The unit system
A "unit" is the standardized stake size you use across all bets, defined as a percentage of total bankroll. The standard recreational range is 1-2%.
Example: $1,000 bankroll, 1.5% unit size = $15 per bet. Track results in units, not dollars. "+12 units this month" is more meaningful than "+$240" because the unit metric is invariant to bankroll changes.
The math behind 1-2% per bet
If you bet 1% per wager and lose 10 in a row (a normal occurrence over a long sample), you've drawn down 9.6% of your bankroll. Recoverable.
Bet 5% per wager, lose 10 in a row, and you've drawn down 40% of your bankroll. To get back to even from -40%, you need to win 67%, not 40%. Drawdowns compound asymmetrically.
Bet 10% per wager, lose 10 in a row, and you're down 65%. To recover you need to triple your remaining bankroll. Most bettors don't survive this.
| Per-bet size | 10-loss streak drawdown | Required gain to recover |
|---|---|---|
| 1% | -9.6% | +10.6% |
| 2% | -18.3% | +22.4% |
| 5% | -40.1% | +67.0% |
| 10% | -65.1% | +186.7% |
| 25% | -94.4% | +1685% |
The Kelly Criterion
The Kelly Criterion is the mathematically optimal bet sizing for a known edge. It maximizes long-run bankroll growth.
Formula: f = (bp - q) / b
- f = fraction of bankroll to bet
- b = decimal odds minus 1 (the profit ratio)
- p = your estimated true probability of winning
- q = 1 - p (probability of losing)
Worked example: an NFL team is priced at +110 (decimal 2.10, b = 1.10). You estimate true probability is 50% (you think the line is fair-but-no-edge would be -110, so +110 carries 4.5% edge). Kelly: f = (1.10 × 0.50 - 0.50) / 1.10 = 0.05 / 1.10 = 4.5% of bankroll.
Use our Kelly Criterion calculator to compute this for any American-odds price + edge estimate.
Why most pros use half-Kelly or quarter-Kelly
Full Kelly maximizes median long-run growth but carries painful variance. A 50/50 chance of a -50% drawdown is mathematically optimal under Kelly assumptions, but most humans cannot psychologically endure that.
Half-Kelly (multiply Kelly's output by 0.5) gives you ~75% of the long-run growth with only ~50% of the variance. Quarter-Kelly gives you ~50% of the growth with ~25% of the variance. Most pros run somewhere between half- and quarter-Kelly.
The chasing-losses death spiral
You start a session with $1,000. You lose your $20 bet. The chase mindset:
- Bet 2: $40 (recover the $20 lost). Lose. Down $60.
- Bet 3: $80 (recover the $60). Lose. Down $140.
- Bet 4: $160. Lose. Down $300.
- Bet 5: $320. Lose. Down $620.
- Bet 6: $640. You're now risking 64% of original bankroll on a single bet.
Even if every bet was a true coin flip, the chance of hitting 6 losses in a row is 1.6%. That's a 1-in-64 outcome, but you'll experience it inside any 100-session sample. Chasers go bust regularly because they don't survive the unfortunate-but-inevitable streak.
Stop-loss and stop-win discipline
Set both before every session:
- Stop-loss: "I will stop betting today if I'm down 10% of bankroll." Prevents chase-mode escalation.
- Stop-win: "I will stop betting today if I'm up 25%." Prevents give-back-the-profits mode that comes from over-confidence after winning.
Both are emotional, not strictly mathematical. The math says continue betting +EV bets indefinitely. But humans are not perfectly rational, and most blowups happen during emotional sessions. Stop-loss and stop-win discipline prevents 80%+ of avoidable bankroll destruction.
Bankroll separation from life money
Keep your sports-betting bankroll in a separate account. Specifically: not in the checking account that pays your bills.
This is partly psychological (separation makes loss feel less catastrophic) and partly practical (operators may freeze accounts during disputes; you don't want your rent stuck in escrow). Most pros use a dedicated bank or fintech account funded specifically for betting.
Tools to use
Use our Kelly Criterion calculator for stake sizing on +EV bets. Use our parlay calculator + no-vig calculator to evaluate edge before sizing. Track every bet in a spreadsheet — the closing-line value (CLV) over a 100+ bet sample is the only reliable signal that you have an edge.