Free sports betting tools
Compare odds, calculate payouts, check expected value, and explore stake sizing with free calculators for any sport. No account required. Arbitrage checking is manual; optional model-data shortcuts currently cover MLB only.
No-Vig Calculator
Strip sportsbook juice from 2-way and 3-way markets to see fair-odds implied probability.
Start here if you want to know what a book is really pricing.
Parlay Calculator
Multi-leg parlay payout with a correlation warning for same-game stacks.
Use when building parlays — especially if you're mixing props from the same game.
Arbitrage Finder
Free manual calculator for checking odds across two sportsbooks.
Use when you have accounts at multiple books and want locked profit opportunities.
Kelly Calculator
Stake sizing via the Kelly criterion, with fractional-Kelly presets (full / half / quarter / tenth).
Use once you have an edge estimate and want to size the bet responsibly.
EV Calculator
Expected value, EV%, and breakeven probability for a single bet.
Use to check whether a specific bet has long-run positive expected value.
Enter your own odds and probability estimates in the no-vig, parlay, Kelly, and EV calculators. For MLB, optional model-data shortcuts can fill in available odds or estimates from the Mongoose Bets simulation. Enter prices from two books in the Arbitrage Finder to check a potential arbitrage; live scanning is not currently available.
How to use the tools together
Use the calculators to work through a bet and compare prices you find at different books:
- Find a line. The sportsbook quotes odds; those odds contain a 4-6% built-in margin (the vig). Before you do anything else, run the line through the No-Vig Calculator to get the vig-free fair probability. That's what the market actually believes.
- Estimate true probability. If you disagree with the market — because you have a model, sharper information, or a sim-derived number — that's the edge you're betting on. For MLB, our model-data shortcuts provide one source of probability estimates.
- Check expected value. Run the offered odds + your probability estimate through the EV Calculator. Positive? Proceed. Negative? Pass.
- Size the bet. The Kelly Calculator tells you what fraction of your bankroll to stake. Almost everyone should size at quarter Kelly, not full.
- Cross-check for arbitrage. If you have accounts at multiple books, check the Arbitrage Finder to see if any line discrepancy creates a locked-profit opportunity across books.
- Stack bets. Multi-leg parlays compound your edge (or your loss). Use the Parlay Calculator and watch for the correlation warnings — correlated legs break the independence assumption that naive parlay math relies on.
Who these tools are for
These tools are for anyone who wants to understand the numbers behind a sports bet. Bring odds from the sport you follow and compare how price, probability, and stake affect the calculation. The MLB data shortcuts are optional; you can use the manual calculators without them.
If you're new to +EV betting, the tools also function as a teaching rail. Each has an explainer section below the widget that walks through the math, gives worked examples, and lists common mistakes. You can use them to internalize the workflow before betting real money.
What these tools are not
They're not prediction engines. They're not a model. They don't tell you which bets to place. They're deterministic math: given your inputs, they return the right number. Where the inputs come from is on you — that's the hard part of betting, and no calculator fixes it.
For prediction-engine output (who wins, who covers, what's the edge on each market), the main Best Bets page is the right destination. For the methodology behind how those predictions are generated, read the methodology page.
Frequently asked questions
- Are these tools really free?
- Yes. All five tools are free and work without an account. The optional sim-integration features (loading today's closing line into the no-vig calculator, auto-filling win probability into Kelly or EV, loading Mongoose picks into the parlay calculator) read from the same public Supabase data that powers the main Mongoose Bets site. No login, no paywall, no freemium gotcha.
- What data do the sim hooks use?
- Sim hooks pull from the Mongoose Bets moneyline_bets table: today's best odds and sim-derived win probabilities for every MLB game on the slate. The data is refreshed every cycle of our main simulation pipeline — typically 2-6 times per day depending on news and lineup updates. See the methodology page for the full pipeline description.
- Are these calculators accurate?
- The math is standard industry math — the same formulas used by every sports betting calculator online. We use proportional devig (not Shin), basic American/decimal odds conversion, the Kelly criterion as originally derived by John Kelly, and the textbook expected-value formula. Where our tools differ from competitors' is the sim-integration option: our numbers come from the same 2,500-sim model that powers the main site, not guesswork.
- Can I use these tools for sports other than MLB?
- Yes. Enter odds and probability estimates from any sport in the no-vig, parlay, Kelly, and EV calculators. Their outputs depend on your inputs and each tool's assumptions. The Arbitrage Finder is a manual calculator; live scanning is not currently available. Optional model-data shortcuts currently cover MLB only.
Explore research for the sports you follow
Want to go beyond the calculators? Review the models, research tools, and sport coverage included in each plan.
View plans and coverageBuilt on the Mongoose Bets methodology
These calculators are free and work without an account. The data they optionally pull — today's closing lines, our sim-derived fair odds, the +EV bets we've flagged — comes from the same pipeline described on our methodology page.