Know exactly how likely you are to go broke. Essential knowledge for every serious bettor.
What is Risk of Ruin?
Risk of Ruin (RoR) is the mathematical probability of losing your entire bankroll before your edge has time to compound. Even with a 55% win rate, betting too large a percentage of your bankroll can give you a 50%+ chance of going broke. This calculator uses the proven gambler's ruin formula adapted for sports betting to show your exact bust probability.
Your total betting bankroll
Amount you risk per bet
Your historical or expected win percentage
Standard juice is -110
Leave empty for lifetime risk
Risk of Ruin isn't just an abstract concept. It's cold math that determines whether you'll still be betting next year or watching from the sidelines. Even winning bettors go broke all the time because they don't understand this calculation.
The formula we use is derived from the classic gambler's ruin probability, adapted for the asymmetric payouts common in sports betting:
Breaking this down:
The key insight: your Risk of Ruin decreases exponentially as you increase your bankroll relative to your bet size. Going from 10 units to 20 units doesn't cut your risk in half. It squares the reduction. This is why proper bankroll sizing matters so much.
Here's a scenario that plays out constantly: A bettor hits 55% over 200 bets. They're up $2,000. They feel invincible. They start betting bigger. Then variance hits, they lose 12 of 15, and suddenly they're broke.
This isn't bad luck. It's math. At 55% with -110 odds, you have roughly a 2.3% edge per bet. That's real, but it's not huge. Variance can easily produce 12-of-15 losing streaks even with a 55% true win rate. The question is: can your bankroll survive it?
Your edge tells you that you'll win in the long run. Your bet size determines whether you survive to see the long run. These two concepts work together but serve different purposes.
| Bet Size (% of Bankroll) | Bankroll in Units | RoR at 55% / -110 | Assessment |
|---|---|---|---|
| 10% | 10 units | ~35% | Dangerous |
| 5% | 20 units | ~12% | Risky |
| 2% | 50 units | ~0.5% | Safe |
| 1% | 100 units | ~0.002% | Very Safe |
Notice how the drop from 5% to 2% bet sizing takes your Risk of Ruin from 12% to 0.5%. That's a 24x improvement in survivability just by being a bit more patient with your bet sizes.
This is harder than it sounds. You need at least 500 tracked bets, ideally 1,000+, to have a reliable estimate of your actual win rate. Short-term results are dominated by variance. If you've only tracked 100 bets and you're at 58%, you might actually be a 52% bettor who got lucky.
Different bettors have different tolerance levels. Here's a general framework:
Work backwards from your desired RoR. If you want a 1% Risk of Ruin and you have a 55% win rate at -110 odds, you need roughly 100 units in your bankroll. That means betting 1% of your bankroll per bet.
Pro Tip: When in doubt, bet smaller. The cost of betting too small is slower growth. The cost of betting too big is going broke. One of these is recoverable.
This is the silent killer. If you think you're a 55% bettor but you're actually 52%, your Risk of Ruin calculation is way off. The RoR formula is very sensitive to edge estimates. Always be conservative in your self-assessment.
If you bet 5 games on Sunday's NFL slate, those bets aren't independent. Weather, referee tendencies, and other factors can make them move together. Your effective bet size might be higher than you think. Some bettors cap their total exposure on any single day to account for this.
Nothing destroys bankrolls faster than increasing bet sizes after a loss. Your RoR calculation assumes consistent bet sizing. The moment you double up to chase, you've thrown the math out the window and increased your ruin probability dramatically.
If your bankroll grows from $1,000 to $5,000, your bet size should increase proportionally to maintain the same RoR. Similarly, if you lose half your bankroll, you should cut your bet size in half. This is the principle behind Kelly betting.
Critical Warning: The RoR calculation assumes your win rate estimate is accurate. If you're overconfident in your edge, your true Risk of Ruin is much higher than calculated. Most bettors overestimate their edge. Be honest with yourself.
These two concepts are closely related. Kelly Criterion tells you the optimal bet size for maximum bankroll growth. Risk of Ruin tells you the probability of going broke at any given bet size.
At full Kelly, your theoretical Risk of Ruin is 0% (you never bet more than you can afford to lose relative to your edge). But full Kelly is extremely volatile. Most bettors use fractional Kelly (usually 25-50% of Kelly) to smooth out variance while accepting a small but non-zero Risk of Ruin.
The relationship:
This is why overbetting is so dangerous. You don't just increase risk. You actually decrease expected growth while taking on more risk. It's the worst of both worlds.
Risk of Ruin isn't just an academic exercise. It's the difference between being a winning bettor who stays in the game and a winning bettor who goes broke before their edge plays out.
The math is unforgiving. Variance doesn't care about your confidence or your system. It will throw 15-bet losing streaks at you eventually. The only question is whether your bankroll can absorb those hits.
Use this calculator before you place another bet. Know your number. If it's higher than you're comfortable with, reduce your bet size. There's no shame in betting 1% per play. The sharps who've been doing this for decades often bet even less.
Survival is the prerequisite to success. Get your Risk of Ruin under control, and let your edge do the rest.