Calculate day counts between two dates using ACT/ACT, ACT/360, ACT/365, 30/360, and 30E/360. Compare all methods at a glance — with optional interest amount calculation.
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Explanation
How Day Count Calculator works
The day count calculator computes the number of interest days between two dates using various international day count conventions. These methods determine how many days are used for interest calculation and which annual divisor (denominator) is applied.
Interest formulaInterest = principal × interest rate × (interest days / denominator)
The year fraction (interest days / denominator) indicates the portion of the year for which interest is calculated. Depending on the method, the same period can lead to different results.
The year fraction (interest days / denominator) indicates the portion of the year for which interest is calculated. Depending on the method, the same period can lead to different results.
Knowledge
Actual vs. notional days
The central difference between ACT methods and 30/360 methods lies in the day counting approach.
In interest calculation, there are two fundamentally different approaches to day counting: actual calendar days (ACT) and notional 30-day months (30/360).
ACT methods count each calendar day individually. A month with 31 days yields more interest days than one with 28 days.
30/360 methods assume a flat 30 days per month. February, March, and July all have the same number of interest days.
The difference can be significant for short terms: February has only 28 or 29 days with ACT but always 30 with 30/360.
For the denominator (annual basis): ACT/ACT uses 365 or 366, ACT/360 and 30/360 use 360, ACT/365 uses 365.
Knowledge
Day count methods overview
All five common day count methods with numerator, denominator, and typical usage.
The interest formula is: Interest = Principal x Rate x (Interest days / Denominator). The method determines how interest days and denominator are derived.
ACT/ACT — Numerator: actual days, Denominator: 365 or 366. Usage: Government bonds, ISDA.
Late payment interest (BGB Section 288): ACT/ACT (actual days in year).
Building society loans: Often 30/360 (German commercial method).
Practice
Practical examples
For illustration only – your own calculation is what counts.
6 months method comparison
Comparison of all day count methods for the period 01.01.2024 to 01.07.2024. Since 2024 is a leap year, different methods yield different day counts.
Result: ACT/ACT: 182 days (denominator 366), ACT/360: 182 days (denominator 360), ACT/365: 182 days (denominator 365), 30/360: 180 days (denominator 360), 30E/360: 180 days (denominator 360).
Loan term with ACT/360 (Euro method)
A loan of EUR 50,000 at 4.5% runs from 15.03.2024 to 15.09.2024. The bank uses ACT/360. What are the interest days and amount?
Result: 184 days with denominator 360. Year fraction: 0.511111. Interest amount: EUR 1,150.00.
Bond coupon with 30/360 (German method)
A bond pays semi-annual coupons. Day count calculation from 01.02.2024 to 01.08.2024 using 30/360 with a par value of EUR 100,000 and 3.2% coupon.
Result: 180 days with denominator 360. Year fraction: 0.5. Interest amount: EUR 1,600.00.
Notes
Common mistakes
Using the wrong day count method for the product
The day count method is not freely choosable but determined by contract, market convention, or law. ACT/360 for money market, 30E/360 for bonds, ACT/ACT for late payment interest. Wrong method = wrong result.
Take the correct method from the contract or market convention: Money market = ACT/360, Bonds = 30E/360, Late payment interest = ACT/ACT.
Not considering leap year effects
In a leap year, February has 29 days. With ACT methods, this leads to one additional interest day. With ACT/ACT, the denominator also changes (366 instead of 365). With 30/360, leap years play no role since February always counts as 30 days.
Consider leap years with ACT methods — the calculator accounts for them automatically.
Miscounting start and end days
The convention is: The start day is not counted, the end day is counted (exclusive start, inclusive end). From 01.01 to 01.02 is 31 days, not 32. Some sources use different conventions — the contract terms are authoritative.
Follow the exclusive start / inclusive end convention — when in doubt, check the contract terms.
FAQ
Frequently asked questions
What are interest days?
Interest days are the number of days for which interest is calculated. Depending on the method, either actual calendar days are counted or notional days (e.g., 30 days per month) are used. The number of interest days, together with the annual divisor (denominator), determines the year fraction used in the interest formula.
What is the difference between the day count methods?
The methods differ in two aspects: how days are counted (numerator) and which divisor is used for the year (denominator). ACT/ACT: Actual days / actual days in the year (365 or 366). ACT/360: Actual days / 360 (Euro method). ACT/365: Actual days / 365 (Great Britain). 30/360: Every month has 30 days / 360 (German commercial method). 30E/360: Similar to 30/360 but with adjusted treatment of the 31st day (Eurobond method).
When should I use which method?
ACT/360 is standard in the European money market (loans, overnight deposits). 30/360 and 30E/360 are used for bonds and fixed-income securities. ACT/365 is common in Great Britain. ACT/ACT is considered the most exact method and is used for government bonds and in ISDA documentation. When in doubt, the contract specifies the applicable method.
What does ACT/360 (Euro method) mean?
With ACT/360, actual calendar days are counted but divided by 360. This means a full year (365 or 366 days) yields a year fraction greater than 1. For borrowers, this means slightly higher interest compared to ACT/365 or ACT/ACT, since the denominator is smaller.
Why does the 30/360 method exist?
The 30/360 method dates from a time before computers when calculating with actual calendar days was laborious. By assuming 30 days per month and 360 days per year, calculation is greatly simplified. Today it is used primarily for bonds and fixed-income securities, as it produces uniform coupon periods.
How does the 30/360 method handle the 31st of a month?
In the German 30/360 method, the 31st is set to the 30th when the start day is also the 30th or 31st. In the 30E/360 method (Eurobond), the 31st is always set to the 30th regardless of the start day. This distinction can make a one-day difference for short periods.
Which method does my bank use?
Most German banks use ACT/360 (Euro method) for loans and overnight deposits. Fixed deposit contracts and bonds often use 30E/360. Late payment interest under BGB is calculated with ACT/ACT. The applicable method is stated in the contract or the bank's terms.
Why does ACT/360 lead to higher interest than ACT/365?
With ACT/360, division is by 360 instead of 365. This yields a larger year fraction and thus higher interest. Over a full year (365 days), the year fraction with ACT/360 is about 1.0139 instead of exactly 1.0 — the borrower pays approx. 1.4% more interest than with ACT/365.
Limits
Limitations
Contract terms are authoritative: Individual contracts may provide different rules for day counting.
No bank holidays considered: In practice, payment dates may shift due to holidays (Business Day Convention).
Simplified 30/360 variants: There are multiple variants (US, European, German). The calculator implements the most common ones but not all.
Sources
Sources and references
ISDA — Day Count Fraction DefinitionsInternational Swaps and Derivatives Association (ISDA)
International standard definitions of day count methods by the International Swaps and Derivatives Association.
View sourceRetrieved: 07/22/2026 · Verified on: 07/22/2026
PAngV — Preisangabenverordnung (Effektivzinsberechnung)Bundesministerium der Justiz
German Price Indication Regulation with requirements for the day count method in effective interest rate disclosure.
View sourceRetrieved: 07/22/2026 · Verified on: 07/22/2026 · Primary source
Deutsche Bundesbank — GeldmarktkonventionenDeutsche Bundesbank
Conventions of the Deutsche Bundesbank for money market transactions specifying ACT/360 as the standard method.
View sourceRetrieved: 07/22/2026 · Verified on: 07/22/2026 · Primary source
EZB — Euro Money Market ConventionsEuropäische Zentralbank (EZB)
Conventions of the European Central Bank for the euro money market with ACT/360 as the standard.
View sourceRetrieved: 07/22/2026 · Verified on: 07/22/2026 · Primary source
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