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Overdraft interest calculator

Going overdrawn is one of the more expensive ways to borrow. This calculator works out, to the day, what an overdraft costs over the period you choose, and scales it to a month and a year. You can also compare it with a rate you enter yourself — say a personal loan. It holds no live overdraft rates and recommends no bank or product. Everything runs in your browser; nothing is stored.

All defaults are example assumptions; enter your own overdraft rate. Interest is worked out to the day (actual/365 convention). The calculation runs in your browser; nothing is stored.

What your result means

The headline is the interest for the period you entered; the cards also scale it to a month (30 days) and a year (365 days). If you added a comparison rate, a second block shows the interest at that rate over the same period and the difference. Read the yearly figure as a projection at a constant balance, not a fixed charge — and remember it is a simple interest cost, not a quoted APR.

How this calculator works

The interest is worked out as simple, day-count interest on the actual/365 convention: interest = amount × rate/100 × days / 365. For the projections it uses 30 days (a month) and 365 days (a year).

If you enter a comparison rate, the calculator works out the interest for the same period at that rate and shows the difference. The comparison rate is your own input (for example a personal loan rate) — no current overdraft rates are held and no products are compared.

This is a simple daily-interest cost, not a regulated APR/EAR figure and not the same as a credit-card or standard loan rate. In reality the overdrawn balance usually changes from day to day; the calculator assumes it is constant over the period.

Formula and variables

Interest=Amount × Rate / 100 × Days / 365
Per month=Amount × Rate / 100 × 30 / 365
Per year=Amount × Rate / 100
Amount
the overdrawn amount
Rate
the overdraft rate per year in per cent (your input)
Days
how long the account is overdrawn, in days

Worked examples

Example: £1,200 overdrawn for 18 days

Example inputs
Overdrawn amount€1,200.00
Overdraft rate39.9% p.a.
Duration18 days
Comparison rate9.9% p.a.
Example results
Interest for the period€23.61
Per month (30 days)€39.35
Per year (365 days)€478.80
Interest at the comparison rate€5.86
Saving over the period€17.75

At an assumed 39.9% a year, being €1,200.00 overdrawn for 18 days costs €1,200.00 × 39.9% × 18/365 ≈ €23.61. Over a full year at a constant balance that would be €478.80. At a 9.9% comparison rate the same 18 days would cost €5.86 — a €17.75 difference. The rates are example assumptions.

Assumptions

  • A constant overdrawn balance over the period (a simplification).
  • Day-count interest on the actual/365 convention.
  • The rate is your input, not a current market figure.

Limitations of this calculator

  • Real account balances change; an overdraft usually varies day to day.
  • No current overdraft rates are held, and no product is recommended.
  • An unarranged (over-the-limit) overdraft can cost more — that is not modelled.
  • A simple interest cost, not a regulated APR/EAR figure.

Common misconceptions

  • Reading it as an APR: This is a simple daily-interest cost on the amount and days you enter, not a regulated APR/EAR or a credit-card rate.
  • Assuming a constant balance: In reality the overdrawn amount changes most days; a real bill is the sum of each day's interest, which this simplifies.
  • Treating the rate as current: The rate is your own input, not a live or typical figure; check your bank's terms for the actual rate and day-count.

Frequently asked questions

Which day-count convention does it use?

Actual/365 — the actual number of days divided by 365. Some banks use a 360-day year, which gives slightly higher amounts. Your bank's terms are what count.

Can I compare an overdraft with a personal loan?

Yes, using the optional comparison-rate field. Enter a rate of your own and the calculator shows the interest for both over the same period. It recommends no specific product or provider.

Is this the APR of my overdraft?

No. It is a simple daily-interest cost from the amount, rate and days you enter. A regulated APR or EAR follows a set method and can differ; check the figure your bank quotes.

Are my inputs stored?

No. The calculation runs entirely in your browser. Your inputs are not stored and are not sent to any server.

Sources and further reading

Official and independent sources on this topic. The links open each website in a new tab; no content is loaded from them into this page.

Spotted an error in the calculation or the text?

If you notice something that is wrong or unclear: let us know via the contact page. We review every report.

Last reviewed: 01/08/2026 · All calculations run in your browser – inputs are not stored. ·How we check our calculators