Why Your Order Didn't Fill: FIFO vs Pro Rata Explained

The Invisible Rule Behind Every Fill

Imagine a busy tea stall. Three people are waiting, all willing to pay the same price. A customer walks up and buys two cups. Who gets served? The person who arrived first? Or everyone gets a splash, in proportion to how many cups they asked for?

That is essentially the choice exchanges make every microsecond. It is called the matching algorithm, and it decides which resting orders — your limit orders sitting in the book, waiting — actually get filled when a market order arrives. Two of the most common rules are FIFO and pro rata. They sound like plumbing. They are not. They quietly decide whether your strategy works or just looks good on paper.

FIFO: The Queue Everyone Understands

FIFO stands for First In, First Out. It is also called price-time priority, because two things matter: price first, then time of arrival. The best price wins; among orders at the same price, whoever got there earliest fills first.

If a market sell order for 50 lots arrives and there are three buy limit orders at that price, FIFO fills them in timestamp order. The earliest order gets filled completely, then the next, and so on until the incoming order runs out.

You already know this rule from daily life. It is the supermarket queue. As one research paper puts it, we queue for buses, we queue on customer service calls, and we generally find it fair because everyone follows the same waiting procedure. That instinct is why FIFO is the default on many exchanges, including CME equity index products like the E-mini S&P 500, most energy contracts, and the most liquid FX futures.

One practical quirk worth knowing: on CME Globex, if you modify an order — increase its size, change its price, or change the account number — it loses its timestamp and goes to the back of the queue. Under FIFO, your place in line is an asset. Treat it accordingly.

Pro Rata: Everyone Gets a Slice

Pro rata does not care when you arrived. It allocates the incoming order in proportion to the size of each resting order at that price level.

A simple example: three sell orders rest at the same price — 100 lots, 200 lots, and 50 lots — for a total of 350. A buy order for 350 arrives. Under pro rata, the first gets 100, the second gets 200, the third gets 50. Everyone fills, in proportion to what they posted.

Notice what this does to incentives. Under FIFO, being early is everything. Under pro rata, being big is what counts. The exchange's own documentation makes the point plainly: in the pro rata step, larger orders receive a larger percentage of a fill.

This is not a loophole. Each contract carries equal weight, so a trader with ten times the size gets ten times the fill — strictly proportional, nothing more. A 100-lot order among 1,000 resting lots receives 10% of the incoming quantity. Fair in principle. The trouble starts in practice.

Where Pro Rata Gets Messy

Contracts come in whole units. But proportions produce fractions. A trader with 100 lots out of 1,100, hit by a 100-lot market order, is entitled to about 9.09 contracts. There is no such thing as 0.09 of a contract, so the exchange needs a second rule to handle the leftover.

That is why pro rata is never the final step in a CME algorithm. It is always followed by a FIFO step to allocate the residual lots left over from rounding. A common design also sets a pro rata minimum — a threshold below which an allocation is simply rounded to zero. On CME's AUD/USD futures, that minimum is two lots. Imagine the small order that qualifies for nothing at all because the maths said 1.9.

Here is the subtle part. Whichever way the rounding rules lean, traders adapt. If the exchange rounds up and rewards many small orders, splitting a big order into single lots becomes attractive. Research on the London International Financial Futures Exchange's Time Pro Rata system for short-term interest rate contracts found just that: a theoretical case for heavy order splitting, followed by empirical evidence of more single-contract additions and cancellations. If it rounds down, size wins, and you get a size race instead.

So a rule designed to treat everyone proportionally ends up being something traders can tune their order sizes around — not through wrongdoing, simply through mathematics.

What It Means for Your Fill Quality

Fill quality is not one number. It is a bundle: how often you get filled, how much you get filled, and how confident you can be beforehand.

Under FIFO, your position is knowable. You can see how much volume sits ahead of you at your price. If you are first in line, you fill first. What you cannot rely on is that the queue works exactly as advertised at nanosecond scale — one academic analysis argues that hardware and networking limits mean exchanges cannot guarantee genuine first-in-first-out processing, and that unequal access to speed means not everyone has an equal shot at the front of the line.

Under pro rata, your expected fill is straightforward: your share of resting depth times the incoming quantity. What you cannot know is the size of any single fill. A resting order representing 10% of the level has a 10% chance per contract under a randomised variant. Across many trades the average lands where the proportion says, but each individual trade can swing widely. That is not a flaw; it is the nature of allocating fractions. It just means your position sizing has to survive a bad draw, not only the average one.

Hybrids: The Practical Reality

Most real markets do not pick a pure version. They blend.

CME's Threshold Pro Rata assigns a top-order allocation — often to the order that first improved the price — then allocates pro rata with a minimum, then hands any residual to FIFO. Split FIFO/Pro Rata divides quantity between the two by product-specific parameters. Lead Market Maker steps allocate a configurable slice to designated liquidity providers before FIFO begins; on CME, these apply to options rather than futures.

Pure FIFO still covers the largest share of activity — equity index, energy, and the major Treasury futures. But the short-term interest rate complex, agricultural products, and various FX and spread markets run on some pro rata variant. Same exchange, different rules, different behaviour. If you trade more than one product, you are playing more than one game.

Why This Matters Beyond the Fill

Matching rules shape behaviour in ways that ripple outward. FIFO rewards speed, which is why so much effort and money goes into shaving microseconds — one analysis describes institutions investing heavily to be faster than competitors. Pro rata dulls that particular race but creates its own incentive to post size, and to tune that size paper-thin around rounding thresholds.

This is why venues treat their algorithm as a design choice tied to the market's character. Low-volatility, liquidity-hungry products lean pro rata. High-volatility products lean FIFO. And exchanges generally do not change these rules casually, because traders build expectations around them.

It is also worth saying plainly: rules vary by exchange, by product, and sometimes by contract month. Anything you read about CME may not describe Eurex, and anything you read today may change after the next exchange notice. Check the specification for the specific instrument you trade. That is not a formality — it is the difference between understanding your fills and guessing at them.

The Takeaway in Plain Language

FIFO is the queue. Time is your asset, so avoid unnecessary modifications that cost you your timestamp, and understand that speed at the margin is contested territory. Pro rata is the pie. Size is your asset, but the slices come out uneven per trade, and rounding rules create incentives that are worth knowing before you size an order.

Neither is better. They are different grammars, and fluency in the one your market uses is not optional. The trader who understands the rule knows why a fill happened. The one who does not is left refreshing the screen, wondering what the tea stall is doing.