Odds to Probability Converter
Convert between odds and probability in either direction, with reduced odds for and against and a visual share breakdown.
Conversion mode
Chance the event occurs
20%
chanceFavorable
20%
Unfavorable
80%
- 1
Total outcomes = a + b
1 + 4 = 5 - 2
Probability p = a ÷ (a + b)
1 ÷ 5 = 0.2 - 3
Probability (%)
0.2 × 100 = 20
How does this calculator work?
Odds of a:b convert to a probability of p = a / (a + b); for example 1:4 equals 0.2 or 20%. To go the other way, a probability p becomes odds for of p:(1 − p) and odds against of (1 − p):p, each reduced to whole numbers. This tool converts both directions instantly.
Formula
How this is calculated
Pick a conversion mode. In "odds to probability" mode you enter the odds as two numbers, a (favorable outcomes) and b (unfavorable outcomes). The probability is p = a / (a + b), and the percentage is 100 × p. The odds for are reported as a:b and the odds against as b:a, each reduced by their greatest common divisor.
In "probability to odds" mode you enter a single probability p as a decimal between 0 and 1. The complementary probability is q = 1 − p. The odds for the event are p : (1 − p) and the odds against are (1 − p) : p. To present whole-number ratios, p and q are scaled to integers and reduced by their greatest common divisor.
Inputs must be non-negative, the total a + b must be greater than zero to avoid dividing by zero, and a probability must fall within [0, 1]. Odds here are unitless ratios; probability is a pure number between 0 and 1, shown also as a percentage. Extreme cases like p = 0 or p = 1 give degenerate odds (0:1 or 1:0).
Frequently asked questions
Probability is the fraction of all outcomes that are favorable, p = a / (a + b). Odds compare favorable to unfavorable outcomes directly as a ratio a:b, so odds of 1:4 mean a probability of 1/5 = 0.2.
Odds for are p:(1 − p) = 0.25:0.75, which reduces to 1:3. Odds against are the reverse, 3:1.
Odds against swap the ratio so unfavorable outcomes come first: if odds for are 1:3, the odds against are 3:1.
Also known as
TG we-Calculate Editorial Team. (2026). Odds to Probability Converter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/odds-to-probability-calculator
TG we-Calculate Editorial Team. "Odds to Probability Converter." TG we-Calculate. 2026. https://we-calculate.com/calculator/odds-to-probability-calculator.
TG we-Calculate Editorial Team, "Odds to Probability Converter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/odds-to-probability-calculator
@misc{wecalculate_odds_to_probability_calculator, title = {Odds to Probability Converter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/odds-to-probability-calculator}}, year = {2026}, note = {TG we-Calculate} }
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