30/360 German: Definition & Meaning | Bondfish
30/360 German — also known as 30E/360 ISDA — is a day count convention that treats every month as 30 days and the year as 360 days, counting both the 31st of a month and the last day of February as day 30 when accruing bond interest.
What it means
A day count convention tells you how many days of interest have built up between two dates. Under the German method, the calendar is simplified: any start or end date falling on the 31st is moved back to the 30th, and a date falling on the last day of February (the 28th, or the 29th in a leap year) is also treated as the 30th — with one exception, the bond’s maturity date in February keeps its real day.
The result is a perfectly regular calendar of twelve 30-day months. The convention is defined in the ISDA definitions as 30E/360 (ISDA) and is traditional in German and other continental European markets, which is where the nickname comes from.
Why it matters for bond investors
The day count convention determines the accrued interest you pay when buying a bond between coupon dates and the interest you receive when selling. Two bonds with identical coupons can accrue slightly different amounts depending on whether they use 30/360 German, 30/360 US, or an actual-days method — the difference shows up around month-ends and February.
Example
Days = 360 × (Y2−Y1) + 30 × (M2−M1) + (D2−D1)
From Feb 28, 2025 to Mar 31, 2025: Feb 28 → 30, Mar 31 → 30
Days = 30 × 1 + (30 − 30) = 30 days (vs 33 under 30/360 ISDA)
Related terms
This definition is for general information only and is not investment advice. Bond investing involves risk, including possible loss of principal.