Market Orders
The simplest order type: what a market order guarantees, what it doesn’t, and how slippage happens when an order walks the order book.
Speed over price
A market order tells your broker to buy or sell right away at the best price currently available. It is the simplest order there is, and in a heavily traded stock during regular hours it usually fills in a fraction of a second.
The trade-off is in the name. A market order guarantees that you get filled, but it does not guarantee the price. You accept whatever the market offers at that moment.
Where the fill comes from
When you buy at market, your order is matched against the sellers waiting on the ask side of the order book, starting with the lowest ask. When you sell at market, you are matched against the highest bid. If your order is bigger than the shares available at the best price, it keeps going to the next price level, and the next, until it is fully filled. This is called walking the book.
Slippage in numbers
Slippage is the difference between the price you expected and the price you actually got. Suppose the best ask is $50.02 for 200 shares, then $50.03 for 300 shares, then $50.05 for 500 shares, and you send a market order to buy 700 shares.
| Price level | Shares filled | Cost |
|---|---|---|
| $50.02 | 200 | $10,004 |
| $50.03 | 300 | $15,009 |
| $50.05 | 200 | $10,010 |
| Total | 700 | $35,023 (average ≈ $50.03) |
An illustrative order book. In very liquid stocks the effect is usually tiny; in thin stocks it can be large.
When market orders make sense
- Heavily traded stocks and ETFs with tight bid-ask spreads, during regular hours.
- Small orders relative to the shares available at the best prices.
- Situations where getting out matters more than the exact price, such as exiting a trade that has hit your stop.
When to avoid them
Market orders are riskiest when the order book is thin or moving fast. That includes small, lightly traded stocks, the first few minutes after the open, the moments right after big news, and extended-hours sessions, when fewer participants are trading. In these conditions the spread can be wide and the next price level can be far away, so a market order can fill at a surprisingly bad price.
A common rule of thumb is simple: if you would be upset by a fill a few percent away from the last price you saw, use a limit order instead.
The spread is a cost too
Even without slippage, buying at the ask and later selling at the bid means you pay the spread. In a stock with a bid of $19.95 and an ask of $20.05, the spread is $0.10, or about 0.5% of the price. Round trip, you start the trade slightly behind. For frequent traders, spreads add up, which is why many prefer very liquid stocks.
Checklist before you hit “buy at market”
- Check the bid, the ask and the spread.
- Compare your order size with the average daily volume.
- Avoid the first minutes after the open and moments right after news if you can.
- Consider a limit order if the stock is thinly traded or the spread is wide.
Market orders in fast markets
In a fast market, such as right after major news, the price you see on screen may already be out of date by the time your order arrives. A market order will still fill, but possibly well away from the last quote. That is when a marketable limit order, a limit slightly above the ask, gives you speed with a safety net.
Common mistakes
- Using market orders in thinly traded stocks.
- Placing market orders before the open, where they fill at the opening price, whatever it turns out to be.
- Selling at market in a panic during a sharp drop.
- Ignoring the spread when working out the true cost of a trade.
A sensible default
For liquid, heavily traded stocks during regular hours, market orders are usually fine. For everything else, a limit order is the safer default. Many brokers show an estimated cost before you submit, so take a second to check it. When in doubt, use a limit.
Key takeaways
- A market order fills right away at the best available price.
- It guarantees execution, not price.
- Large orders can walk the book and suffer slippage.
- Use market orders mainly in liquid stocks during regular hours.
- Avoid them in thin stocks, fast markets, and extended hours.