Interest Calculator

Before you calculate

Use this calculator when the balance can receive annual or monthly contributions and compounding frequency matters. Use the simple interest calculator for non-compounding interest, the compound interest calculator for principal-only annual compounding, the savings calculator for growing deposits, and the interest rate calculator when you need to solve for the rate.

Formula summary

The calculator converts the entered nominal annual rate to an effective monthly rate based on the selected compounding frequency. It then runs a month-by-month projection: beginning contributions are added before monthly interest, end contributions are added after monthly interest, tax drag reduces credited monthly interest, and the inflation-adjusted balance divides the final balance by the entered inflation assumption.

Worked example

Example: $25,000 plus $5,000 per year at 5 percent

With the default inputs, a $25,000 starting balance, $5,000 added at the beginning of each year, 5 percent nominal annual interest compounded monthly, 5 years, 0 percent tax, and 3 percent inflation produce an ending balance of about $61,193. Total principal is $50,000, so the model generates about $11,193 of gross interest before any real-world fees, product rules, or taxes beyond the entered tax field.

The same balance can change materially if contributions move from beginning to end timing, if monthly deposits are added, or if the entered tax and inflation assumptions are changed.

Assumptions

• The initial investment, annual contribution, monthly contribution, annual rate, compounding frequency, tax rate, inflation rate, and contribution timing stay unchanged for the entered term.
• Tax rate and inflation rate are user-entered planning assumptions. The calculator does not look up official tax rules, CPI data, live rates, product fees, provider terms, withdrawals, market volatility, or account limits.
• Results use the numbers entered on the page, round currency outputs to two decimals, and are educational estimates rather than financial, legal, tax, lending, investment, or regulated advice.

Common mistakes to avoid

• Treating Total interest as after-tax interest. In this calculator, Total interest is the gross interest generated by the model, while the tax-rate input reduces the interest credited to the ending balance.
• Mixing annual and monthly deposits without checking timing. Beginning contributions get more time to earn interest than end contributions, and annual contributions are applied on yearly boundaries.
• Reading the inflation-adjusted balance as a forecast. It only restates the ending balance in today's buying-power terms using the inflation rate you entered.

Methodology and sources

This calculator is an original implementation based on documented formulas, app-specific assumptions, deterministic fixtures, edge cases, rounding policy tests, and internal validation. It is not copied from a single source.

Outputs are checked with deterministic fixtures, edge cases, rounding policy tests, and internal independent comparator checks where overlapping outputs are available. The result remains an educational estimate, not a quote, approval, tax answer, or personalised advice.

Formula version 2026.05.21-generic-interest-simulation. The version marks the calculation logic and validation fixture set used for this estimate.

This calculator uses generic financial math, so there is no single official source for the formula.

Estimate only

The result is educational and is not financial, tax, legal, lending, investment, or regulated advice. Real provider terms, fees, rates, taxes, and personal circumstances can change the final answer.

Formula and help: read the full interest calculator methodology notes.

Related reading

• Compound interest for beginners
• How Calcs.finance validates calculator formulas
• Estimate limitations and not-advice disclaimer

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