OKX spot market order size: tgtCcy, quote vs base currency

Spot market orders look simple, but the unit of the size field is easy to get wrong. A request that was meant to buy 0.01 BTC can spend 0.01 USDT, or one that was meant to spend 100 USDT can try to buy 100 BTC. Both exchanges that matter here, OKX and Bybit, default to a quote-currency amount for market buys.

How does tgtCcy work on OKX?

In POST /api/v5/trade/order, sz for a spot market order is set by tgtCcy. The OKX documentation says it applies to spot market orders only, and “Default is quote_ccy for buy, base_ccy for sell”.

For BTC-USDT:

Side	tgtCcy	sz	Result
Buy	quote_ccy (default)	100	Spend 100 USDT on BTC
Buy	base_ccy	0.001	Buy 0.001 BTC at market
Sell	base_ccy (default)	0.001	Sell 0.001 BTC
Sell	quote_ccy	100	Sell BTC until you receive 100 USDT

Limit orders are different. For spot and margin limit orders, sz is always the quantity in base currency. Margin market buys use quote currency and margin market sells use base currency.

What happens if the amount does not fit the balance?

OKX documents two cases for the non-default combinations (a base-currency market buy or a quote-currency market sell):

• If the amount you enter is greater than what you can buy or sell, OKX executes the order at your maximum buyable or sellable quantity. Use a limit order if you need the exact quantity.
• If the price moves while the balance is locked, OKX adjusts the quantity on a best-effort basis from your balance. It tries to lock a little extra to avoid this.

Attaching TP/SL to the order is not supported for a market buy with tgtCcy=base_ccy or a market sell with tgtCcy=quote_ccy.

What do accFillSz and fillSz mean?

Order endpoints and the WebSocket order channel return both fields:

• fillSz is the quantity of the most recent individual fill. It is not cumulative.
• accFillSz is the running total of filled quantity since the order was created. In the WebSocket order channel it always holds the cumulative total, not the increment since the last push.

On spot and margin the unit of both fields is the base currency. On futures and swaps the unit is contracts. Order rows also carry tgtCcy, so read it together with sz: for a quote-sized market buy, sz is in USDT while accFillSz is in BTC, and the two numbers cannot be compared directly. For the filled amount of an order, use accFillSz.

Does Bybit work the same way?

Mostly. For Bybit V5 spot, the create-order documentation says a market buy is placed by value by default. The marketUnit parameter picks the unit for qty on spot market orders:

marketUnit	qty unit
baseCoin	Base coin, for example BTC on BTCUSDT
quoteCoin	Quote coin, for example USDT

Set it explicitly when you want a fixed amount of the base coin. See the Bybit available balance guide for what balance a spot order can use.

How Ninja Terminal handles this

Ninja Terminal sends the unit explicitly instead of relying on exchange defaults:

• OKX spot market orders are sent with tgtCcy set, to base_ccy unless a quote-sized market order is requested.
• Bybit spot market orders are sent with marketUnit, set to baseCoin unless a quote-sized plain market order is requested.
• Spot market orders from Quick Entry and the order ticket reach both exchanges in base-coin units by default.
• Filled quantity on OKX orders is read from accFillSz, so partial fills show the running total.

A market order fills at the best available prices and can fill worse than the last price. See slippage. This guide is not investment advice.