30/360 US (Bond Basis)

API value: thirty_360

Also known as: 30/360, 30/360 Bond Basis, 30/360 ISDA, 360/360

30/360 Bond Basis is the 30/360 variant defined in ISDA documentation and common in US-dollar bonds and loans. It uses a 360-day year, but its rule for the end of the period is asymmetric: whether the 31st counts as the 30th depends on where the period started.

Rules

The formulas use these symbols:

Step 1: adjust the days. Before counting:

• If $D_1$ is the 31st, it becomes 30.
• If $D_2$ is the 31st and $D_1$ (after adjustment) is 30, $D_2$ becomes 30. Otherwise $D_2$ stays 31.
• The end of February is not adjusted.

Step 2: count the interest days.

Formula:

$$d = 360 \cdot (Y_2 - Y_1) + 30 \cdot (M_2 - M_1) + (D_2 - D_1)$$

With the terms:

$$\text{Days} = 360 \cdot (\text{End year} - \text{Start year}) + 30 \cdot (\text{End month} - \text{Start month}) + (\text{End day} - \text{Start day})$$

With the example numbers: period from 28 February 2026 to 31 March 2026. $D_1 = 28$ stays 28, because February is not adjusted. $D_2 = 31$ stays 31, because $D_1$ is not 30.

$$d = 360 \cdot (2026 - 2026) + 30 \cdot (3 - 2) + (31 - 28) = 0 + 30 + 3 = 33$$

Step 3: calculate the interest.

Formula:

$$I = B \cdot r \cdot \frac{d}{360}$$

With the terms:

$$\text{Interest} = \text{Balance} \cdot \text{Rate} \cdot \frac{\text{Days}}{360}$$

With the example numbers: 100,000 EUR at 4.00 % for the 33 days above.

$$I = 100{,}000 \cdot 0.04 \cdot \frac{33}{360} = 366.67$$

What this means in practice

• Periods running from month-end to month-end in 30- and 31-day months count 30 days, like other 30/360 methods.
• A period that starts before the 30th and ends on a 31st counts one day more: 15 March → 31 March is 16 days, not 15.
• Around February this adds up: a period from 28 February to 31 March counts 33 days, because 28 February is not adjusted and the 31st then stays 31.
• On a month-end schedule the year can therefore total 361 days instead of 360. A 31st that stays 31 at the end of one period is reset to 30 at the start of the next, so the day is not given back.

Example

100,000 EUR at 4.00 % p.a., balance unchanged.

How to recognise it in a contract

• "30/360", "30/360 (Bond Basis)" or "360/360" in a facility agreement governed by US or English law, especially for USD loans.
• Reference to the day count fractions of the 2006 ISDA Definitions.

Related

• Guides overview
• 30E/360 (Eurobond)
• German 30/360