[Calculators](https://pecuvo.eu/en/calculators/) · Compound interest # Compound interest calculator — with monthly contributions and the rule of 72 Interest on interest: enter a starting amount, a monthly contribution, an annual rate and the years. The calculator shows the final amount — how much of it is your own money and how much is interest. The numbers stay in your browser and are not sent anywhere. Knowledge, not financial advice — for a specific decision, read the contract and ask an independent specialist. ## How it is calculated - Every month: amount = amount × (1 + annual rate / 12) + the contribution. - **The rule of 72:** 72 ÷ the annual percentage ≈ years to double (at 6% — about 12 years). The mathematics is the same in every country. Knowledge, not investment advice: returns on investments are not guaranteed, and inflation reduces the real value. Tax on interest differs between countries. ## The terms in the Glossary - [Compound interest (compounding and capitalisation)](https://pecuvo.eu/en/glossary/compound-interest/) — compound interest is interest charged not only on the starting amount but also on interest already added — which is why - [Interest](https://pecuvo.eu/en/glossary/interest/) — interest is the price paid for using someone else's money — what a borrower pays a lender, or a bank pays a saver, for t - [Rule of 72](https://pecuvo.eu/en/glossary/rule-of-72/) — the rule of 72 is a quick approximate way to work out how many years it takes for a sum to double: divide 72 by the annu - [Inflation](https://pecuvo.eu/en/glossary/inflation/) — Inflation is the general rise in prices over time, so the same amount of money buys less. - [Real and nominal interest rate](https://pecuvo.eu/en/glossary/real-interest-rate/) — the nominal interest rate is the percentage written in the contract; the real interest rate is that percentage after inf ## In depth in the modules - [Module 7 · Interest, compounding, capitalisation](https://pecuvo.eu/en/modules/interest-compounding-capitalisation/) ## More calculators - [Net salary (example: Bulgaria)](https://pecuvo.eu/en/calculators/net-salary-bulgaria/) - [Loan and APR](https://pecuvo.eu/en/calculators/loan-repayment-apr/) - [Saving for a goal](https://pecuvo.eu/en/calculators/savings-goal/) - [Emergency fund](https://pecuvo.eu/en/calculators/emergency-fund/) - [Pay yourself first](https://pecuvo.eu/en/calculators/pay-yourself-first/) - [Budget](https://pecuvo.eu/en/calculators/budget/) - [Inflation](https://pecuvo.eu/en/calculators/inflation/) - [Bulgarian lev and euro](https://pecuvo.eu/en/calculators/lev-to-euro/)