Betting margins and value: overround, fair odds and CLV

Betting margins decide whether a bet can be good value. A bookmaker does not need to know who will win: it sets prices that add up to more than 100% probability, and that extra is its margin, the overround in the UK or the vig or juice in the US. A bet is value only when its price is longer than the fair price once the margin is removed. Over many bets, the margin is what you pay. This guide shows how to measure it, how to remove it to find fair odds, and why it matters more than any single result.

Implied probability and the overround

Every price implies a probability. In decimal odds, implied probability = 1 ÷ odds. A price of 2.00 implies 50%; 4.00 implies 25%. Read our guide to how to read betting odds if any format is unfamiliar, or use the converter:

Add the implied probabilities of every outcome in a market and you get the overround:

• Two-way market at 1.91 / 1.91: 52.36% + 52.36% = 104.7%. The margin is 4.7%.
• US odds -110 / -110: -110 is decimal 1.909. The overround is about 104.8%. If both sides are truly 50/50, a $110 bet wins $100 half the time and loses $110 half the time, an average loss of $5 per $110, about 4.55%.
• Football 1X2 at 2.10 / 3.40 / 3.80: 47.62% + 29.41% + 26.32% = 103.35%.

Markets with many outcomes, such as outrights and correct scores, can add up to far more than 100%. Always check before you bet.

Removing the margin: fair odds

The simplest method divides each implied probability by the overround so they add up to 100%. Using the 1X2 market above (overround 1.0335):

Fair odds = offered odds × overround. For the home side: 2.10 × 1.0335 = 2.17.

This proportional method assumes the margin is spread evenly across outcomes. If a bookmaker puts more of its margin on the outsiders, their true chance is lower than this estimate, so their fair odds are longer. It is still a useful first check.

What value means

A bet has value when the price you take is longer than the fair price, meaning the bookmaker has underestimated the chance. If you think the home side above has a 50% chance, its fair price is 2.00 and 2.10 looks like value. If the true chance is 46%, it is not.

The hard part is the estimate. Bookmakers price markets with data, models and the weight of money from other bettors. Your view has to be better than theirs, after the margin, to have an edge. Most people's is not.

Closing line value

The closing line is the last price before the event starts. On big markets it reflects the most information, so it is often treated as the best available estimate of the true chance.

Closing line value (CLV) compares your price with that closing price, after removing the margin.

Example (illustrative, our calculation). You back a team at 2.20. At kick-off, the two-way market closes at 2.00 / 1.87, an overround of 103.5%. The fair closing price for your team is 2.00 × 1.035 = 2.07. You took 2.20 against a fair 2.07: about 6.3% better.

One bet proves nothing; it can beat the close and still lose. Over hundreds of bets, consistently beating the fair closing price is one of the few signs that prices were well chosen. Consistently getting worse prices than the close means you are paying more than the margin.

Why most bettors lose over time

Each bet carries the margin. A 4.5% margin does not sound like much, but it applies to every stake, and turnover adds up. On our calculation, betting £20 a week at a 4.5% average cost is about £47 a year in expected losses, before any bad run. Parlays and bet builders multiply the margin, as our parlays guide shows.

The UK Gambling Commission published data in July 2025 covering almost 15 million betting accounts that placed at least one bet in 2024: 72.54% of active customers were in loss and 25.42% in profit, measured on their settled bets. Among accounts the operators had restricted, 46.78% were in profit, so profitable accounts make up a far larger share of restricted accounts than of accounts overall.

Bankroll basics: a budget, not an investment

Treat betting money as spending on entertainment, not as an investment.

• Set a monthly budget you can afford to lose completely, from money left after bills and savings. Decide it before you bet, not during.
• Use small, fixed stakes. With a £100 monthly budget, £2 bets give you 50 bets for the month. Raising stakes after losses to make them up is the fastest way to overspend.
• Keep a record. Note every bet, price and result. A record shows what betting actually costs you, and whether your prices beat the close.
• Use the tools. Set deposit limits and take time-outs where a site offers them. Our safer gambling page explains how they work.

No staking plan changes the margin. Systems that raise stakes after losses, such as Martingale, change the size of your swings, not your expected loss.

Mistakes to avoid

• Judging a bet by the result. A losing bet at a good price was still a good price.
• Ignoring the margin on props, outrights and bet builders.
• Treating betting as income. The numbers say most accounts lose.
• Raising stakes to recover losses.

Where to bet

See our UK betting site listings, New Jersey sportsbook listings and the sports betting markets hub. Related reading: sports bet types explained, betting exchanges explained and odds boosts and betting promotions.