What is a Betting Unit?
Stop betting $50 on Tuesday night MACtion and $500 on Sunday Night Football. That's how you go broke. A "unit" is a standardized percentage of your total betting bankroll. For sharps, a single unit is typically 1% to 2% of their total roll. If you have $10,000, your base unit is $100.
Most squares over-bet their edge. They treat their bankroll like a weekend Vegas budget. But to survive the brutal variance of sports betting, you need mathematical discipline.
Never risk more than 3% to 5% of your bankroll on a single day's card, regardless of how many games you play. Capital protection is your primary job. Winning bets is secondary.
Flat Betting: The Sharp Standard
Flat betting means wagering the exact same amount (one unit) on every single play. No matter how much you "love" a spot. It's boring. It's repetitive. And it keeps you alive.
If you're betting -110 NFL spreads, flat betting ensures you only need a 52.38% win rate to break even against the vig. Start chasing losses or doubling down on locks, and you suddenly need to hit 55% just to tread water.
Sharp tip: Base your unit size on your starting bankroll for a season or month, not your daily fluctuating balance. Don't recalculate your unit size after every single win or loss.
The Kelly Criterion
When you have a massive edge, you should bet more. When the edge is thin, bet less. That's the core idea behind the Kelly Criterion. It calculates the mathematically optimal percentage of your bankroll to wager based on your perceived edge.
The formula looks like this:
f* = (bp - q) / b Where: f* = Fraction of bankroll to wager b = Decimal odds - 1 (the net payout) p = Probability of winning q = Probability of losing (1 - p)
Let's say you're betting a +100 (2.0 decimal) underdog. You cap the true probability of winning at 55%. b = 1.0. p = 0.55. q = 0.45. f* = (1.0 * 0.55 - 0.45) / 1.0 = 0.10. Kelly tells you to bet 10% of your bankroll.
Fractional Kelly
Do not actually bet 10% of your bankroll on one game. Full Kelly assumes you know your exact edge with 100% certainty. You don't. Your model has noise. Market CLV moves against you. You overestimate your advantage.
Professional bettors use Quarter Kelly or Eighth Kelly. If full Kelly suggests 10%, a Quarter Kelly bettor wagers 2.5%. It gives you most of the mathematical growth while drastically cutting your variance and risk of ruin.
Fixed vs Variable Sizing
Should you use flat units or scale based on edge? If your model produces highly accurate win probabilities, variable sizing (fractional Kelly) maximizes ROI. But for most bettors — especially those betting derivatives or props where true probabilities are hard to pin down — flat betting is safer.
Start with a flat 1% unit. Once you have a 1,000-bet sample size proving your edge, then you can experiment with variable sizing.