TL;DRWhen your order is executed by the exchange. Your fill price is the actual price at which the transaction occurred.
A fill is the execution of your order. When your buy order matches with a seller on the exchange, you've been filled. Your fill price is the exact price of the transaction.
Traders use fill as both noun and verb: 'I got a fill at 5,200' or 'I'm waiting for my limit to fill.'
Market orders fill immediately at the best available price. Limit orders fill only at your price or better and might not fill at all. Stop orders don't fill until triggered, then become market orders.
Partial fills occur when there isn't enough volume at a price for your entire order. Buying 5 contracts when only 3 are available at your price gives a partial fill of 3.
A good fill matches or beats your expected price. Fill quality depends on market conditions (RTH vs. overnight), liquidity, and broker routing.
Direct market access (DMA) brokers send orders directly to the exchange. Brokers routing through intermediaries may add delay, which matters in fast markets.
Clean limit fill
Buy limit at 5,195. ES trades down to 5,195 and fills.
Filled at your exact price. Ideal limit order scenario.
Partial fill
Buy limit for 10 contracts at 5,195. Only 6 available at that price.
Partial fill of 6 at 5,195. Remaining 4 stay working. If price moves away, you have a smaller position than planned.
Assuming a fill means the trade was a good idea
Getting filled just means the order executed. It says nothing about whether the trade will be profitable.
Not checking fill price after execution
Especially with market and stop orders, your fill may differ from expected. Always verify.
Chasing after a missed limit fill
A missed fill is not a loss. Don't switch to a market order at a worse price out of frustration.