30/360 ISDA: Definition & Meaning | Bondfish
30/360 ISDA — also called 30/360 Bond Basis — is a day count convention that treats every month as 30 days and the year as 360 days: a start date on the 31st becomes the 30th, an end date on the 31st becomes the 30th only if the start date is the 30th or 31st, and February gets no special treatment.
What it means
A day count convention is the rulebook for counting days of interest between two dates. The 30/360 ISDA version, set out in the ISDA definitions used for swaps and many bonds, applies just two adjustments: if the first date falls on the 31st, count it as the 30th; if the second date falls on the 31st and the first date is on the 30th or 31st, count it as the 30th too.
Crucially, there is no February rule: February 28 (or 29) keeps its real day number. That is the practical difference from 30/360 German, which treats the end of February as day 30, and from 30/360 US, which adds end-of-February adjustments for bonds whose coupons fall on month-ends.
Why it matters for bond investors
The convention drives the accrued interest added to a bond’s price when you buy or sell between coupon dates. Around February and month-ends, 30/360 ISDA, German, and US variants can each produce a slightly different day count — small money on one trade, but the reason two calculators can disagree on the same bond.
Example
Days = 360 × (Y2−Y1) + 30 × (M2−M1) + (D2−D1)
From Feb 28, 2025 to Mar 31, 2025: no adjustment applies (D1 = 28)
Days = 30 × 1 + (31 − 28) = 33 days (vs 30 under 30/360 German)
Related terms
This definition is for general information only and is not investment advice. Bond investing involves risk, including possible loss of principal.