Coin Flip Streak Calculator
Enter how many flips in a row you want and get the probability of the streak — plus the matching 1-in-X odds — for a fair coin.
Probability and odds at once
Enter a streak length and the calculator returns the probability (1/2)ⁿ and the matching "1 in 2ⁿ" odds together.
Assumes a fair coin
The result holds for a perfectly fair 50/50 coin with independent flips — a biased coin or linked flips change the numbers.
What is a coin flip streak probability?
The chance of the same result in a row
This coin flip probability calculator tells you how unlikely it is to land the same side several times in a row with a fair coin. Each flip is an independent 50/50 event, so the chance of a streak of n matching flips is one half multiplied by itself n times: P(n) = (1/2)ⁿ. The same answer can be read as odds — 1 in 2ⁿ — which is often the more intuitive way to picture rare runs like "ten heads in a row". It is the number behind party bets, probability homework, and any time you wonder just how surprising a long run of the same result really is.
Enter the number of flips in a row to get the probability and the 1-in-X odds instantly.
The probability of a streak is one half raised to the number of flips, and the odds are 1 in two raised to the same power.
P(n) = (1/2)ⁿBecause each flip is independent, you multiply a fresh one-half for every flip in the streak. Counting the other way round, n flips produce 2ⁿ equally likely sequences, and only one of them is the all-matching run — so the odds are 1 in 2ⁿ. The two outputs are the same fact in two forms: a probability between 0 and 1, and the headline "1 in X" figure.
Suppose you want the chance of flipping heads five times in a row.
Count the flips
n = 5 — five matching flips in a row is the streak you are after.
Raise one half to that power
(1/2)⁵ = 1/32 = 0.03125 — the probability of the streak.
Read it as odds
2⁵ = 32, so the odds are 1 in 32 — about a 3.1% chance.
The two outputs answer the same question in different languages. The probability (0.03125 for five flips) is the share of the time the streak happens; the odds (1 in 32) reframe it as "expect it about once in 32 attempts". The key idea is that the chances of independent events multiply, so the probability halves with every flip you add: 1 in 2, then 1 in 4, 1 in 8, 1 in 16, 1 in 32, and so on. That doubling makes long streaks vanish fast — ten in a row is already 1 in 1,024, and twenty is over 1 in a million. It also exposes the gambler's fallacy: after four heads, the next flip is still a plain 50/50, because the coin has no memory. The (1/2)ⁿ figure is the chance of a streak measured before you start flipping, not the odds of the next single flip once a run is already underway.
The formula is exact, but it rests on two assumptions worth keeping in mind.
Assumes a fair coin and independent flips
This calculator assumes a perfectly fair coin — a true 50/50 chance on every flip — and that the flips are independent, meaning no flip influences the next. A biased or worn coin, a flipping method that favours one side, or any link between flips would change the real-world numbers. It also gives the chance of one specific run (all heads, or all tails); the chance of any matching streak, heads or tails, is twice as large.