Overdraft Interest Calculator

What is the Overdraft Interest Calculator?

In the United States, some banks offer an overdraft line of credit or reserve line connected to a checking account. The real cost depends less on the month-end number and more on how much of the line stayed outstanding on each day, so the timing of each draw and repayment matters.

This calculator takes a start date, end date, APR, opening balance, and draw or repayment history, then builds a day-by-day running balance using 365 or 366 days automatically. It is designed to estimate the interest component of a revolving overdraft line in USD and to give you a 30-day equivalent cost for comparison.

When this helps

If you repeatedly draw short-term funds for living expenses, business cash needs, or operations and then repay them in stages, looking only at the month-end balance can easily understate the real cost. A daily-balance calculation helps separate the effect of draw timing and repayment timing so you can see which stretch of the month created the sharpest cost increase.

Cleaning up the transaction history before a bank consultation or refinance review makes rate negotiations, repayment schedule adjustments, and product comparisons much faster. If you also carry high-rate short-term debt such as card loans or cash advances, tracking the daily balance trend alongside this result is especially helpful for spotting cash-flow risk early.

Key features

The calculator combines a start date, end date, and transaction history (draws and repayments) to build a daily running-balance model. When multiple transactions happen on the same day, they are applied in entry order. If the period crosses into a different year, the tool automatically switches between 365 and 366 days as needed. Quick presets for the last 7 days, last 30 days, this month, and previous month speed up scenario checks.

Results are delivered as summary cards, a chart, a detailed table, and an Excel download. Along with the total interest for the selected period, the tool also shows a 30-day equivalent interest estimate based on the average balance. If the chart library fails to load, a text notice and fallback table appear immediately so the numeric review is not interrupted.

How to use it

First choose one of the quick presets (last 7 days, last 30 days, this month, or previous month) or type the start and end dates manually. Then set the annual rate and the opening balance. In the transaction table, you only need to enter the dates when the balance actually changed. On days with no activity, the previous day’s balance carries forward automatically.

After you press Calculate, the page scrolls to the result area automatically. There you can immediately review the total interest, average balance, peak balance, 30-day equivalent interest, and ending balance. Then use the chart to inspect daily movement, confirm the applied balance, daily interest, and cumulative interest in the detail table, and save the result as Excel or CSV if you need documentation.

Overdraft interest calculator details (formula, assumptions, official references)

This tool repeats Daily interest = daily balance × (APR/100) ÷ days in that year (365/366) for each day in the selected period and sums the result. Transactions are assumed to affect the balance before that day’s interest is calculated.

Actual charges can differ by bank because U.S. institutions may combine interest-bearing overdraft lines, non-sufficient-funds handling, linked transfers, flat overdraft fees, and different posting cut-off rules. This calculator models the interest portion only, so the agreement and statement should remain the final source.

Official reference links: CFPB: Know your overdraft options, CFPB overdraft lending rule, U.S. Bank Reserve Line of Credit.

Frequently asked questions