A free bet is worth exactly 1 − 1/odds.
Matched betting places a bookmaker's promotional bet and buys the opposite outcome on a market, so the same figure comes back whichever side wins. This page works one all the way through — a €50 free bet at decimal odds of 4.00, covered at 75¢ — shows why the answer is €37.50 and not a range, and then spends the second half on the ways it goes wrong.
What matched betting is actually doing
It is not finding an edge. It is converting a lottery ticket into cash at its fair value, and the bookmaker is the one who paid for the ticket.
A free bet cannot be withdrawn. It can only be staked, the stake is not returned with the winnings, and if it loses you get nothing. So a €50 free bet at decimal odds of 4.00 is a 25% chance of €150 and a 75% chance of zero. Its expected value is €37.50 — but you collect it one time in four and get nothing the other three.
Matched betting takes that same €37.50 and makes it certain. You place the free bet on outcome A at the bookmaker, and with your own money you buy enough of outcome B on a prediction market that both branches pay the same. The variance goes to the market. The value stays with you.
Nothing is created. The free bet's expected value is what you keep — matched betting only removes the coin flip standing between you and it.
The whole trade, in numbers
Bookmaker: €50 free bet on outcome A at 4.00. If A wins it returns €150 of winnings and the €50 stake is not returned, because it was never your money.
Market: outcome B — the exact complement of A — trades at 75¢. A share costs 75¢ and pays €1 if B happens, €0 if it doesn't.
- Shares to buy = stake × (odds − 1) = 50 × 3 = 150 shares
- Cover cost = 150 × €0.75 = €112.50 of your own money
- Locked profit = 150 × (1 − 0.75) = €37.50
Written out as a ledger, both branches land on the same figure — which is the entire point, and the only thing worth checking before you commit.
The fair price is the whole scoreboard
Bookmaker odds imply a probability: 4.00 means a 25% chance, so the opposite outcome is implied at 75%. That 75¢ is the fair cover price — the price at which the two venues agree exactly, and the price at which the identity above holds.
The formula is fair = 1 − 1/odds, and it does double duty: it is also the fraction of face value you keep. Pay less than fair and you keep more than the free bet was worth; pay more and the difference is the market's, not yours.
The number nobody mentions: the cover is the big one
The percentages below are the share of face value you keep, and they are simply the fair price again. Every guide quotes the profit. Almost none quote the €112.50 you have to have sitting there to collect it. At fair prices the cover is (odds − 1) times the locked profit — so longer odds keep more of the free bet and tie up dramatically more of your own money doing it.
| Odds | Shares | Cover at fair | You keep | Capital / €1 |
|---|---|---|---|---|
| 2.00 | 50 | €25.00 | €25.0050% | €1.00 |
| 4.00 | 150 | €112.50 | €37.5075% | €3.00 |
| 8.00 | 350 | €306.25 | €43.7587.5% | €7.00 |
| 21.00 | 1,000 | €952.38 | €47.6295.2% | €20.00 |
All four rows are the same €50 free bet. Going from 4.00 to 21.00 adds €10.12 to the result and asks for €840 more capital, locked until the market resolves. Capital, not promotions, is usually what caps how many of these you can run at once.
Qualifying bets: the leg where you can lose
Most promotions don't hand over the free bet first. They ask for a qualifying bet with your own money, and the free bet arrives after it settles. The arithmetic changes in one important way: your stake is returned if it wins, so you need shares = stake × odds, and your own stake is now part of the loss column.
Take a €20 qualifying bet at 3.00. Fair price is 1 − 1/3 = 66.7¢. You buy 60 shares of the opposite outcome, and:
| Cover price | Cover cost | Net, either way |
|---|---|---|
| 66.7¢fair | €40.00 | €0.00 |
| 68¢ | €40.80 | −€0.80 |
| 70¢ | €42.00 | −€2.00 |
| 72¢ | €43.20 | −€3.20 |
At exactly fair, a qualifying bet is free — it breaks even whichever side wins, and the promotion costs you nothing to unlock. Every cent above fair is a real, banked loss. That loss belongs to the free bet it unlocks: a €2.00 qualifier that releases a €50 free bet at 4.00 nets €35.50, not €37.50.
