Day Count Convention Calculator

Work out day count convention instantly with clear inputs, formula shown and shareable results.

About Day Count Convention Calculator

Day count conventions decide how a period converts into a year fraction for interest. From 15 February to 31 August 2026 is 197 actual days, giving 0.53973 of a year on actual/365 but only 196 days and 0.54444 on 30/360, because that convention treats every month as thirty days. The difference moves real money on large notionals.

Formulas

Actual conventions

year fraction = actual days / 365, 360 or the actual year length

30/360

days = 360(y2-y1) + 30(m2-m1) + (d2-d1), with day capping at 30

Worked example

With the default values below, Day Count Convention Calculator returns Year Fraction of 0.53973. Change any input above and every figure here is recalculated in your browser.

Inputs used

Period start: 2026-02-15
Period end: 2026-08-31
Day count conventionone of Actual/365 (fixed), Actual/360, 30/360 (bond basis), Actual/Actual: Actual/365 (fixed)

Results

Year Fraction: 0.53973
Days Under The Convention: 197
Actual Calendar Days: 197
Convention Applied: Actual/365 fixed

Step by step

1Values used
2Actual conventions
330/360
4Year Fraction= 0.53973
5Days Under The Convention= 197
6Actual Calendar Days= 197
7Convention Applied= Actual/365 fixed

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does 30/360 exist?

It predates computers - assuming equal months made bond coupon arithmetic possible by hand and gives every period an identical accrual.

Which convention is most common?

It depends on the instrument: 30/360 for US corporate bonds, actual/actual for government bonds, actual/360 for money markets and actual/365 for sterling.