Simple interest is calculated on the principal only.
Formula: Interest = Principal × Rate × Time.
Use this calculator before making any financial commitment that depends on this type of calculation. Running the numbers in advance lets you evaluate options without the pressure of a live negotiation or decision deadline.
A frequent error is using annual rates where monthly rates are required (or vice versa). Simply dividing an annual rate by 12 is only an approximation — the correct conversion for compound calculations uses the (1 + r)^(1/12) − 1 formula.
A first-time buyer models three scenarios before making an offer: 10%, 15%, and 20% deposit on a £280,000 property. The calculator shows exactly how the monthly payment and total interest cost change with each deposit level, making the decision visible rather than speculative.
With simple interest, the interest base does not grow over time.
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