Interest stays tied to original principal
Simple interest uses the starting amount for every period. Accrued interest does not become part of the base used for later interest.
Simple Interest Calculator
Solve balance, principal, rate, or term under simple interest with annual, monthly, or daily rate bases, years, months, or days, 365- and 360-day conventions, fees, taxes, inflation, effective annual cost or yield, compound benchmarks, Chart.js paths, and a detailed ledger.
Simple-interest solver, convention check, and compound benchmark
Solve the formula, then test what the quoted rate and time units actually mean
Find balance, principal, rate, or term; convert annual, monthly, or daily rates and year, month, or day terms; choose a 365- or 360-day convention; add fees, taxes, inflation, effective annual cost or yield, compound benchmarks, Chart.js paths, and a calculation ledger.
Plan with context
Simple interest is a linear formula, but real contracts add conventions and cash-flow timing.
The equation remains straightforward because accrued interest never joins principal. Correct use still requires matching the quoted rate's time basis, the term units, the day-count convention, and every charge around the transaction.
Rate and term units must agree
An annual percentage applied to years differs from a monthly percentage applied to months. Converting both to a common annual-and-year basis prevents a twelvefold or day-count error.
Day-count convention changes accrual
Exact 365-day and commercial 360-day calculations can produce different interest for the same number of days. The contract determines which convention governs.
Add-on interest can mask cost
A lender may calculate interest from original principal and then collect the total through installments. The resulting cash-flow APR can exceed the headline simple rate because principal declines while interest was calculated upfront.
Fees change effective cost or yield
An upfront charge reduces borrower proceeds or increases a saver's investment, while recurring charges continue with time. Economic annualization should use net cash rather than the formula rate alone.
Compound interest creates a widening gap
When earned interest is retained, it begins producing additional interest. The difference from a simple path grows with rate, time, and compounding frequency.
Included
Four-variable solver for balance, principal, rate, or term, saver and borrower perspectives, annual, monthly, or daily rate bases, year, month, or day terms, 365- and 360-day conventions, upfront and recurring charges, tax on earned interest, inflation, fee-inclusive annualized result, annual, monthly, or daily compound benchmark, formula substitution, Chart.js paths, and a calculation ledger.
Not included
Dated cash flows, declining-principal installment schedules, true add-on-loan APR, bank discount and proceeds formulas, exact actual/actual leap-year conventions, 30/360 variants, compounding inside the primary simple-interest result, variable rates, late charges, minimum interest, regulated APR or APY disclosure, withholding, or jurisdiction-specific tax.
Use it well
Identify whether the quote is annual, monthly, or daily, copy the contract's day-count basis, distinguish interest paid at maturity from periodic installments, include every required charge, and use the amortization or APR tools when principal declines during the term.