Units and Fractional Kelly Explained

If bankroll is the foundation of smart betting, bet sizing is the structure you build on top of it.

Most bettors obsess over what they’re betting—sides, totals, props—while treating how much they bet as an afterthought. A little more after a win. A little less after a loss. Sometimes a lot more when something “feels good.” That approach feels intuitive, even responsible. It’s also one of the fastest ways to sabotage long-term results.

In Part 1 of this series, we defined what a bankroll really is. In Part 2, we’re going a step further. We’re talking about units, why serious bettors think in units instead of dollars, and how fractional Kelly sizing creates discipline in a game dominated by variance. This is where betting stops feeling emotional and starts behaving like a system.

What a “Unit” Really Represents — and Why Pros Don’t Think in Dollars

A unit is not a dollar amount. It’s a percentage of your bankroll.

That distinction matters more than most bettors realize. Dollars anchor you to emotion. Units anchor you to math. When a bet is “$50,” your brain immediately assigns meaning: that’s dinner out, groceries, or a bill payment. When a bet is “1 unit,” it’s simply a risk allocation within a larger plan.

Think of units the same way investors think about position sizing in a portfolio. They’re not asking, “How much money do I feel like putting on this stock?” They’re asking, “How large should this position be relative to my total capital and the expected return?”

For most bettors, one unit falls somewhere between 0.5% and 2% of bankroll. The exact percentage matters less than the consistency. Once units are defined, every bet becomes comparable. A losing streak isn’t “I lost $600.” It’s “I’m down 6 units.” A winning run isn’t hype—it’s data.

This shift alone changes behavior. It removes ego. It removes impulse. It forces you to think in probabilities instead of paydays.

Why Flat Betting Feels Safe — but Often Misprices Risk

Flat betting is simple. Every bet is the same size. One unit, no matter what.

That simplicity is why so many bettors gravitate toward it. It feels controlled. Predictable. Safe. But flat betting quietly assumes something that isn’t true: that every bet carries the same edge.

In reality, edges vary. A bet priced at +110 when your fair odds are +105 is not the same as a bet priced at +120 when your fair odds are +100. Treating them identically underweights strong edges and overweights thin ones.

Imagine owning two businesses. One reliably earns a 20% margin. The other barely clears 2%. You wouldn’t invest the same amount of capital into both just because they’re “businesses.” Flat betting does exactly that.

Flat betting isn’t reckless. It’s just blunt. It ignores information you already have: your estimated edge.

The Core Intuition Behind the Kelly Criterion

The Kelly Criterion answers one simple question:
How much of my bankroll should I bet to maximize long-term growth while avoiding ruin?

At its core, Kelly balances two forces. Bet too small, and you grow slowly. Bet too large, and variance can wipe you out before your edge has time to work.

Kelly sizing increases bet size when your edge is larger and decreases it when your edge is smaller. Not emotionally. Mathematically.

The insight most bettors miss is this: Kelly is not about winning more often. It’s about growing capital efficiently over thousands of bets. It assumes losses will happen. It plans for them.

If you’ve ever wondered why sharp bettors survive brutal downswings while casual bettors don’t, this is a big reason why.

Why Fractional Kelly Exists — and Why Full Kelly Is Usually a Mistake

Full Kelly is aggressive. It assumes your edge estimates are perfect. In real-world betting, they never are.

Even small errors in estimating true odds can make full Kelly volatile to the point of being unusable. Drawdowns become severe. Psychology breaks before math does.

That’s why fractional Kelly exists.

Half Kelly, quarter Kelly, even eighth Kelly all sacrifice some theoretical growth in exchange for dramatically smoother bankroll paths. The expected value remains positive. The variance becomes survivable.

For most bettors—especially newer ones—fractional Kelly isn’t conservative. It’s practical. It acknowledges uncertainty. It respects the emotional reality of betting through losing streaks.

This is where most bettors get it wrong. They hear “optimal growth” and think bigger bets. In practice, optimal survival beats theoretical perfection every time.

A Practical Example: Emotion vs. Structure

Consider two bettors with identical bankrolls and identical betting skill.

The first bettor increases bet size after wins. A hot streak feels like confirmation. Confidence creeps in. Bets grow larger, not because the edge increased, but because recent results did. When variance inevitably swings back, the losses are larger too.

The second bettor sizes every bet in units, tied to fractional Kelly. A stronger edge gets a slightly larger bet. A marginal edge stays small. Wins don’t change behavior. Losses don’t either.

Over a short stretch, either bettor might look smarter. Over hundreds of bets, the difference becomes stark. The emotional bettor experiences wild swings, frequent stress, and occasional blow-ups. The structured bettor compounds quietly, rarely feeling invincible and rarely feeling desperate.

Same picks. Same odds. Completely different outcomes.

How Betting For Value Uses Units and Kelly-Based Thinking

At Betting For Value, bets are expressed in units—not dollars—by design. It keeps the focus on decision quality, not bankroll size. A one-unit play means the same thing whether your bankroll is $200 or $20,000.

Kelly-based sizing aligns naturally with a +EV mindset. When the edge is small, exposure stays small. When the edge is larger, the bankroll works harder—but never recklessly.

Just as important, this approach reinforces responsible betting. Units cap risk. Fractional Kelly reduces drawdowns. Together, they help ensure that variance doesn’t force bettors out of the game before their edge has time to show itself.

Smart betting isn’t about swinging harder. It’s about staying standing long enough to let the math work.

Closing: Discipline Is the Real Edge

Units and fractional Kelly aren’t flashy. They won’t make screenshots look impressive. They won’t turn a bad month into a miracle.

What they do is create discipline. Consistency. Survival.

They turn betting from a series of emotional decisions into a long-term system. They help you endure variance without abandoning strategy. They protect the bankroll so it can actually grow.

Before the next bet, pause and think about how you’re sizing it—and why. That moment of reflection is where smart betting begins.

In the next post in this series, we’ll dig deeper into Risk of Ruin and how poor bet sizing quietly eliminates even good bettors. Until then, remember the goal isn’t just to win bets. It’s to build a process that lasts.

This is how we redefine smart betting.