Let’s say I buy 100k of stock on margin. And on the same day, I sell 100k of stock so that I’m back to zero. Do I get charged interest on 100k (interest measured on intra-day high water mark) or zero (taking amount at end of the day, or I guess maybe T+1)? Or something else?
Here is what AI says:
If you buy USD 100,000 of stock on margin and sell it again the same day, IBKR generally does not charge one full day of margin interest on the intraday peak of USD 100,000. Margin interest is calculated from the relevant end-of-day settled cash balance, not an intraday high-water mark. If the resulting settled debit is zero, the margin-interest charge should normally be zero.
Why
IBKR’s published process is:
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At the end of each day, it determines the ending settled cash balance for each currency and account segment.
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It calculates an adjusted cash balance from those settled balances.
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Debit interest is then calculated on the resulting negative cash balance, using the applicable currency rate and tier.
IBKR explicitly states that only settled money is considered for interest-rate purposes. A stock purchase and sale can therefore create timing effects depending on the product’s settlement cycle.
Example
Assume:
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You have USD 50,000 of settled cash.
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You buy USD 100,000 of stock.
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You sell the USD 100,000 position later the same day.
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Both trades execute normally and there are no other transactions, FX balances, short positions, or fees.
The purchase temporarily creates a financing requirement during the day, and IBKR’s real-time margin system will monitor it. However, after the sale, the net cash obligation should generally be back to approximately the original amount. If the end-of-day settled cash balance is not negative, there should be no margin-loan interest for that day.
This is different from margin requirements: IBKR may show a margin requirement while you hold the stock, but a margin requirement by itself does not mean that you are borrowing money or paying interest. Interest arises when the account actually has a qualifying debit balance.
Important exceptions
You could still see a charge or a related cost if:
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The purchase and sale are in different currencies, leaving a negative balance in one currency.
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Commissions, exchange fees, or other charges make the final balance slightly negative.
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You sell a different security, or the trade does not fully execute.
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A short position or another account segment creates a separate financing balance.
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The transaction involves a product with different settlement rules.
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You are trading in a way that creates a settlement-related debit even though the platform’s trade-date cash figure appears positive. IBKR notes that unsettled stock proceeds and timing differences can make the displayed trade-date balance differ from the balance relevant for determining whether a loan exists.
Practical answer
For an ordinary same-day buy-and-sell of the same USD stock:
| Situation at the relevant daily calculation | Likely margin interest |
|---|---|
| Net settled cash balance is zero or positive | USD 0 |
| Net settled cash balance is negative | Interest on the negative balance |
| Intraday debit, but no qualifying end-of-day debit | Normally USD 0 |
| Debit caused by commissions, FX, or another position | Interest on the resulting debit |
IBKR accrues interest daily and posts the finalized amount monthly, generally on the third business day of the following month. To verify a particular trade, check the daily statement’s Ending Settled Cash and the monthly statement’s Debit Interest Charged rather than relying only on the real-time “cash available” or margin figures.