Promissory Note Calculator (Interest & Maturity Value)
Calculate the interest, maturity value and maturity date of a simple-interest promissory note from the principal, rate, issue date and term in days — plus late (default) interest if it is paid after the due date.
How it works
Interest = principal × rate × days ÷ basis (365 or 360) · maturity value = principal + interest · maturity date = issue date + term in days · late interest = maturity value × default rate × days late ÷ basis
A promissory note is a written promise to pay a sum of money, usually with simple interest, on a set date. Short-term notes count interest by the exact number of days: actual/365 (exact interest) or actual/360 (ordinary or banker’s interest), and the 360-day basis makes the same rate cost about 1.4% more. The maturity value is what the maker owes on the due date; many notes add a higher default rate after that date, often charged on the full amount due. Selling a note to a bank before maturity is a separate calculation — the bank discount.
Worked example
A 10,000 note at 8% issued on January 15, 2026 for 180 days earns 10,000 × 0.08 × 180 ÷ 365 = 394.52 of interest (2.19 a day), so the maturity value due on July 14, 2026 is 10,394.52. On an actual/360 basis the interest would be 400.00. The equivalent effective annual rate is 8.16%.
Sources: Wikipedia — Promissory note · Wikipedia — Day count convention
Frequently asked questions
How do you calculate interest on a promissory note?
Multiply the principal by the annual rate and by the number of days, then divide by 365 (or 360 if the note uses banker’s interest). A 10,000 note at 8% for 180 days earns 394.52 on a 365-day basis.
How do I find the maturity date?
Add the term in days to the issue date, counting the calendar exactly. A 180-day note issued on January 15, 2026 matures on July 14, 2026.
Does a promissory note need to charge interest?
No, but loans between family members or related parties may have tax consequences if the rate is below the minimum rate set by the tax authority, so check local rules before writing an interest-free note.
What happens if the note is paid late?
If the note has a default-interest clause, the higher late rate applies from the due date until it is paid. Enter the late rate and the days late to see the extra interest and the total due.
Related calculators
Educational estimate, not financial advice. Real contracts, fees, taxes, day-count and rounding rules differ — check the official cost disclosure and your agreement before deciding.