Calculator

Break-Even Win Rate Calculator

About this calculator

By Alex Thornton · author of the MIT-licensed teachersbettextbook library · Last reviewed 24 July 2026

Two questions on one page: what win rate does this price demand, and how many bets before your record means anything?

Grade the same bets on a fraction of the sample →

A break-even win rate is how often a bet at this price has to win just to get your money back. At −110, that is 52.38%. Above it is edge; below it is a slow loss.

The second half is the one people skip

Break-even is the easy part. One divided by the decimal odds. −110 needs 52.38%, and every point above that is edge.

Knowing whether you actually have it is the hard part. A two-point edge takes hundreds of settled bets to separate from luck, and most people call it after twenty.

So this runs the real test: a proportion test, a Wilson interval, and the sample size an edge that size would need. It is usually a bigger number than anybody wants it to be.

Closing line value gets to the same answer far sooner, which is the entire reason it exists.

Want more math?the Nerd Corner

q = \dfrac{1}{d} = \dfrac{1}{1 + b}, \qquad q_{\text{ml}<0} = \dfrac{|\text{ml}|}{|\text{ml}| + 100}, \qquad q_{\text{ml}>0} = \dfrac{100}{\text{ml} + 100}

where:

$q$ : the break-even win rate, 0 to 1. Numerically identical to the price’s vig-inclusive implied probability.
$d$ : decimal odds, total return per unit staked including the stake. −110 is $d = 1.909$ .
$b = d - 1$ : net profit per unit staked. −110 is $b = 0.909$ .
$\text{ml}$ : the American moneyline, in whole units either side of ±100.

A fee moves the same figure on a prediction market, because it is paid either way:

q_{\text{fee}} = P + r\,P\,(1 - P), \qquad r = 0.07\ \text{(taker)}, \quad r = 0.0175\ \text{(maker)}

$P$ is the contract price in dollars: a 50¢ contract is $P = 0.50$ . The Kalshi fee calculator handles the round trip.

Worked example: the numbers in the form below

A −110 price, and a 500-bet record with 270 wins:

d = 1 + \tfrac{100}{110} = 1.9091 \quad\Rightarrow\quad q = \tfrac{1}{1.9091} = 52.38\%

\hat{p} = \tfrac{270}{500} = 54.00\%, \qquad \hat{p} - q = +1.62 \text{ points}

z = \dfrac{0.5400 - 0.5238}{\sqrt{0.5238 \times 0.4762 / 500}} = \dfrac{0.0162}{0.02234} = 0.72, \qquad p_{\text{2-sided}} = 0.47

The record is above break-even, but the 95% interval runs 49.62% to 58.32% and still contains 52.38%. A two-sided test returns p = 0.47, about what a fair coin returns.

Break-even at the prices you actually see

American	Decimal	Break-even win rate	Profit per $100
−200	1.500	66.67%	$50.00
−150	1.667	60.00%	$66.67
−120	1.833	54.55%	$83.33
−110	1.909	52.38%	$90.91
+100	2.000	50.00%	$100.00
+150	2.500	40.00%	$150.00
+200	3.000	33.33%	$200.00
+300	4.000	25.00%	$300.00

−110 is the one worth memorising: 52.38%, not the 52.4% that gets quoted. Those 0.019 points are $364 of the wrong sign on 10,000 bets of $100: the tell of a number copied rather than computed.

Break-even after prediction-market fees

Exchanges charge on the fill rather than in the line, so the break-even rises above the contract price by the fee: most at 50¢, least at the tails, the inverse of sportsbook vig.

Contract price	Price implies	Taker break-even	Maker break-even
10¢	10.00%	10.63%	10.16%
25¢	25.00%	26.31%	25.33%
50¢	50.00%	51.75%	50.44%
75¢	75.00%	76.31%	75.33%
90¢	90.00%	90.63%	90.16%

At the same fair probability, a coin flip costs 2.38 points of break-even at a −110 line and 1.75 as a Kalshi taker, 0.44 as a maker. Books vs. exchanges covers the rest.

How many bets before a record means anything

A win rate is the sample mean of a Bernoulli variable, and Bernoulli variables are noisy. One bet tells you nothing; the question is how many make it something.

