How to arbitrage on Polymarket and Kalshi

Prediction market arbitrage sounds simple on paper. One platform shows one price, another shows a different price, and the gap looks like free money. In practice, most bad trades happen before the first click. Traders rush into a spread that looks attractive, only to realize that the markets are not truly equivalent, the rules do not match, or one leg fills while the other does not.

That is why this guide is not a generic explainer of prediction market arbitrage. This page is about one specific setup: how to compare and trade opportunities between Polymarket and Kalshi without mistaking a false spread for a real edge. If you want the broader theory first, start with our guide to prediction market arbitrage.

Who this guide is for

This guide is for readers who already understand the basics of prediction markets and want to move from theory to execution. You do not need to be a professional trader, but you do need to approach this carefully. Arbitrage only works when the setup is clean, the market match is real, and the full trade can be executed without turning into a directional bet.

If you are still getting comfortable with the basics, it helps to first read what a prediction market is and how prediction markets work before moving into cross-platform setups.

It is also for people who keep seeing screenshots of price gaps between Polymarket and Kalshi and assume that every gap is an opportunity. Most are not. A spread only matters after you verify the contract wording, event timing, settlement rules, liquidity, fees, and the path to getting both sides filled.

Step 1: make sure the two markets are actually the same market

This is where most mistakes begin. Two event contracts can look similar while still resolving differently. A headline-level match is not enough. You need both markets to refer to the same real-world outcome, measured over the same time frame, and governed by compatible rules.

At the contract level, this comes down to understanding what an event contract is in trading and how different platforms define resolution.

Start by checking the exact wording of each contract. Look at the event itself, the cutoff date, the time zone, the source used to resolve the market, and whether the contract settles on a final confirmed value or an earlier published estimate. Small differences here can completely kill the trade.

For example, one market may ask whether inflation will come in above a certain level based on an official release, while another may reference a related but not identical release. One platform may resolve based on a final certified result, while the other may settle on a preliminary number. If those rules differ, the spread is not safe just because the subject looks similar.

Example: one contract might resolve on the final certified CPI release, while the other resolves on the first published estimate. Even if the topic looks identical, those are not the same trade.

It is also worth understanding prediction market regulation in the US, especially when you compare platforms with different structures, access rules, and compliance frameworks.

Market-matching checklist

Before you compare prices, confirm all of this:

• the event is identical
• the resolution date is identical
• the time zone does not shift the outcome window
• the data source is compatible
• the wording does not create edge cases
• the contract settles in a way that truly matches the other side

If even one of these points feels unclear, skip the trade. A trade you do not take is often better than a trade you misunderstand.

Step 2: check whether the spread is real after friction

A visible price gap is only the starting point. The real question is whether the spread survives friction. That means fees, slippage, fill risk, and the possibility that one leg moves while you are entering the other.

A lot of "great" opportunities disappear the moment you stop looking at screenshots and start thinking in executable numbers. Maybe the theoretical edge is `2%`, but the book is thin and the true fill price is worse. Maybe one platform lets you enter size quickly while the other has only shallow liquidity. Maybe the edge disappears before you complete the second leg.

The clean way to think about it is simple: do not calculate your trade based on `top-of-book` fantasy. Calculate it based on realistic fills. If your trade works only in perfect conditions, it probably does not work at all.

What to account for before entering

Your spread calculation should include:

• realistic entry prices on both platforms
• fees or platform costs
• slippage on available size
• delay between the first and second leg
• capital tied up until resolution, if relevant
• exit risk if you cannot hold to settlement

If the edge becomes thin after all of that, it is not a strong setup.

Step 3: decide whether this is true arbitrage or just a directional trade in disguise

One of the biggest traps in prediction market arbitrage is thinking you are market-neutral when you are not. This usually happens when the contracts do not fully offset each other or when execution is incomplete.

A real arbitrage setup should not depend on your personal view of the outcome. It should work because the contracts are misaligned, not because you think one market is wrong and the other is right. The moment your `PnL` starts depending on a specific event result rather than the spread itself, you are no longer doing clean arbitrage.

That is why paired execution matters so much. If you can fill only one side, or if one market is clearly more fragile than the other, you may be stepping into a trade that looks hedged but behaves like speculation. This is also one reason prediction markets and sports betting should not be treated as interchangeable, even when the pricing may look similar at first glance.

If your outcome depends on one side moving in your favor after entry, you are no longer running **clean arbitrage**. You are carrying market risk.

