Irish Calculators
Loans & mortgages

Overdraft Interest and Fee Calculator

Calculate simple daily debit interest for non-overlapping balance periods and add known fees. Supply the statement balances and contractual rates; interest is not automatically capitalised.

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Your numbers

Each line: start date, exclusive end date, account balance. Negative balance means overdraft; do not overlap periods.
Your figures stay on this device.

How the calculation works

Estimate overdraft charges from dated account-balance periods. Borrowing is split between the authorised limit and excess debt, with supplied annual rates and known fees applied without automatic interest capitalisation.

For each non-overlapping period, interest = days / day-count denominator × (authorised debt × authorised rate + excess debt × excess rate).

  1. Identify the debt

    For each constant-balance period, take the borrowing represented by a negative balance. A positive balance incurs no debit interest and does not cancel earlier interest.

  2. Apply the appropriate rates

    Charge the authorised rate on debt within the limit and the full excess rate on debt above it. The excess-rate input is not an added surcharge.

  3. Accrue and total

    Multiply rate-weighted debt by actual days divided by the chosen denominator. Sum interest over the entered periods and add the known fees once.

Worked example: Ten actual days at one euro per day

Sample inputs demonstrate the method. They are not a forecast or product offer.

The example uses minus €1,000 from 1 January to the exclusive end of 11 January: ten days. At 36.5% on a 365 basis, interest is €1 daily. Ten days cost €10; €2 fees bring the total to €12.

Balance periods
2026-01-01,2026-01-11,-1000
Authorised annual debit rate
36.5
Excess annual debit rate
36.5
Authorised overdraft limit
1000
Day-count denominator
Actual days / 365
Known fees for these periods
2

Expected result

Debit interest
€10.00
Total cost
€12.00
Worked example: Excess debt has a separate rate

Sample inputs demonstrate the method. They are not a forecast or product offer.

Balance periods
2026-01-01,2026-01-11,-1000
Authorised annual debit rate
36.5
Excess annual debit rate
73
Authorised overdraft limit
500
Day-count denominator
Actual days / 365
Known fees for these periods
0

Expected result

Debit interest
€15.00
Total cost
€15.00

Understanding your result

The estimate covers only periods supplied. Missing days, rate changes or limit changes are not inferred, so statement reconstruction needs a suitable constant-balance breakdown.

Interest stays separate from principal. If a lender capitalises charges, reflect the changed debt in the following balance periods rather than assuming the tool rolls it forward.

Questions you might have

Does a credit balance cancel earlier overdraft interest?

No. Only the debt on each date accrues debit interest; a later credit balance does not erase it.

Choosing the right inputs

Balance periods
Enter start date,exclusive end date,account balance per row. Negative means overdraft; positive balances earn no credit interest here. Periods must not overlap.
Authorised overdraft limit
Borrowing up to the authorised limit uses the normal rate. Only the amount above that limit uses the excess rate.
Day-count denominator
Choose the contractual 365 or 360 denominator. Both count actual elapsed days before dividing the annual interest amount by that denominator.
Known fees for these periods
Enter known fees for these periods. They are separate costs rather than automatic additions to daily borrowing balances.

Common mistakes to avoid

  • Do not enter positive balances for overdraft borrowing.
  • Do not count the exclusive end date as an extra day or overlap adjoining periods.

Assumptions to check

  • Actual dates are counted in UTC. This mode does not capitalize interest or infer missing statement balances.

Sources and method

Read our calculation and testing approach.