Partial Fill

Context & Mechanics

Definition and cause

A partial fill arises when the order book holds less counter-volume at the desired price than the order demands. Example: limit buy of 1,000 shares at $50, but the ask side offers only 400 shares at that price – 400 are filled, 600 remain in the book as an open residual order. This particularly affects limit orders in illiquid instruments, orders that are large relative to typical volume and fast markets in which liquidity vanishes between two ticks.

Order qualifiers control the behaviour

What happens to the remainder is determined by time-in-force qualifiers: GTC (good-till-cancelled) leaves the rest in the book until it fills or is cancelled. IOC (immediate-or-cancel) executes what is possible immediately and deletes the rest. FOK (fill-or-kill) accepts only complete execution – otherwise nothing happens. The choice is strategic: IOC avoids forgotten residual orders, FOK protects against dwarf positions, GTC maximises the fill chance at the cost of control.

Practical consequences

Three effects are frequently overlooked: first, position sizing no longer matches – planning $1,000 risk on 1,000 shares but holding 400 means only $400 risk, distorting the R calculation. Second, multiple partial executions can cost multiple minimum commissions depending on the fee model. Third, stop and target orders must be adjusted to the real share count – especially critical with OCO brackets, whose platform behaviour on partial fills varies.

Why it matters for traders

Partial fills are not an error but market mechanics – the key is planning for them in the workflow: aligning order size with real liquidity, choosing time-in-force deliberately, checking fills. In the GlanWick simulator it can be traced by way of example how order size and book depth influence execution – GlanWick is a training and simulation tool and not a prop firm itself.

Execution Example

On a $100,000 account a trader plans to buy 1,000 shares of a thinly traded small-cap stock at a $50 limit (planned risk $1,000 with a stop at $49). Only 400 shares are offered at the limit.

Execution: 400 shares fill at $50 – partial fill; 600 shares remain in the book as a GTC remainder.
Risk check: instead of $1,000 the real risk is only $400 (0.4R) – the stop order must be adjusted from 1,000 to 400 shares, otherwise it would sell 600 shares short.
Later fill: price returns after two hours and fills the remaining 600 shares – the position is now complete, but time-shifted and with a second commission.
Alternative: with IOC the remainder would have been deleted immediately (a deliberate 400-share position), with FOK nothing would have filled at all – the choice of qualifier is part of the setup.

Execution Risk & Errors

Not adjusting stop and target orders to the real share count after a partial fill

Forgetting open GTC remainders that fill unexpectedly hours later

Choosing order sizes that are a multiple of typical book depth

Not using time-in-force qualifiers (GTC/IOC/FOK) deliberately

Overlooking multiple commissions on split executions in the fee model

Frequently Asked

Why is my order only partially executed?

Because there is not enough counter-volume in the order book at the execution price. Mainly affected are limit orders in illiquid instruments and orders that are large relative to book depth.

What happens to the unexecuted remainder?

The time-in-force qualifier decides: GTC leaves it in the book, IOC deletes it immediately, FOK prevents any partial execution from the outset.

Do I have to adjust my stop after a partial fill?

Yes – to the actually filled share count. A stop for the planned full size would open an unwanted short position when triggered.

How do I avoid partial fills?

Align order size with real liquidity, choose liquid trading hours, split large orders or use FOK when only complete execution makes sense.