Betting ROI and EV Calculator July 2026

Short answer: This calculator estimates expected profit and expected ROI for one proposed cash bet. It combines the odds with the win probability you enter. It cannot verify that probability or tell you whether your model is accurate.

This is not a historical ROI tracker. To measure completed results, divide total net profit or loss by total stakes from your betting records.

How to use the betting ROI calculator

1. Enter the cash stake for the proposed bet.
2. Enter your own estimated win probability between 0 and 100 percent.
3. Choose decimal, American or fractional odds and enter the available price.
4. Read raw implied probability, potential profit if the bet wins, expected profit and expected ROI.

The currency selector changes the display symbol, not the maths. Use the final accepted price and account for any fee that the calculator does not include.

Expected value formula

Let p be your estimated win probability as a decimal, S the stake and O the decimal odds.

Profit if win = S x (O - 1)

Expected profit = p x profit if win - (1 - p) x S

Expected ROI = expected profit / S x 100

The same ROI can also be written as (p x O - 1) x 100. Stake changes the expected cash amount, but it does not change the expected ROI when probability and odds remain the same.

Worked example: 100 at 2.10

Assume a 100 stake, decimal odds of 2.10 and your own estimated win probability of 50 percent.

1. The potential profit if the bet wins is 100 x 1.10 = 110.
2. The weighted winning profit is 0.50 x 110 = 55.
3. The weighted losing amount is 0.50 x 100 = 50.
4. Expected profit is 55 - 50 = 5.
5. Expected ROI is 5 / 100 x 100 = 5 percent.

The output is positive only because the entered 50 percent estimate exceeds the 47.62 percent raw implied by odds of 2.10. It is not evidence that the real chance is 50 percent.

Break-even probability

For decimal odds, the raw break-even probability before fees is:

Break-even probability = 100 / decimal odds

At 2.10, the raw break-even point is about 47.62 percent. If your calibrated estimate is lower, expected ROI is negative. If it is higher, the model shows positive expected ROI.

Bookmaker margin means a quoted implied probability is not automatically a fair probability. Use the vig calculator for a complete market comparison.

Probability sensitivity

Small probability changes can reverse the result. At odds of 2.10 and a 100 stake, a 50 percent estimate produces expected profit of 5. A 45 percent estimate produces expected profit of -5.50 and expected ROI of -5.50 percent.

Do not choose a probability merely to make the result green. Record how the estimate was produced, test it on future events and inspect calibration across a meaningful sample.

Expected ROI versus realized ROI

Expected ROI is a model forecast before the event. Realized ROI measures completed bets:

Realized ROI = total net profit or loss / total stakes x 100

A short winning run can show positive realized ROI even when the decisions had negative expected value. A sound model can also lose over a small sample. Keep the two measures separate and report sample size, period, fees and inclusion rules.

What the calculator excludes

• bookmaker margin beyond the one price entered;
• exchange commission and other account fees;
• tax, currency conversion and payment costs;
• stake limits, rejected bets and price movement;
• void, dead-heat and settlement-rule differences;
• uncertainty or error in your win-probability estimate;
• correlation between bets in a portfolio.

Use the odds converter when you only need format conversion and raw implied probability.

Responsible use

A positive model result does not guarantee a win or long-term profit. Use only money you can afford to lose, cap each stake before the event and never raise your probability estimate to justify chasing a loss.

If gambling affects bills, sleep, work or relationships, stop and use time-out or self-exclusion tools. Our responsible gambling policy lists practical checks and support routes.