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Interest math

Interest rate percentage guide for borrowers and savers

The headline percent on a loan or savings account — and how it differs from the rate you actually realise.

Interest rates are the percentage cost of borrowing money, or the percentage return on lending it. The headline number on a savings account, a mortgage or a credit card is always a percent of the principal — but real products layer compounding, fees, promotional periods and rate resets on top, so the published rate rarely equals the rate you actually pay or receive.

This page walks the simple-interest formula on a $5,000 deposit at 4.5% per year, then expands the discussion to the distinction between APR (annual percentage rate) and APY (annual percentage yield) — the two numbers that decide which loan or account is cheaper in real life.

The simple interest formula

Simple interest accrues on the original principal. For $5,000 at 4.5% per period:

interest = principal × rate ÷ 100 = $5,000 × 0.045 = $225

Compound interest re-applies the rate to the growing balance each period; many real-world products use compounding even when the headline rate is quoted as a simple percent.

APR vs APY in one paragraph

APR (annual percentage rate) is the nominal rate over a year, ignoring compounding within the year. APY (annual percentage yield) bakes the compounding frequency in, so it reflects the actual interest you would earn or pay over a year. For a 4.5% APR compounded monthly, the APY is closer to 4.59%. Always compare APYs when shopping for accounts and APRs when comparing loans — those are the comparisons regulators standardised the disclosures around.

This page is educational and not financial advice; verify product-specific terms with the issuing institution.

Common mistakes to avoid

  • Calling margin "markup" or vice versa. Margin divides by revenue; markup divides by cost.
  • Comparing APR and APY directly without normalising for compounding frequency.
  • Booking commission on invoiced sales but forgetting the clawback when revenue is refunded.
  • Averaging percent growth across periods rather than recomputing from the underlying values.

Calculation tips & best practices

  • Define numerator and denominator explicitly before reporting any business percent.
  • Document the date range; a percent comparison without a date window is decoration, not data.
  • Treat published rates as a starting point; fees, taxes and timing change the realised number.
  • Always confirm material financial figures with a qualified adviser.

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Note: Percentage results are estimates for informational use only. Always verify critical financial, tax, or business calculations with a qualified professional.

Calculate interest in one step

Open the calculator in Basic mode, type your principal and the rate, and the breakdown shows the simple-interest amount before compounding kicks in.

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