Why Your Limit Order Didn’t Fill: Price, Queue and Partial Fills
zhan chenA touched price is not a promised fill. Recalculate a 500 share order, trace queue priority and distinguish a price cap from execution certainty and total trading cost.
A stock trades at your limit price, yet your order remains open. That is not, by itself, evidence of an execution error. A limit specifies an acceptable price; it does not reserve the available shares or put your order at the front of the queue. The useful question is not just “Did the chart touch my price?” but “Was eligible liquidity available to this order, at this venue, after it became active?”
The distinction is measurable. In the example below, a 500 share buy order executes only 250 shares despite offering more than the best displayed ask. A separate queue example shows why hundreds of shares can trade at your price without filling a single share of your order.
Start with the quote you can actually trade against
A buyer seeking immediate execution normally meets sell offers, or asks; a seller meets bids. The last trade is a historical transaction. A midpoint is a calculation. Neither is necessarily a current executable price for your size. A candle’s low is especially weak evidence: it says a price was recorded during an interval, not that enough stock was offered to your order after it joined the market.
Assume a hypothetical dollar denominated stock in a single continuous trading order book. The displayed offers are unchanged while an order arrives. Ignore fees initially, and assume no other orders, cancellations or special allocation rules intervene. Each scenario starts from the same book; these are alternatives, not consecutive orders.
| Ask price | Shares offered | Cumulative shares |
|---|---|---|
| $100.00 | 100 | 100 |
| $100.02 | 150 | 250 |
| $100.05 | 250 | 500 |
A price ceiling is not an instruction to pay that price
A buy limit for 500 shares at $100.02 can take 100 shares at $100.00 and 150 at $100.02. The consideration is $25,003, giving an average fill price of $100.012. The other 250 shares cannot execute against the $100.05 offer under that limit. Whether they remain working or are cancelled depends on the order’s duration and the venue’s rules.
A market order for 500 shares, under these static assumptions, consumes all three levels: $10,000 + $15,003 + $25,012.50 = $50,015.50. Its average is $100.031. Relative to buying all 500 at the initial best ask, the difference is $15.50, or 3.1 basis points of the $50,000 reference amount. This is an illustration of walking the book, not a forecast of real execution: quotes can change before the order arrives.
A buy limit at $100.05 also fills all 500 shares in this example at an average of $100.031. It does not instruct the venue to pay $100.05 for every share. This is a marketable limit: it can trade immediately against eligible offers while retaining a price ceiling. If those offers disappear, however, the ceiling can leave part or all of the order unfilled. A market order removes that particular ceiling, not every obstacle to execution; trading halts and absent liquidity still matter.
The queue can explain a trade at your price
Now consider a different, explicitly first in, first out queue. Your 200 share buy order rests at $99.98 behind 800 eligible shares at the same price. Assume there are no priority changes, cancellations or hidden orders ahead. Incoming sales of 600 shares execute at exactly that price on exactly that venue. All 600 go to earlier orders. Your fill is zero and 200 shares still stand ahead of you.
A further 300 shares sell into that queue. The first 200 clear the remaining earlier interest; the next 100 reach you. You have bought 100 shares and still have 100 outstanding. The two transactions show why a price touch does not prove a missed fill, and why even substantial reported volume need not clear your position.
Do not turn this example into a universal exchange rule. Some products use proportional allocation or hybrid algorithms rather than pure time priority. A price or quantity amendment may also change priority, depending on the venue. Trades reported elsewhere do not automatically deplete your queue. For the separate distinction between reported activity and the data behind it, see FastBull’s guide to trading volume and relative volume.
Duration controls the remainder; fees change the cash cost
Return to the 500 share order capped at $100.02. An immediate or cancel instruction, where supported, can fill the available 250 and cancel the remaining 250. A fill or kill instruction requiring all 500 immediately within the limit instead cancels the entire order if that full quantity is unavailable. A day order can leave a remainder working until its applicable expiry. Extended hours eligibility and broker handling must be checked separately.
The limit also applies to execution prices, not necessarily to the final fee inclusive unit cost. Add an assumed $3 charge to the 250 share partial fill: ($25,003 + $3) ÷ 250 = $100.024 per share. That exceeds the $100.02 limit even though no individual trade breached it. Actual fees may be charged per order, per execution or on another basis; use the confirmation rather than assuming this illustrative fee schedule applies.
Diagnose before changing the order
- Confirm acceptance, not merely submission. Record the order identifier, acknowledgement time, venue or routing instruction, side, limit and quantity. Rejection, a pending state and an active order are different outcomes.
- Match the relevant quote side and time. Check available size after acknowledgement, not a chart print from before the order existed. Verify whether the chart and the execution venue share the same data.
- Read duration and session conditions. Check expiry, immediate or cancel, fill or kill, and whether the order can trade outside the regular session.
- Reconcile each execution and the remaining quantity. Include amendments and any priority consequences. A cancellation request is not a cancellation acknowledgement: check for intervening fills before placing a replacement that could double the intended exposure.
- Separate price from total cost. Recalculate the weighted average fill and add the applicable fees. Preserve the execution report if the result still conflicts with the stated order rules.
Over the counter FX and CFDs require an additional check: a broker’s quote feed is not a consolidated exchange order book. Do not transplant a stock exchange queue calculation to a different execution model without establishing its rules.
A better fill is not automatically a better trade
A passive limit can miss a move entirely. It can also fill just as new information pushes the market through the price. Avoided spread cost is therefore not a guaranteed advantage. Decide in advance which matters more for the specific task: the maximum acceptable price, the minimum required quantity or the deadline. Changing the cap because a candle touched it is not a substitute for that decision.
The same discipline applies to backtests. Treating every candle touch as a complete limit fill assumes away queue position, depth and arrival time. Bars alone cannot establish those facts. Where detailed data are unavailable, label the fill assumption and test less generous alternatives; otherwise an apparent strategy edge may simply be an execution assumption.
