Why did I receive less crypto than I bought on spot?

Because the spot trading fee was taken from the coin you bought. OKX's fee rules say it plainly: when you buy on the order book, the fee is charged in the crypto you bought, so you receive slightly less than the quantity you ordered; when you sell, the fee is charged on the proceeds of the sale. Nothing is missing — the difference is the fee, and it is shown as its own line on each fill in your order history. The exact currency can vary (for example if you pay fees with an exchange token), so read the fee line rather than assuming.

Where the fee comes from on a spot trade

On a spot trade you hand over one asset and receive another. The exchange takes its fee from one side of that swap:

Buying (for example BTC with USDT): OKX charges the fee in the crypto you bought. You ordered a quantity of BTC; the fee is deducted from that BTC, so the amount that lands in your balance is a little smaller.
Selling (BTC for USDT): OKX charges the fee on the proceeds, so you receive slightly fewer USDT than quantity × price.

OKX's spot formula is fee rate × the amount of crypto bought or sold in each fill. With a fee rate of r, a buy of quantity Q leaves you with about Q × (1 − r) of the coin. The rate itself depends on whether the order filled as maker or taker and on your fee tier — see maker vs taker fees.

Why the number looks odd

Several fills, several fees. OKX calculates the fee separately for each fill and then totals it. A market order that walked through three price levels has three fee lines, each on its own fill.
Tiny leftovers. Because the fee is taken in the coin you bought, a later sell of "everything" may leave a dust balance, or fall just short of a round quantity.
A different fee coin. Some exchanges let you pay fees with their own token at a discount — MEXC's MX deduction is one example. With that switched on, the fee line can show the exchange token instead of the coin you traded.
Convert is different. A Convert or instant-swap trade usually shows no fee line at all; its cost sits in the quoted rate instead — see crypto Convert and the spread.

How to check a specific trade

OKX's FAQ advises reading the fee line on the individual fill rather than assuming one fixed currency. On OKX's app, open the trade in your transaction history and look under Fill details; on the web, open Order history, select Details and hover over Fee to see how it was calculated. Other exchanges show the same information in their spot order or trade history — the fill detail lists the fee amount and the coin it was charged in. For a menu-by-menu guide, see where to see trading fees paid.

Futures work differently: on USDT-margined contracts the fee is taken from your margin balance, and on coin-margined contracts in the traded coin — see coin-margined vs USDT-margined fees.

Fee rebates on spot

A fee rebate is a share of the trading fee actually charged, calculated from the commission the exchange reports — the coin the fee was taken in doesn't change what was charged. Whether an exchange counts spot volume toward its affiliate commission is set by its own program, so if you trade spot, compare your rebate history with the fee lines on your fills.

Frequently asked questions

Why is my crypto balance smaller than the amount I bought?

Because the spot fee was deducted from the coin you bought. OKX states that when you buy on the order book the fee is charged in the crypto you bought, so you receive slightly less than the ordered quantity.

When I sell, which currency is the fee taken in?

From the proceeds. OKX charges the sell-side fee on the proceeds of the sale, so you receive slightly less of the quote currency, such as USDT, than quantity × price.

How do I see exactly how much fee was taken?

Open the fill details of the trade in your spot order or trade history. Each fill shows its own fee amount and the coin it was charged in; a single order can have several fills.