Loan Days Calculator
The days a loan actually runs — the calendar count beside the three bank day-count conventions, with the interest each one bills on the same dates.
Loan 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 accrual on one window; amortization and compounding live on the finance pages
In short: 15 January to 15 March 2026 is 59 actual days — but 30/360 books the same window as 60 paper days, so the interest differs: $96.99 on actual/365, $98.33 on actual/360, $100.00 on 30/360 for $10,000 at 6%. Same loan, same rate, three answers — the note's convention clause decides which one is real.
Formula
actual days = b − a; 30/360 days = 360·Δy + 30·Δm + Δd (31st folds to 30th); interest = principal × rate × days ÷ year-basis
A loan's clock is set by its day-count convention, and the note names it in the interest-calculation clause. Actual/365 tracks the calendar on a 365-day year. Actual/360 keeps the calendar days but bills them on a 360-day year — the same stated rate quietly costs more, the money-market habit. 30/360 abandons the calendar for twelve identical 30-day months: the 31st folds into the 30th, so twelve months of accrual always reconcile to exactly one year of interest — the bond-basis shape most mortgages ride. Same dates, same rate, three different interest figures is the honest output.
Worked Example
- Enter the from and to dates.
- Enter the principal and the annual rate.
- Read the calendar count and what each convention bills on it.
Defaults: 15 January to 15 March 2026 — 59 actual days, 60 on the 30/360 paper; $10,000 at 6% bills $96.99 / $98.33 / $100.00 across the three conventions.
Strengths & Limits Of This Model
Where this engine is strong
- All three conventions priced side by side
- The 30/360 fold shown, not hidden
- Negative windows honest
Where it stops
- No amortization schedule — the payment pages own that
Practical Use Cases
Note negotiation
what the clause costs
Payoff interest
the days to the payoff date
Statement checks
which convention billed
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.
Loan Days Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does the same loan have three interest figures?
Because the conventions divide time differently. Actual/365 uses the calendar and a 365-day year; actual/360 keeps the calendar days but bills them over 360, which raises the effective rate a shade above the stated one; 30/360 replaces the calendar with twelve 30-day months. The note's clause picks one — this page prints all three so the clause can be checked, not trusted.
What is the 31st-to-30th fold?
The 30/360 convention has no 31st: a period that starts on the 30th or 31st treats the other side's 31st as a 30th. 31 January to 28 February is therefore 28 days on paper as on the calendar, but 28 February to 31 March books as 33 paper days where the calendar holds 31. The quirk is the convention's, not the page's.
Can the dates run backwards?
Yes — every count and every interest figure goes negative, the sign being the honest answer to a settled-before-started question. Nothing is refused; the cards print what the window says.
Which convention is normal?
None is universal: corporate and interbank paper leans actual/360, bonds and most US mortgages run 30/360, consumer and government paper actual/365. The only safe reading is the clause in your own note.
Does the fold ever cost the borrower?
It can, in either direction: a window that opens 28 February and closes 31 March holds 31 calendar days but books as 33 on the 30/360 paper, so the borrower is billed two days that never happened. The reverse fold bills fewer. The page prints both counts so the spread is visible before the statement arrives.
What about leap years?
The actual-day counts simply run one longer — the calendar is the truth for them. The page's 365 divisor stays fixed in leap years too; the actual/actual bond convention that switches the divisor to 366 is named here as an absence, and the year page prices the leap day itself.
Is 30/360 the only 30-day variant?
No — a European variant folds the 31st unconditionally, where the US bond basis folds it only after a 30th or 31st start. This page prints the US fold; a note written on the European variant can read a day or two differently on the same window.
Which figure should I bill?
The one your note names. The page's job is the spread: three conventions side by side make the clause's choice a decision about money, not a footnote. If the note is silent, actual/365 is the calendar's own arithmetic — the least surprising of the three.