Limit Order

A limit order is an order to buy or sell a security at a specific price. The Securities and Exchange Commission's investor education states what follows from that in a single sentence, which is worth quoting because the asymmetry is where the confusion lives. "A buy limit order can only be executed at the limit price or lower, and a sell limit order can only be executed at the limit price or higher."

Read that as two different objects. On a purchase, the limit is the most you are willing to pay, so the order can fill below it and never above it. On a sale, the limit is the least you are willing to accept, so the order can fill above it and never below it. In both directions the limit protects you from a worse price and leaves the better price available. What it cannot do is make the trade happen. If nobody is willing to meet the price, the order simply sits there, and an order that sits there is not a position.

The thing a limit order buys you is a price, and the thing it costs you is certainty. Those are the only two variables, and every order type is a choice between them. A market order trades certainty of execution for whatever price is available at the moment it reaches the market. A limit order does the opposite. The choice therefore depends on which risk is the one that actually hurts in the situation at hand, and that varies more than people expect. Buying a widely traded fund with money that has to be invested this week, the risk of not trading is the larger one. Buying a thinly traded security whose quoted price moves several percent between trades, the risk of an unexpected price is larger.

A limit order is a standing instruction, and its duration is a separate decision that is being made whether or not you make it. The SEC states the default: "Unless an investor specifies a time frame for the expiration of an order, orders to buy and sell a stock are 'Day' orders, meaning they are good only during that trading day," and a day order that does not execute expires rather than carrying into after-hours or the next session. So an unfilled limit order does not quietly wait for weeks unless it was entered to. Brokerage platforms offer longer durations and other qualifications, and each of those is a further instruction with its own consequences.

Partial fills are ordinary and are the case people fail to plan for. An order for 500 shares can be met by a counterparty offering 200 at an acceptable price, leaving 300 outstanding. The result is a position half the intended size, and, on a platform charging per trade, potentially more than one charge. The general point is that a limit order describes conditions rather than an outcome, so the outcome can be none of it, some of it, or all of it.

A limit order is not a stop-loss order, and treating them as variations of one another is the most consequential mix-up in this area. A stop order is a different instrument with its own trigger mechanics and its own behavior once triggered, covered on its own page. The distinction that matters here is that a limit order is visible as a willingness to trade at a price you have chosen, while a stop order is an instruction that does nothing until a condition occurs. Assuming a stop behaves like a limit, or the reverse, is how an investor ends up believing they are protected in a way they are not.

Where the order goes after you enter it is a further subject. Retail orders are routed by the broker to a venue, and the arrangements governing that routing, along with the mechanics of how resting orders are displayed and matched, sit with their own terms. None of it changes the rule above, which is the part a reader needs before pressing anything.

You choose a side, a quantity, a limit price and a duration. The order goes to the market and rests there as a visible willingness to trade at that price or better. If a counterparty is prepared to meet it, some or all of the order executes. If the duration expires first, the order is canceled and nothing has happened.

A hypothetical illustration of the "or better" half, which is the part that is usually assumed away. A stock is quoted around $50.00 and Theo enters a buy limit at $48.00 for 100 shares. Two things can happen and both are normal. If the price drifts to $48.00 and a seller meets him there, he pays $4,800. If instead the price falls further and the best available offer when his order is reached is $46.50, he pays $4,650, because the limit sets the maximum and not the price. What cannot happen is a fill at $48.01.

The same order on the other side. Theo also holds 100 shares he would sell at $55.00 and enters a sell limit there. If the price reaches $55.00 he receives $5,500; if the market is offering $56.20 when his order is reached, he receives $5,620. And if the price never gets there, he still owns the shares, which is the outcome the limit price quietly chose. All figures are illustrative.

Pros

• You know the worst price you can receive before the order is entered, which no market order can offer.
• The order can fill at a better price than the limit, so the protection costs nothing when the market moves your way.
• It is the natural tool for a thinly traded security, where the gap between the quoted price and the achievable price is widest.
• It allows a decision made calmly to be executed later without being remade at the moment of trading.

Cons

• The order may never execute, and an investor who believes they have bought something has not.
• A partial fill leaves a position smaller than intended and may cost more than one charge on a platform that bills per trade.
• A limit set close to the market can miss by a cent and a limit set far from it may sit unfilled for the whole duration, and neither is a prediction problem the order can solve.
• In a fast-moving market the price can pass through the limit without the order being reached, so avoiding a bad fill and missing a good one look identical afterward.