Testing a record against break-even is a one-sample proportion test:

z = \dfrac{\hat{p} - q}{\sqrt{q\,(1-q)/n}}, \qquad \hat{p} = \dfrac{w}{n}

The honest summary is the interval, not the point estimate. Here that is the Wilson score interval, which behaves near 0 and 1 where the normal one breaks:

\dfrac{\hat{p} + \frac{z_{\alpha/2}^{2}}{2n} \;\pm\; z_{\alpha/2}\sqrt{\dfrac{\hat{p}(1-\hat{p})}{n} + \dfrac{z_{\alpha/2}^{2}}{4n^{2}}}}{1 + \frac{z_{\alpha/2}^{2}}{n}}

Run forward, the same relationship gives the standard sample-size formula, at 95% confidence and 80% power:

n \;\approx\; \dfrac{\Big[\, z_{\alpha/2}\sqrt{q(1-q)} \;+\; z_{\beta}\sqrt{p(1-p)} \,\Big]^{2}}{(p - q)^{2}}, \qquad z_{\alpha/2} = 1.960,\ \ z_{\beta} = 0.842

The denominator is the killer. Edge enters squared, so halving the edge quadruples the sample:

True win rate at −110	Edge over 52.38%	ROI per bet	Bets to detect it
53.4%	+1.02 pts	+1.9%	18,845
54.0%	+1.62 pts	+3.1%	7,462
55.0%	+2.62 pts	+5.0%	2,849
57.4%	+5.02 pts	+9.6%	773
60.0%	+7.62 pts	+14.5%	334

“Bets to detect it” is that test at 80% power, the same figure the panel above prints for a record you enter.

A 2.6-point edge (a 5% ROI, genuinely good) needs 2,849 settled bets: nearly three years at 20 bets a week. A one-point edge needs 19,570.

Why closing line value gets there faster

Same skill, different instrument. A win rate observes one bit per bet and pays full Bernoulli variance for it; closing line value observes a continuous number: the probability points between the price taken and the close.

Over 100 bets the 95% interval on a win rate spans roughly ±9.8 points, wide enough to contain almost any hypothesis. The same 100 bets pin mean CLV to about ±0.6.

That is the argument for the CLV report card: the same claim about skill, measured with far less noise per observation.

What a break-even win rate is not

Not a fair probability. It is the vig-inclusive number; the fair one is what is left after de-vigging.
Not defined across mixed prices. One rate exists for one price at one stake. A book of bets at −110, +150 and −300 has no single break-even win rate: group by price, or grade on expected value.
Not affected by pushes. A push returns the stake and belongs in neither column; it comes out of the count rather than counting as half a win.
Not fee-free on an exchange. A 50¢ contract needs 51.75% as a taker, and a round trip pays that twice.
Not preserved by a parlay. Legs multiply the decimal odds, so four −110 legs break even at 7.53%, compounded here.
Not a licence to wait. The table says how long a win rate takes to become evidence, not how long an edge takes to be real.

Check this number yourself

One function call in the MIT-licensed teachersbettextbook package; the fee-adjusted version has its own:

pip install teachersbettextbook

>>> from teachersbettextbook import moneyline_to_prob, kalshi_breakeven
>>> moneyline_to_prob(-110)
0.5238095238095238
>>> kalshi_breakeven(0.50)               # 50-cent contract, taker
0.5175
>>> from math import sqrt
>>> q, n, w = moneyline_to_prob(-110), 500, 270
>>> (w/n - q) / sqrt(q*(1-q)/n)          # the z the page reports
0.7248824356...

Same inputs, same digits, no account. More: open source.

Break-even is the floor. De-vig the line to see how much of it is margin, expected value to price a gap above it, Kelly to size one, closing line value to learn sooner whether it was real.

Frequently asked questions

What is a break-even win rate?

A break-even win rate is the share of bets at a given price that must win for the money staked to come back exactly. It is one divided by the decimal odds, which is the same number as the price’s vig-inclusive implied probability. At −110 that is 52.38%.

What is the break-even win rate at −110?

52.38%. A −110 price is decimal 1.909, and 1 / 1.909 = 0.5238. Staking 100 units on each of 1,000 bets at −110 and winning 523 of them returns 99,845 units against 100,000 risked; 524 wins returns 100,036. The break-even count sits between them, at 523.81.

How many bets does it take to know whether you beat the break-even rate?

More than most records contain. Detecting a true 55% win rate against a 52.38% break-even at conventional 95% confidence and 80% power takes 2,849 settled bets. A flat one-point edge takes 19,570. A 500-bet record at 54% carries a 95% interval of 49.6% to 58.3%, which still contains break-even.

What is the break-even win rate on a prediction market?

The contract price plus the fee. On Kalshi’s standard schedule a taker pays 0.07 × P × (1 − P) per contract, so a 50¢ contract breaks even at 51.75% rather than 50%, and a 25¢ contract at 26.31%. A resting maker order pays a quarter of that, putting the 50¢ break-even at 50.44%.

Does beating the break-even rate prove an edge?

Not on its own, and usually not on the sample sizes people have. A 270–230 record over 500 bets at −110 is 54%, 1.62 points above break-even, and a two-sided test returns p = 0.47, roughly what a coin does. Closing line value measures the same skill on far fewer bets because each bet contributes a continuous number instead of a win or a loss.