- A simple paper fill is priced at the level that triggered it, while a live order fills against whatever the order book holds at the moment it arrives.
- The largest gap sits on stop-losses, because a live stop becomes a market order and can fill well past its level in a fast move, so paper results flatter the exit a risk plan depends on.
- Mark paper results down per order type rather than by one percentage, since a strategy with a thin paper edge can lose money live once costs and stop gaps are priced in.
A paper account is at its most generous on the exit you hoped you would never need. Open your trade history and find the stop-loss exits. If every one filled at exactly its stop price, you were never charged for the costliest move a live market makes, when price falls through your stop faster than anyone will buy from you.
So does paper trading include slippage? Partly. Some simulators add a modeled cost to market orders, the slippage you accept the moment you click. Everything else tends to be priced at the level that triggered it. And no simulator can push your order through a real order book, which is where a live fill gets its price.
We'll follow Eunha through her own history instead. She reads markets from the seam between structure and emotion, and she gets to the truth by asking questions until one of them comes back with a number. When live results trail paper ones, the verdict people reach for is nerves, and it is a heavy one to carry. Eunha checks the fills first. Before she calls any part of the gap psychology, she wants to know how much was mechanics. So she walks each kind of order through a live book. Then she prices what the gap does to a strategy.
Three months of her breakout strategy sit in the simulator's trade log. The account charged her on the way in but not on the way out. Each market-order entry filled a little above the quote, and the running slippage total in her portfolio grew with it. Each stop-loss exit printed at exactly the stop she set. So she writes one question at the top of a notepad. Which of these fills could a live market have refused?
How does paper trading price a fill?
Eunha sorts the log into three piles before she judges any of it: market orders, limit orders and stop-losses. The piles matter, since each was priced by a different shortcut. A paper engine has nobody to trade against. So it prices every fill from a number it already holds. Market orders get the quoted or last traded price, sometimes with a modeled cost on top, while limit orders and stops fill at their own price the moment the market reaches it.
All three shortcuts share one assumption: the price on the screen was a price somebody would have given you. A live market makes no such promise. It fills your order from whatever is resting in the order book at the instant the order arrives. Slippage is the distance between the price you expected and the price that book actually paid you.
Eunha taps the stop-loss pile. "Every one of these is a price I saw. I want to know which ones I could have had," she says.
Her question splits the piles along a line the log hides. A market order always fills live, just at a worse price when the book is thin. A limit order may never fill at all. A stop, once triggered, is all but certain to fill, and its price is the one part nobody guaranteed.
A market order pays for its size, not the quote
Only the market pile was charged for slippage, which makes its cost the easiest to see. A quote is not the price of your order but the price of its first unit. The rest is priced by the offers stacked behind it. Take an illustrative book around a $100 midpoint, with 20 units offered at $100.05, 30 more at $100.15 and 50 at $100.40. A 100-unit market buy clears all three levels and averages $100.255. That is a quarter of a percent above the midpoint. On a book with 200 units at the best offer, the same order pays $100.05 for every unit, half the ten-cent spread and nothing more.
Nothing about the order changed between those two books except the depth, and the depth decided the fill.
So Eunha writes down the rule a live order follows anyway. You add up the offers, or the bids when you sell, down to the worst price you would accept. If your size doesn't fit, the order fills past it. How deep the book is at that instant settles the rest.
The extreme version played out on 22 September 2026. A market sell of roughly $8 million in USDe hit a shallow book on Binance's USDE/USDT pair. It printed $0.9202, about 8% below the dollar the token is built to track. Within minutes it was back near $0.9996, Cryptopolitan reported. A market order fills against the one book it is sent to, at whatever depth that book holds in that second. For a few minutes, that book held very little.
Eunha's own entries were small against deep books, and each one already carries a modeled cost. A live market would have refused none of them. It would only have charged more on a thinner day.
A touched limit order is not a filled one
Eunha trusts the limit pile less, even though it looks cleaner. Several of her limit entries filled at a candle's exact low, the tick where price touched her level and turned. Live, that touch would only have started a queue. On a standard exchange book, orders resting at the same price fill in the order they arrived. A touch fills the orders placed at your level before yours, and if the selling runs out before the line reaches you, price leaves without you.
That error has a direction, which makes it expensive. The touches that turn right at your level are the trades a paper account books as its best entries. A live queue is likeliest to skip them. The fills you can count on live come when price trades through your level, which means it is still moving against you. Miss enough of those touches and the live strategy keeps its losing fills while its best winners happen without it.
"So my best paper entries are my least certain live ones," Eunha says, and circles four of them.
Her rule for this pile is strict. A paper limit fill counts only where price traded through her level. A bare touch gets marked as a trade that may never have happened.
Why does a stop-loss fill below the stop?
Eunha's question gets its sharpest answer in the stop pile, because a live stop almost always fills. What it cannot promise is where. "A stop order becomes a market order once the stop price is reached," FINRA's guidance on stop orders explains. The price you receive, it adds, "could be markedly different than your stop price" when the market moves fast. A stop is not a promise of a price but an instruction to trade at whatever price comes next.
