Skip to content
calculatorzone
UK · 2025/26

Compound Interest Calculator

Calculate how compound interest grows savings or increases debt over time. Enter principal, rate, compounding frequency and term to project your final balance.

Last reviewed: 30 July 2025Source: FCA — Investment basicsUpdated every: tax year
Compound Interest Calculator · UKInvestments & Savings
Try:

Total

£19,671.51

Interest Earned

£9,671.51

Rates & sources

Compound growth assumes reinvested returns and no platform fees. Past performance is not a guide to future returns.

Source: FCA — Investment basics — figures refreshed at the start of each tax year.

Upgrade · £9.99

Full 2025/26 tax summary as a PDF

A personalised 4-page report with your band breakdown, NI, pension relief and sources — delivered to your inbox. No subscription.

When to use this calculator

  • Before choosing between saving, investing, or increasing your monthly contribution.
  • When you want to compare best-case, base-case, and cautious return assumptions.
  • When you need a quick projection before making a longer-term portfolio decision.
  • When you are deciding how many more years of contributions are needed to reach a specific target balance.
  • When you want to see whether starting earlier versus contributing more each month produces a bigger outcome.

Example: seeing compounding do the heavy lifting

Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.

Starting amount

£15,000

Monthly contribution

£250

Annual return assumption

6.5%

Investment period

10 years

This example works because it shows how a steady contribution plan compounds more meaningfully in the later years. That makes the trade-off between time and return easier to judge.

How to read your results

Total

This is the headline outcome of the calculation, but it is most useful when read alongside the supporting metrics below it rather than in isolation. Try changing one input at a time and watching how this total moves to understand which driver has the biggest impact.

Interest Earned

Use this to separate genuine growth from your original capital so you can compare savings or investment scenarios on an equal footing. It is also the figure you may need to declare on a self-assessment return if it exceeds the personal savings allowance.

Method & assumptionsAuthoritative sources

This calculator uses the standard compound interest formula: A = P(1 + r/n)^(nt), where P is your principal, r is the annual interest rate expressed as a decimal, n is the number of compounding periods per year, and t is the number of years. It assumes a constant interest rate throughout, which is rarely guaranteed in practice — savings rates change, and investment returns fluctuate. The results are nominal figures and do not account for inflation, tax, or product charges. UK savers should note that the AER figure on savings products already standardises for compounding frequency, making it the fairest basis for comparison. This tool is for illustrative purposes only and does not constitute financial advice.

Common mistakes

  • !Assuming a constant return without checking a more conservative growth rate.
  • !Forgetting to include ongoing contributions, fees, or tax wrappers where relevant.
  • !Focusing only on the final balance instead of the path required to reach it.
  • !Ignoring the drag of platform fees or fund charges, which can reduce the real compounded return significantly over ten or more years.
  • !Comparing ISA and general investment account projections without adjusting for the tax treatment of interest, dividends, or capital gains.

What to do next

  • Test a cautious, expected, and optimistic growth rate instead of relying on a single projection.
  • Compare this result with related savings or retirement tools before committing more money.
  • Use the linked guides to understand which assumptions matter most over longer periods.
  • Consider running the same figures in an ISA and a general account scenario to see how the tax treatment changes the outcome over ten or more years.
  • If the projected balance falls short of your target, use the tool to work backwards — increase the monthly contribution until the result meets your goal.

Go deeper — 2 guides reference this calculator

Frequently asked

The compound interest calculator applies your interest rate compounded at regular intervals (daily, monthly, quarterly, or annually) over your investment period. Interest earned is added to principal, then earns interest itself, creating exponential growth.

Use arrow keys to navigate items, Enter or Space to expand/collapse.

End-of-article next steps

Where to go after this calculator

Move to a focused hub instead of restarting your research from scratch.

Best Tools for Small Business

Pick software, validate unit economics, and execute workflows with one connected path.

Open hub →

Startup Resource Center

A startup stack for software decisions, runway math, and practical launch workflows.

Open hub →

Creator Toolkit

Choose creator platforms, calculate revenue targets, and ship assets faster.

Open hub →

Money Management Tools

A practical path for financial product choices, scenario math, and planning utilities.

Open hub →