Calculate simple interest from principal, annual rate and an explicit days, months or years duration with a selected day-count basis.
A quick decision brief for this specific tool
Principal, annual simple rate, positive duration, explicit days/months/years unit and 360- or 365-day basis
Converted time in years, simple interest and principal-plus-interest arithmetic
Converts days by the selected basis, months by 12 or uses entered years, then applies I = P × r × t exactly once with no compounding
Choose your path
Calculate principal-times-annual-rate-times-explicit-time simple interest with a stated duration unit and day-count basis, without implying payments, compounding or product costs.
Convert exact days to a documented year fraction.
Enter principal, annual simple rate, days and the agreement day basis.
Can reproduce interest and principal-plus-interest without mistaking it for a payment schedule.
See how months or years become t.
Choose the duration unit and read each formula step.
Distinguishes annual rate, time in years, interest and ending balance.
Avoid substituting a simplified worksheet for disclosures.
Use the arithmetic only if the agreement uses simple interest, then verify fees, APR, compounding and accrual rules externally.
Does not use the estimate as a quote or payoff statement.
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
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Days use the selected day-count denominator, whole months use 12 and years use the entered value directly.
t = days ÷ basis, months ÷ 12, or entered yearsThe annual rate is divided by 100 and multiplied by principal and time exactly once. Interest is not added back into a compounding balance.
I = P × (annual rate ÷ 100) × tPrincipal plus calculated interest is displayed as arithmetic only. Payments, fees, APR, changing rates, taxes, compounding and lender-specific accrual or payoff rules are excluded.
ending balance = P + IUpdated: August 2026
$10,000 at a 5% annual simple rate for 90 days converts to 90/365 years before applying I = P × r × t.
→ Simple interest: $123.29
A reviewer selects 360 only because the agreement specifies that convention and records the visible difference from a 365-day calculation.
A borrower uses the result as a formula check, then reads the actual APR, fees, payment schedule and payoff rules rather than treating the worksheet as a quote.
Choose the duration unit explicitly and inspect the converted time-in-years step.
Use a payment/amortization or compound-interest tool only when those product mechanics actually apply.
Enter principal and an annual simple rate, then state the exact duration unit. For days, choose the day-count basis used by the agreement. The result is arithmetic, not a quote, payment schedule or payoff statement.