Slippage is when you click Buy at 60¢ and your order fills at an average of 63¢. It happens with market orders on markets where few contracts are offered at the top price, so your order spills over into worse prices. Volume tells you how much a market has traded, which hints at liquidity, but the order book depth is what actually predicts slippage. The fix is boring and effective: use limit orders.
What slippage is
Kalshi matches your order against other users' orders in an order book. At any moment there's a best price with some number of contracts offered at it, then a worse price with more, and so on. A market order says "fill me now at whatever's there." If you want 200 contracts and only 50 are offered at 60¢, you get those 50, then 100 at 62¢, then 50 at 65¢. Your displayed price was 60¢; your average fill was 62.75¢. That gap is slippage.
It isn't a fee and it isn't Kalshi taking something. It's the cost of demanding immediate execution in a market where not many people are offering.
Why it's worse on some markets
- Thin markets. A niche market (a specific player prop, a minor political question) might have a few dozen contracts at each price level. A big election or NFL game has thousands.
- Near expiry or during news. Prices move fast, orders get pulled, and the book thins out.
- Size. 10 contracts almost never slip. 1,000 contracts often do.
What volume tells you, and doesn't
Each market shows volume: the total contracts traded, counted at $1 notional each. High volume generally means an active market with tighter spreads, so it's a decent first filter. But volume is history. It doesn't tell you how many contracts are offered right now at the price you want. For that, look at the order book (the price ladder) before trading size: if the top level shows 40 contracts and you want 400, expect slippage.
Kalshi's headline volume numbers have a similar quirk: a $1 combo that could pay $100 counts as $100 of volume. Real money, but not the amount that changed hands.
How to avoid slippage
- Use limit orders. Set the price you'll pay. Either it fills at that price or it doesn't fill. On most markets a resting limit order is also fee-free (see fees), so you save twice.
- Check the ticket. For larger market orders Kalshi shows the average price you'd get. If it's materially worse than the top price, cut the size or switch to a limit.
- Split big orders. Several smaller limit orders at a few price levels fill more of your size without paying up for all of it.
- Trade liquid markets. If the edge is small, it isn't worth paying 3¢ of slippage to get in.
Slippage and the spread
Related but different: the spread is the gap between the best Yes price and the best No price (or best bid and ask). A market showing Yes 58¢ / No 45¢ has a 3¢ spread; buying and immediately selling costs you that 3¢ plus fees. Wide spreads and slippage go together, and both come from the same thing: not enough people on the other side.