Methodology

What does "fair price" mean on BeatVig?

A fair price is a sportsbook's or exchange's own two-sided quote with its built-in margin removed, so what's left is BeatVig's estimate of the true probability rather than the shaded number a venue actually posts. The primary method is multiplicative de-vigging: each side's implied probability is divided by the pair's combined total, so a -110/-110 quote (52.38% + 52.38% = 104.76% implied) reduces to exactly 50%/50%.

Where a page shows a range rather than one number, that range comes from re-running the same venue's own pair through three more models — an even split of the margin, and two models (Shin, and a power curve) that attribute more of the margin to informed trading. BeatVig never mixes one venue's "over" against another venue's "under" to build a fair price; every method runs on one venue's own two prices, or it isn't shown.

How is vig calculated?

Vig is the amount a two-sided quote's implied probabilities add up to past 100%: vig = (over_implied + under_implied) − 1. A -110/-110 market carries 4.76% of vig by this measure. BeatVig reports the median vig per venue and per market rather than the average, because a handful of stale or thinly-traded quotes can run far higher than anything a reader would actually be shown, and a median isn't pulled off course by them.

Right now, across BeatVig's own live board, the typical sportsbook charges 7.72% vig and the typical prediction-market spread is 9.00%. See the full breakdown on Vig by book.

At a sportsbook the vig IS the margin — it's built into the line and charged whether the bet wins or loses. At an exchange the vig is the bid/ask spread between the two sides of an order book; the venue's own trading fee is charged separately, on top of it (see "exchange fees" below).

What does "EV %" mean?

EV % is a venue's de-vigged fair probability minus what that venue is actually charging: edge = fair_probability − venue_implied_probability. A positive number means the venue is pricing the outcome cheaper than BeatVig's own fair estimate says it's worth. Rows are withheld, not shown with a caveat, when there's no fair price to compare against yet or when the fair estimate sits at an extreme BeatVig's own plausibility ceiling rejects.

Where the cheaper venue is an exchange, the headline number is shown net of that exchange's own trading fee (see below) rather than the raw price gap — a posted quote isn't what the trade actually costs, and an edge that the fee erases outright is not published as if it were free money.

How is arbitrage calculated?

An arbitrage is both sides of the same bet, bought at two different venues, for less than $1 of certain payout: if the two sides' net implied probabilities (after each venue's own fee) add up to less than 100%, the margin is (1 − sum) / sum, staked in proportion to each side so either outcome pays the same amount back.

Because this is a claim that a risk-free position exists right now, it carries the tightest guards on the site: both legs must come from a quote that still describes one coherent two-sided market, both legs must have been priced within the last five minutes, and a candidate is capped against a plausibility ceiling rather than published on faith. A pair that fails any of these checks is not shown at a discount or with a warning — it's simply not shown. See Responsible Gambling: prices move and a venue can limit or void a bet before a position settles, so nothing on this page is a promise of profit.

What do exchanges charge in fees?

Sportsbooks charge no separate fee — their margin is already inside the two prices they post, which is the vig above. Exchanges charge on top of the quote, in one of two ways, published on BeatVig's own fee schedule and used to net every EV % and arbitrage figure the site shows:

• Kalshi and Polymarket charge a per-contract trading fee that peaks at an even-money price and shrinks toward zero at both extremes — the coin flip is the most expensive trade on either venue. Polymarket's fee is capped per contract; Novig charges nothing on a pregame straight trade and a flat rate on a live one.
• ProphetX charges a commission on net winnings only, rather than a fee on the trade itself.

Every rate used here is taken from each venue's own published fee schedule, kept in one config file so a rate change is a one-line edit rather than a search through formulas.

How fresh is the data, and when is a price marked stale?

Every live price on BeatVig shows its own age. A live EV alert older than 30 seconds is marked stale, the same rule applied everywhere a single quote is timestamped. Arbitrage rows are held to a tighter standard still: both legs of a pair must have been priced within the last five minutes of each other, because two quotes of very different ages are the classic way a frozen price manufactures an arbitrage that was never really there.

Prices are never cached — every price-driven page is rebuilt from BeatVig's own current price data on every load, which is why the numbers above move between visits. (A settled, past-slate page like the weekly report is the one exception, since its numbers stop changing once that week closes.)