound growth calculator using a 7% average annual return illustrates the same principle: contributions grow tax-free and qualified withdrawals in retirement are also tax-free. For 2026, the Roth IRA contribution limit is $7,000 per year ($8,000 if aged 50+), as set by the IRS. Compounding inside a 401(k) is tax-deferred — you pay tax on withdrawal rather than on growth — which still dramatically improves long-run outcomes compared to a taxable brokerage account. The SEC's investor.gov site provides useful guidance on compound growth in the context of US retirement accounts.
What are the most common compound interest mistakes?
The most common mistake is underestimating the difference between real and nominal returns. A savings account advertising 4.5% looks attractive, but if inflation is running at 3.5%, the real return is only about 1%. Run the same calculation in real terms and the "final balance" is far less impressive in terms of actual purchasing power. Always ask what your money will actually buy in future, not just what the nominal number says.
A second common error is treating the interest rate as the primary variable and ignoring time. An extra five years at the start of an investment journey produces more wealth than doubling the contribution rate for the same five years at the end. This is why the compound interest calculator for 1099 contractors and freelancers who delay investing until their income is more stable often produces a sobering comparison — the opportunity cost of waiting even three to five years is significant.
Finally, many savers withdraw interest rather than letting it compound. If your savings account pays interest into a separate current account and you spend it, you are receiving simple interest — the compounding effect is broken. Always use accumulation-mode funds or accounts where interest is credited back to the principal. The FIRE calculator explores this in the context of building long-term financial independence.
Frequently Asked Questions
What is the formula for compound interest?
The compound interest formula is A = P(1 + r/n)^(nt), where A is the final balance, P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is time in years. For example, £10,000 at 5% compounded monthly for 10 years gives: A = 10,000 × (1 + 0.05/12)^(12×10) = £16,470. The formula is universal — it applies identically to ISAs, 401(k)s, and any fixed-rate savings vehicle worldwide.
How much will I have if I save £100 or $100 a month for 20 years?
At 5% annual rate compounded monthly, saving £100/$100 per month for 20 years produces approximately £41,100/$41,100 — compared to just £24,000/$24,000 contributed. That means £17,100/$17,100 was generated by compound interest alone. At 7%, the final balance rises to around £52,400/$52,400. The impact of rate and time compounds significantly: the same £100/$100/month started 10 years earlier produces over £90,000/$90,000 at 5%.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual return to estimate how many years it takes to double your money. At 6% it takes roughly 12 years (72 ÷ 6 = 12); at 8%, about 9 years; at 4%, 18 years. The same rule applies to debt — a credit card balance at 24% APR doubles in just 3 years if unpaid. The Rule of 72 is an approximation but is accurate enough for planning and comparison purposes.
How does daily compounding work compared to annual compounding?
Daily compounding applies a tiny fraction of the annual rate every single day, so each day's interest becomes part of the next day's base immediately. On £10,000 at 5% over 10 years: annual compounding gives £16,289, while daily compounding gives £16,487 — a difference of just £198. The interest rate itself matters far more than compounding frequency. Selecting an account with daily compounding but a lower interest rate almost always produces a worse outcome than one with monthly compounding at a higher rate.
What happens if I stop adding money to my investment after 10 years?
Compounding continues regardless of whether you make new contributions. If you invest £200/month for 10 years at 6% and then stop, your accumulated pot of roughly £32,000 continues to grow for the remaining years. By year 30, that pot alone (with no further contributions) is worth approximately £115,000. Early contributions compound for the longest time — stopping contributions is not ideal, but the money already invested still works hard. Use this calculator to model both phases: the accumulation period and the subsequent compound-only growth phase.
Is interest on a UK ISA compounded daily or monthly?
It depends on the provider. Most UK Cash ISAs compound interest monthly, with some compounding daily. Stocks and Shares ISAs compound via fund accumulation units — dividends are automatically reinvested into more units, which effectively creates continuous compounding. Always check your provider's terms and conditions. In practice, the frequency difference has a small numerical impact; the primary advantage of the ISA is tax-free compounding, not the specific compounding interval.
Does a 401(k) or pension earn compound interest?
Yes — both US 401(k)s and UK workplace pensions invested in funds earn compound returns through reinvested dividends and capital appreciation. The return is not guaranteed at a fixed rate (unlike a savings account), but historically diversified equity funds have returned roughly 7–10% annually before inflation over long periods. Tax-deferred compounding inside a 401(k) or a UK SIPP means you pay no tax on dividends or gains until withdrawal, which significantly improves long-run outcomes by allowing the full gross return to compound year after year.
How long does it take money to double with compound interest?
Use the Rule of 72: divide 72 by your annual return. At 6% it takes 12 years; at 8%, about 9 years; at 10%, roughly 7.2 years. A more precise calculation uses t = ln(2) ÷ ln(1 + r). For reference: a UK Cash ISA at 4.5% AER doubles in approximately 16 years; a globally diversified equity portfolio averaging 7% real return doubles in around 10 years. Compound interest in a high-yield savings account in the US (currently 4–5% APY) doubles in 14–18 years.