Slippage is a fee you don't see on the receipt
It doesn't show up as a fee line, but a bad fill costs just as much. Big orders in thin markets pay the most. Add it to your true cost with the futures fees calculator.
The difference between the price you saw and the price you paid
Slippage happens when your market order moves through the order book. On BTC/USDT with deep liquidity, a $10,000 market buy might execute at 0.01–0.02% above the quoted price. On a thin altcoin at $50k volume/day, a $5,000 buy could move the price 1–3% against you before filling.
Formula: slippage% = (actual_fill − quoted_price) ÷ quoted_price × 100. Negative for buys (you paid more than quoted), positive for sells (you received less). Combined with taker fees, total cost = fee% + slippage%.
Limit orders eliminate slippage but introduce non-fill risk — if price gaps past your limit, you don't get filled. For large positions on illiquid pairs: TWAP (time-weighted average price) orders split the trade into smaller chunks over time. The TWAP calculator shows estimated slippage reduction from splitting.
Related: true trade cost, maker vs taker, TWAP execution calculator.