What Is a Reserve (Iceberg) Order?
Most order types show their full size to anyone watching a stock’s order book. A reserve order deliberately doesn’t, revealing only a slice of the total at a time.
The short answer
A reserve order, often called an iceberg order, is a large order broken up so that only a portion of the total quantity is visible on the public order book at any given moment. As each visible slice fills, another slice from the hidden portion is automatically displayed, continuing until the full order is complete or canceled. The purpose is to avoid signaling the true size of a large trade to the rest of the market.
Why hiding size can matter
If other market participants see a very large order sitting on the book, they may adjust their own buying or selling behavior in response, sometimes pushing the price away from the large order before it can be fully filled. This is a version of the market impact concern that also motivates not-held orders, where a broker gets discretion over timing specifically to reduce that same kind of signaling effect. A reserve order tackles the problem differently: instead of relying on a broker’s judgment, it uses an order type that automatically limits how much size is shown at once.
How the mechanics generally work
A trader placing a reserve order specifies the total quantity along with a smaller “display” amount that’s allowed to show publicly. Only that display amount appears in the market order or limit order book that other participants can see, and it typically operates similarly to a limit order in that it won’t execute below or above the trader’s specified price. Once that visible slice fills, a new slice of the same display size is shown, drawn from the remaining hidden quantity, and the process repeats until the whole order is done. From the outside, it can look like a series of smaller trades rather than one large one.
Who tends to use reserve orders
- Traders moving large positions. Anyone trying to buy or sell a substantial number of shares without visibly moving the market benefits most from this structure.
- Institutional-style trading desks. Reserve orders are more commonly available through platforms built for larger or more active trading rather than typical retail brokerage tools.
- Traders in less liquid securities. Stocks with thinner trading volume are more susceptible to price swings from a visibly large order, making concealment more valuable there.
How this relates to routing decisions
A reserve order still has to be routed and executed like any other order once each visible slice is active, so it works alongside, rather than instead of, the usual order routing process a brokerage follows. The concealment happens at the display level, controlling what other participants can see, while the underlying execution mechanics for each slice proceed the same way they would for a standalone order of that smaller size.
What to weigh
A reserve order trades some transparency for protection against market impact, which can be a reasonable trade-off for a trade large enough to move a stock’s price on its own. For an everyday, modestly sized trade, the concealment this order type offers usually isn’t necessary, since ordinary order sizes rarely move the market meaningfully in the first place. Recognizing which problem an order type is designed to solve is generally more useful than assuming a more complex order is automatically a better one.