Work out day count convention instantly with clear inputs, formula shown and shareable results.
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.
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
It predates computers - assuming equal months made bond coupon arithmetic possible by hand and gives every period an identical accrual.
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.