Step 4: build your execution plan before placing anything

Good arbitrage trading is less about theory and more about workflow. You should know exactly what you are doing before you click into the first leg. Which side do you enter first? What size can both books support? What will you do if price moves between legs? Under what conditions will you stop?

You do not want to improvise after the first order is live. Once one side fills, your clock starts ticking. Even a short delay can change the economics of the setup, especially in fast-moving markets or thin books.

A basic execution plan should answer four questions:

1. Which platform and which side will you enter first?
1. What is your minimum acceptable net spread?
1. How much size can you actually fill without moving the book?
1. What is your stop condition if the second leg does not fill as planned?

If you cannot answer those questions before entering, the setup is not ready.

Step 5: size the trade based on the weaker side

A common beginner mistake is sizing off the more liquid leg and then discovering that the other platform cannot absorb the trade cleanly. That creates partial fill risk, which is one of the fastest ways to turn arbitrage into stress.

Always size according to the weaker side of the setup. The thinner market determines how much you can actually trade. If one side supports only a small size at your target price, then your practical opportunity is small, no matter how attractive the other side looks.

This is also why it helps to treat your first few trades as process training rather than profit maximization. A smaller trade that fully executes teaches you more than a larger trade that leaves you exposed.

Step 6: know when not to take the trade

This part matters more than people think. Not every mismatch deserves action. In fact, the best arbitrage traders are often just very good at filtering out bad setups.

Walk away when:

• the contract wording is close but not identical
• settlement rules feel vague
• one order book is too thin
• the spread disappears under realistic fills
• execution requires too much speed relative to the available edge
• the trade works only if you assume everything will go perfectly

That last point matters a lot. Prediction market arbitrage is not about finding screenshots that look clever. It is about finding trades that still make sense after friction, delay, and imperfect execution.

Step 7: watch for false positives that trap beginners

False positives are everywhere in this niche. A market may look mispriced simply because one platform updates faster. Or because the visible price on one side reflects tiny size. Or because traders on one venue care more about speed than precision.

Sometimes the gap exists for a reason. One platform may attract a different user base, react at a different speed, have a different liquidity profile, or behave differently around event uncertainty. That does not always create a clean arbitrage window. Sometimes it just creates noise.

The best way to avoid false positives is to slow down and ask one question: if I had to explain exactly why this spread exists and exactly how it closes, could I do it in plain English? If the answer is no, the setup is not mature enough.

A spread without a clear explanation is usually noise, not edge.

Step 8: decide whether you are holding to resolution or managing the position earlier

Not every arbitrage trade needs to be held all the way to settlement. Some traders prefer to capture compression in the spread before resolution and exit earlier. Others are comfortable holding until the contracts settle, assuming the structure is truly clean.

Both approaches can work, but the important thing is to decide this before entry. If you plan to hold to resolution, you need confidence in contract matching and capital lock-up. If you plan to exit earlier, you need to understand whether the spread is likely to normalize and whether both markets will stay liquid enough for an orderly exit.

The wrong approach is entering without clarity and then improvising later. That usually leads to bad decisions, especially if the market becomes more volatile or the event gets closer.

Common mistakes to avoid

Beginners usually do not lose because they misunderstand the concept of arbitrage. They lose because they rush the setup.

The most common mistakes are:

• treating similar contracts as identical
• calculating the spread without using realistic fills
• ignoring partial fill risk
• entering one leg before knowing how to complete the second
• oversizing based on the stronger book
• mistaking platform noise for a genuine edge

Most of these mistakes are avoidable. The solution is not to trade faster. The solution is to trade more cleanly.

A simple pre-trade checklist

Before you place a trade between Polymarket and Kalshi, ask yourself:

• Are these two contracts truly equivalent?
• Do I understand exactly how each one resolves?
• Does the net spread still work after fees and slippage?
• Can I fill both legs at a realistic size?
• Do I know what I will do if one side moves or fails to fill?
• Am I taking arbitrage, or am I quietly taking market risk?

If you cannot answer yes to all of those, you are not looking at a finished setup yet.

Final thoughts

Arbitrage between Polymarket and Kalshi is not about spotting any random price gap and jumping in. It is about discipline. The traders who survive in this space are usually the ones who spend more time filtering than trading. They check the contract match, the resolution logic, the real spread, the fill path, and the conditions under which the idea breaks.

That may sound less exciting than the promise of free money, but it is the difference between a structured trade and a sloppy bet with extra steps.

If you want the broader mechanics behind this setup, go back to our full guide to prediction market arbitrage.

Further reading

• How betting odds work
• How to read moneyline odds
• Odds formats explained