Eunha puts numbers on it. Her example entry is $100, with a stop at $98, a 2% risk. A fast move trades straight through $98, and the next price anyone will pay is $97.50. The live exit fills there, for a 2.5% loss, a quarter more than the plan allowed. If that stop was sized to lose 1% of the account, the account just lost 1.25%. That sizing rule capped the damage on paper only. For a short, the same thing happens upward, with the stop filling above its level. The order is called a stop, which describes the plan better than the outcome.
A paper account books that exit at exactly $98.
Paper flatters risk control here more than anywhere else, because a risk plan is built on this one number. Underneath it sits a transfer of control that no order ticket shows. You choose the level. Whoever is still bidding when price gets there chooses the price.
How much slippage should you add to paper trading results?
Whether the strategy goes live at all comes down to Eunha's next question: how much of her edge survives the three piles? She strips the strategy down to round numbers. It has a 2% target, a 2% stop and a 55% win rate. Before any costs, that earns 0.20% per trade.
Then she charges 0.15% a round trip for fees and spread. Wins shrink to 1.85% and losses grow to 2.15%. The expectancy falls to 0.05% per trade, and the break-even win rate climbs from 50% to 53.75%, so a strategy that looked comfortable now rests on a margin of a point and a quarter. Next she lets half of her stop exits fill 0.5% past the stop, the way the fast ones would have. The average loss becomes 2.40%, and the expectancy turns negative, about 0.06% lost per trade. Break-even moves to roughly 56.5%.
The same rules that make money on paper lose it live, and not one entry changed.
The gap isn't always that large. One public live log shows the calm end of it. In a 2020 comparison of paper and live fills, a quant ran a medium-frequency mean-reversion strategy on Bybit. Across 23 buys and 28 sells, volume-weighted slippage was "around 0.000557 %", effectively zero. Live profit came in at 0.08867% per trade against the simulation's 0.1%, which the author read as about what the simulation predicted. The author's own caveat is that the strategy "still needs to trade for longer and also during times of serious market stress," which is exactly when stops get expensive.
Small orders in a quiet, liquid market keep it small. A stop in a fast market does not, and neither does a size that outruns the book.
The mark-down can't be a single percentage, then. One number would overcharge her calm market entries and still flatter her stops, so Eunha writes a separate rule for each pile:
| Order | How a simple paper fill is priced | How it fills live | How to mark the paper result |
|---|---|---|---|
| Market | At the quoted or last price, sometimes plus a modeled cost | Half the spread, plus your size walked through the book | Add half the spread plus what your size would take from the visible book |
| Limit | At your price, as soon as price touches it | Behind the orders already queued at that price, or not at all | Count it only if price traded through your level |
| Stop | At the stop price | As a market order at the next available price | Use the worst price of the minute that triggered it: the low for a long, the high for a short |
Re-marked that way, her record loses its best-looking trades first. The limit entries that touched and turned go, and so do the stop exits that printed at a level the market skipped. What remains is smaller and more honest. It is also the only version of the record worth sizing a live account on.
What the simulator charges, and where it is still generous
Every mark she has made corrects a fill the simulator's own rules wrote. The simulator is not a book but a model of one. It charges market orders a modeled slippage that scales with trade size and liquidity tier. Before you confirm, the ticket shows that charge in a Total Trading Cost line that lists the trading fee and the slippage, with the coin's liquidity tier beneath them. The portfolio keeps a running Slippage total, so her market entries never hid their cost.
Because it estimates depth rather than reading a live book, the simulator can't reproduce a one-off print like USDe's. Other order types don't even get the estimate: limits, stops and take-profits pay no slippage and fill at exactly the price you set once the simulator's price check finds the level crossed. So a stop that a fast market skips past still fills at the stop here. A wick between checks may not trigger it at all.
Paper never runs out of buyers at your stop, which is its least realistic habit and its most comfortable one.
On market orders, the simulator's cost rises with size and on thinner coins, the same direction a live cost moves. On stops it hands you the best case, and every simulated stop exit should be read as one.
Paper trading trains decisions, not fills
None of this makes Eunha's three months worthless. The account rehearsed her decisions: where she entered, where the stop went, and whether she honored it when it hurt. That is what paper trading trains, and it trained them well. What paper cannot rehearse is not the decision but the counterparty. Slippage is the price the other side of a live trade charges for being there at the moment you need to leave. A paper account has nobody on the other side.
She closes the notepad with the question still on top. The strategy that goes live will be whatever still earns after every fill a live market could have refused has been marked down.
If nothing survives the marks, the paper account did its best work early: it lost the money before any of it was real.
Before your next paper trade, price the same $500 market buy twice without confirming either one: once on a coin the ticket rates high liquidity, once on a coin it rates low. Read the Total Trading Cost line on both, then try four times the size and see which percentage climbs. Nothing fills until you confirm, and the $5,000 balance you start with is simulated.
