Interest Days Calculator
The day count that simple interest rides on — two dates, the exact days between, and the interest those days earn at your rate.
Interest Days Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Dates 1900 to 2100 — reversed windows read negative by design
- Simple interest, 365-day year, no compounding
In short: 2026-01-01 to 2026-09-18 is 260 days — the counting convention: the last day arrives unpaid. $10,000 at 5% for those 260 days earns $356.16 on the 365-day year. The page also offers the both-endpoints count (261), because some notes charge the day they mature, and the only honest answer names the convention beside the number.
Formula
days = b − a · interest = principal × rate × days ÷ 365
Simple interest pays for time held, and the time is a day count: to minus from, the maturity day itself excluded — borrow on the first and settle on the 31st and you hold thirty days, not thirty-one. The both-endpoints twin counts the maturity day too, because some notes and some courts charge it; the page prints both so the convention is chosen, not smuggled. The interest runs on the plain 365-day year: principal times rate times days over 365, no compounding — interest on interest is the compound calculators' argument.
Worked Example
- Enter the from and to dates.
- Enter the annual rate and the principal.
- Read the days and the interest they earn.
Defaults: 1 January to 18 September 2026 — 260 days, 261 with both endpoints; $10,000 at 5% earns $356.16 on 260 days.
Strengths & Limits Of This Model
Where this engine is strong
- Both counting conventions printed
- Negative windows honest
- The 365 vs 360 quirk named
Where it stops
- No 360-day banker's year — scale by 365/360
Practical Use Cases
Loan notes
the days a note actually ran
Late-payment interest
the invoice days priced
Deposit windows
what the term earns, plainly
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Interest Days Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Which day does the count start on?
The from day is held and the to day arrives unpaid: 1 January to 31 January is thirty days, the 31st itself excluded. The both-endpoints card adds the maturity day for the notes that charge it — what matters is that the convention is named where both sides can see it.
What about February and leap years?
The count is exact calendar arithmetic — February contributes 28 or 29 as the calendar says, and a leap day inside the window counts like any other day. Only the divisor stays fixed at 365, which is exactly the simplification the banker's-year note below names.
Why divide by 365?
Because simple interest on a yearly rate prices a year at 365 days. Some bonds and banker's practice use 360 — which pays slightly MORE for the same rate and length, a quirk of the denominators. If your note says 360, scale the result by 365/360; the page prints the 365 form and says which it printed.
Does it compound?
No — this is simple interest: principal times rate times time, once. Interest earned does not itself earn. For the compound case the finance calculators own the arithmetic; mixing the two understates a multi-year hold badly.
What if the dates run backwards?
The count goes negative and so does the interest — the sign is the honest answer to a settled-after-started question, useful for back-dated adjustments. The card prints the negative rather than refusing it.
Can the rate be zero?
Yes, and the interest reads $0.00 with the day count still exact — a genuine interest-free window priced honestly. The day count is the point; the money card is optional arithmetic on top.
What is this different from business days?
Everything rests on which days count. This page counts CALENDAR days — the interest does not sleep on weekends. The business-days page walks working days only, the convention for statutory deadlines. Pick by the contract's word, not by habit.
How precise is the cents figure?
Rounded half-up to the cent at the end, never en route: the full-precision product runs once and lands on two decimals. The rate takes decimals — 7.99 computes as 7.99, not 7 or 8.