The Calculator App
Compound Interest
Calculator App
The best free compound interest calculator app. Model monthly or weekly contributions, lump sum deposits, and 30-year growth projections. No download. Works on iPhone and Android.
Investment tools in the app
Compound Interest Calculator
Model growth with monthly contributions, lump sums and compounding frequency
Savings Calculator
See how your savings grow with regular deposits and compound AER
ISA Calculator
Project ISA growth with annual allowance top-ups
ROI Calculator
Calculate return on investment and annualised ROI
Retirement Calculator
Plan your pension pot using compound growth projections
APY Calculator
Convert between APR and APY across compounding frequencies
What is a compound interest calculator app?
A compound interest calculator app projects how your investments or savings grow over time when interest is earned on both your original deposit and the accumulated interest. The best apps let you model regular contributions, one-off lump sums, and different compounding frequencies — showing the gap between weekly and monthly investing, which can be worth £50,000+ over 30 years.
The formula
Worked example — weekly vs monthly investing
Monthly — £500/month
Rate: 8% annualised
Period: 30 years
Final balance: £745,180
Weekly — £115/week
Rate: 8% annualised
Period: 30 years
Final balance: £813,240
Weekly investing generates £68,060 more — same money, same rate, just invested more frequently. Model your own numbers in the calculator.
Frequently asked questions
Is there a free compound interest calculator app?
Yes. The Calculator App is a free compound interest calculator that works on iPhone and Android with no download. It supports monthly contributions, lump sum deposits, and different compounding frequencies.
What is the best compound interest app in the UK?
The best UK compound interest app lets you model weekly or monthly contributions, add one-off lump sums, and see the effect of inflation. The Calculator App does all of this — free, with no sign-up.
Can I calculate weekly investing vs monthly in the app?
Yes. The Calculator App's compound interest tool lets you switch between weekly and monthly contribution frequency, so you can see the exact difference in final balance over 10, 20, or 30 years.
Does the compound interest app work without internet?
Yes. Once loaded, the app works offline on both iPhone and Android. Add it to your homescreen as a PWA and it caches the calculator locally.
How do I use the compound interest calculator?
Enter your starting amount, monthly contribution, annual interest rate, and number of years. The calculator instantly shows your final balance, total contributions, and total interest earned. You can also add a one-off lump sum deposit in a specific month and year.
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