The frictions that are real
Order minimums
Venues will not accept arbitrarily small orders. Polymarket's minimum is 5 shares per order — at a 75¢ cover that's €3.75, and it means the cover for a small promo at short odds may simply not be placeable. A €5 free bet at 1.50 needs 5 × 0.50 = 2.5 shares of cover. There is no way to place it.
Fees, where they exist
Polymarket sets a taker fee per market and most markets are set to zero. Where it isn't, the fee is charged on top of the price rather than deducted from winnings, and it scales with how close to even money the price is: the rate is applied to min(price, 1 − price) of the money you spend. At a 75¢ cover, min(0.75, 0.25) = 0.25, so a 10% market rate costs 2.5% of the cover — €2.81 on €112.50, straight off the €37.50, leaving €34.69. Check the market's fee before sizing, not after.
Two currencies
The bookmaker leg is usually in your local currency; the market leg settles in a dollar stablecoin. Between placing the cover and cashing out, the exchange rate moves, and nothing in the matched bet hedges it. On a €112.50 cover a 2% move is €2.25 — more than half the fee above. It is small, it is real, and it is the reason a "risk-free" label is wrong.
What actually goes wrong
The theory above is airtight. Almost every real loss comes from one of these instead.
- The two legs settle on different words. This is the one that costs money. Dead heats, extra time, a withdrawn competitor, "settled on the official result", a match abandoned at 70 minutes — the bookmaker's rules and the market's resolution criteria are separate documents, and they disagree more often than you'd expect. Read both. If you can't find the market's resolution wording, you don't have a cover.
- The bookmaker voids and the market doesn't. A postponed fixture is the classic. The bookmaker returns the free bet (or expires it), and you are left holding 150 shares as a live directional bet. That is €112.50 of your own money exposed to protect €37.50 — the failure is three times bigger than the trade. You can usually sell the shares back, at whatever the book will pay for them.
- The price moves between the legs. You size at 75¢ and fill at 77¢: €37.50 becomes €34.50. Two cents is a normal amount for a market to move while you switch tabs. Place the cover first or place it fast; never calculate against a price you are no longer looking at.
- The order only partly fills. Order books are thin — often only tens of shares at the best price. Ask for 150 shares as a market order and you may get 96, which leaves you 54 shares under-covered and genuinely exposed in both directions. Check what filled, not what you submitted. How a book fills an order is worth understanding before you place one.
- The promo terms are not what they look like. Minimum odds, an expiry date, a rollover requirement ("wager 3× before withdrawal"), winnings paid as another free bet, or a maximum stake that makes the whole exercise not worth the hour. A free bet with a 3× turnover requirement is not a free bet; it is three free bets' worth of work for one free bet's value.
- The bookmaker limits the account. Accounts that only ever take promotions get stake-restricted or closed. This does not lose you a specific bet; it ends the supply of promotions, which is the entire source of the value. Plan for it rather than being surprised by it.
- The paperwork. Where you live, bookmaker winnings and gains on market positions may sit in completely different tax categories, and the two legs of one matched bet may not offset each other at all. Keep both legs, both prices, both dates. Reconstructing them later from two venues is miserable.
Not risk-free, and not everywhere. Matched betting is for adults (18+). The arithmetic is exact; the execution is not, and the failure modes above are the ordinary case rather than the exception. Prediction markets and bookmakers are both restricted or unavailable in some jurisdictions — availability is checked at sign-up in the places where amparo is offered, and reading this page changes nothing about the rules where you are. Bet only what you can afford to lose. This is not betting, investment or tax advice.
The check, before you commit
- Read the promotion's terms, including the expiry and any rollover.
- Read the market's resolution criteria and confirm they describe the same event, the same period and the same tie-breaks as the bookmaker's rules.
- Work out the fair price,
1 − 1/odds, and compare it to the price actually quoted. The gap is what you are giving up. - Check the depth at that price. If the book cannot fill your whole cover near the touch, size down or use a limit.
- Check the market's fee, and whether the cover clears the venue's order minimum.
- Place the cover, then confirm what filled before you place the bookmaker leg.
- Record both legs, both prices, both dates.
The free-bet calculator on this site does the arithmetic on the page above for any promotion — no account, nothing to install. Placing and sizing the cover automatically is what amparo charges for.