Financial calculators
Interest Calculators
Simple and compound interest, effective rates and how balances grow.
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Compound Interest Calculator
Compounding means interest is added to the balance and then earns interest itself. Enter a starting amount, an optional monthly contribution and a rate to see the projected balance year by year.
Understanding interest maths
Compounding is the reason two accounts paying the same headline rate can end up thousands apart. What matters is how often interest is added to the balance and whether you keep contributing while it grows.
These calculators separate the three forces at work: the money you put in, the return applied to it, and the time it is left alone. Watching the contribution line and the interest line cross is the clearest argument for starting early there is.
All interest calculators
Interest guides
- student-loansHow Student Loan Interest Is Calculated (Daily, Not Monthly)The daily simple-interest formula servicers use, worked on a real balance, plus how capitalisation compounds the effect.
- mortgagesHow to Calculate One Month of Mortgage InterestThe single-line calculation servicers use for each month's interest charge, plus how daily-accrual loans differ from monthly-accrual loans.
- creditHow Credit Card Interest Is CalculatedHow card issuers turn an APR into a daily rate, why the average daily balance matters, and how to check the interest line on your own statement.
- personal-loansPersonal Loan APR vs. Interest Rate: Reading the DisclosurePersonal Loan APR vs. Interest Rate: what to verify, which costs and risks to compare, and which records to keep before deciding.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is charged only on the original amount. Compound interest is charged on the balance including interest already added, so growth accelerates over time.
Does compounding frequency matter much?
It matters more at higher rates and over longer periods. Monthly compounding beats annual compounding at the same nominal rate; the effective annual rate is what makes two offers comparable.
Should I model returns before or after inflation?
Both are useful. A nominal rate shows the balance you will see; subtracting expected inflation shows what that balance will buy.
Are taxes included?
No. Interest and investment returns may be taxable depending on the account and your jurisdiction, which reduces the real outcome.
Other calculator categories
- Mortgage CalculatorsPayments, amortisation, deposits and the running cost of owning a home.
- Loan CalculatorsRepayments, interest and payoff timing on personal and instalment lending.
- Investment CalculatorsProjected returns, contributions and long-run portfolio growth.
- Retirement CalculatorsRetirement pots, contribution levels and sustainable withdrawal income.
- Tax & Income CalculatorsTake-home pay, salary conversions and estimated tax positions.
- Credit & Debt CalculatorsCard payoff timelines, consolidation maths and debt reduction strategies.
- Auto Loan CalculatorsVehicle finance payments, deposits, trade-ins and total cost of ownership.
- Insurance CalculatorsCover levels, premiums, deductibles and what a claim really costs.
- Business CalculatorsMargins, break-even points, cash flow and subscription metrics.
- Savings CalculatorsSavings goals, emergency funds and the effect of regular deposits.
Disclaimer
These calculators produce estimates for information and comparison only. Actual figures depend on the provider, fees, taxes, credit assessment and terms that apply to you. Legamoney does not provide financial advice.