Partial fills: when half your order is a position and half is still an order
A partial fill means only part of your order quantity traded, and the rest is still sitting live as an open order. This happens because an order needs a counterparty at an acceptable price for every unit, and often there is only enough size at your price for part of your request. You end up holding a smaller position than you asked for, at an average price built from the pieces that filled, with a remainder that may fill later, fill worse, or never fill.
A partial fill means part of your order traded and part of it did not. The quantity that traded is now a real position you own, and the quantity that did not is still a live order sitting in the market, waiting.
That split is the whole thing. You are simultaneously in the trade and still trying to get into the trade. Everything confusing about partial fills, the odd average price, the position size that does not match your plan, the stop that leaves you still exposed, comes out of that one fact.
Why an order can only fill as far as the size in front of it
An order is not a request to a vault of infinite inventory. It is a request to match against other people’s orders, and those orders have specific sizes at specific prices.
At any moment, a market has resting orders stacked at various price levels. Suppose a book made of invented quantities, used here only to show the shape of the stack: someone is willing to sell 200 units at one price, someone else 150 units slightly higher, someone else 900 units higher still. When your buy order arrives, it consumes those resting offers from the cheapest upward until one of two things stops it: your quantity runs out, or your price limit runs out. In this made-up book, a buy for 300 units would take the 200 at the cheapest level and 100 of the 150 at the next, and stop there with nothing left to fill.
If your quantity runs out first, you are fully filled. If your price limit runs out first, you are partially filled. The matching engine took every unit it could at prices you accepted, then stopped, because the next available unit costs more than you said you would pay. The remainder does not evaporate. It becomes a resting order of its own, at your limit price, waiting for someone to come sell into it.
This is why order type governs how often you see partials. A limit order caps price and therefore accepts incomplete size: it is designed to stop rather than pay more. A market order caps nothing on price and therefore usually completes, by walking up the book to whatever levels it needs. You rarely get a partial on a market order in a liquid market. You get a worse average price instead, which is slippage. See market orders versus limit orders for how that trade-off plays out.
Three other things make partials more likely: thin books in the instrument you are trading, size that is large relative to what rests at each level, and moments when resting orders get pulled, which tends to happen around scheduled events and session changes.
What the remainder actually does, and what it does to your average price
The remainder’s fate is decided by one setting on your order: time in force. It has nothing to do with luck.
| Time in force | What happens to the unfilled part |
|---|---|
| Good till cancelled | Stays live indefinitely until it fills or you cancel it |
| Day order | Stays live until the session closes, then cancels automatically |
| Immediate or cancel | Fills whatever it can right now, cancels the rest instantly |
| Fill or kill | Fills completely or not at all, so no partial is possible |
The second effect is on your average price, and this is where people lose track. Your entry price is not the number you typed. It is the size-weighted average of every fill you received.
Suppose, and these numbers are invented purely to show the arithmetic, you want 1,000 units and you place a buy limit at 50. The book gives you 300 units at 49.80 and 200 units at 49.95. Then price moves up and leaves 500 units unfilled at your limit.
Your weighted average is ((300 × 49.80) + (200 × 49.95)) ÷ 500 = (14,940 + 9,990) ÷ 500 = 49.86.
So you hold 500 units at an average of 49.86, not 1,000 units at 50. Two consequences follow immediately. Your profit and loss per unit of price movement is half what you planned, because your position is half the size. And your risk calculation is now wrong in a direction that is easy to miss: if your stop was set to risk a fixed amount on 1,000 units, you are now risking roughly half that, which sounds harmless, but it also means your position sizing no longer matches the account-level plan you built. Rerunning the numbers through a position size calculator on the actual filled quantity is the fix, not an adjustment you do in your head.
The worse version is when the remainder fills later at a very different price. Say the last 500 units eventually fill at 50.00 after price drifted back. Your average becomes ((500 × 49.86) + (500 × 50.00)) ÷ 1,000 = 49.93. That is tolerable. But if you chased the remainder with a market order after a sharp move, that second block could fill well above your limit, and your average entry quietly becomes a price you would never have accepted for the whole position in the first place.
The mistake is treating a partial exit as an exit
Most people meet partial fills on entry, shrug, and move on. A smaller position than planned is an annoyance. The real damage happens on the way out, and the specific error is reading the fill notification as confirmation that you are flat.
Suppose, with made-up quantities again, that you placed a sell limit to close 1,000 units and only 400 of them filled. You are not out. 1,000 asked minus 400 filled leaves 600, so you are long 600 units, in a market that just showed you it does not have enough buyers at your price, which is usually the same market condition that made you want to leave. The 600 that did not fill are still a live sell order as well as a live position, so the same quantity shows up twice in your thinking and once in your actual exposure. The reason for the partial and the reason you wanted to exit are frequently the same reason.
The same trap applies to protective stops. A stop limit only protects you as far as resting liquidity reaches your limit price. In a fast move, the book can gap straight through that limit, filling a fraction of your size and leaving the rest exposed while price continues against you. The order looks like it worked. Part of it did. The remainder is a limit order sitting above a market that has already left, and it is doing nothing for you. How stop limit orders behave covers that mechanism in detail.
For leveraged positions this matters more, because residual exposure after a failed exit still counts fully toward your margin and your liquidation level. Half-closing does not half-remove the risk of forced closure if price keeps moving. A liquidation price calculator run on the remaining quantity tells you where you actually stand.
So the habit worth building is small: after any fill notification, read the filled quantity, not the fill event. Then ask two questions. How much do I still hold? And is there an order still live in the market that I have forgotten about? Those two numbers, not the confirmation message, describe your real position.
Education only, not financial advice. Trading carries a risk of loss.
Key facts
- A partial fill splits one order into two states at once: a live position for the filled quantity and a still-open order for the remainder.
- Partials happen because orders match against finite resting size at each price level, not against an unlimited pool.
- Limit orders tend to partial and stop; market orders tend to fill fully by walking to worse prices.
- Your average entry price is the size-weighted average of each individual fill, not the price you clicked.
- Time in force decides what happens to the remainder: stay open, cancel instantly, or cancel at session end.
- A partial fill on the way out leaves residual exposure, which is usually more damaging than a partial entry.
Frequently asked
Do I pay fees twice if my order fills in several pieces?
Fees are usually charged per executed quantity rather than per order, so splitting into pieces does not normally multiply a percentage fee. The exception is any flat per-execution charge, which does scale with the number of fills. If a venue applies a flat amount per fill, five fills cost five times that amount, so check the structure of your fee rather than the number on your ticket.
Should I cancel the unfilled part of my order or leave it working?
The question is whether the original reason for the trade still applies to the remaining size. If price has moved away from your plan, the remainder is now an order you would not place fresh today, so cancelling is consistent. If the setup is intact and you still want the full exposure, leaving it working is reasonable, but treat it as a live decision rather than something you forget about.
Why did my stop order only partly fill?
A stop becomes an active order once triggered, and that active order still has to find size. If it becomes a limit order, it fills only as far as resting liquidity reaches your limit, leaving the rest exposed. If it becomes a market order, it normally fills fully but at progressively worse prices, which is slippage rather than a partial.
Does a partial fill mean my broker did something wrong?
No. A partial fill is the normal result of finite size at a price level, and it is often the honest outcome: you got the quantity that genuinely existed at your price instead of a worse average. What matters is whether the remainder was handled according to the time in force you selected and whether each fill price was consistent with the book at that moment.
Risk warning: trading is high risk and most retail accounts lose money. Education only, not financial advice.