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CalcChef

Coin Flipper

Flip 1 to 10 virtual coins for heads or tails, with a running tally. Each face comes from the browser's secure random source, so both sides are equally likely. How it works

Use the Coin Flipper

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About the Coin Flipper

A coin flip is 50% heads and 50% tails. This coin flipper draws one number from the browser’s crypto.getRandomValues: an even 32-bit number gives HEADS and an odd one gives TAILS. Press Flip Coin and the result appears after a 0.6 second spin, or at once if your device asks for reduced motion. Coins per flip throws up to 10 coins at once, and a tally counts heads and tails for the visit.

How to use this tool

  1. Agree which side stands for which choice before you flip.
  2. Press Flip Coin. The page shows HEADS or TAILS with the flip number for this visit under it.
  3. Press it again for a new, independent flip. The tally under the button counts heads and tails until you press Reset tally or leave the page.

Frequently asked questions

Is an online coin flip really 50/50?
Here it is, to the limits of the browser’s random source. Each flip comes from crypto.getRandomValues, the generator browsers provide for security uses, not from Math.random. Exactly half of the 2^32 possible draws give each face, so no draw is ever thrown away and neither side is favored.
What are the odds of getting heads several times in a row?
Each flip is independent, so n heads in a row have a probability of 1 in 2^n. Three in a row is 1 in 8 (12.5%), five is 1 in 32 (3.125%) and ten is 1 in 1,024 (about 0.098%). After four heads, the next flip is still 50/50.
How do I do best of three with a coin flip?
Agree on the sides first, then press Flip Coin with one coin until one side has two wins. The tally under the button shows the heads and tails so far, and Reset tally clears it for the next game. A best of three ends after two flips half the time and after three flips the other half.
How many heads should I expect from flipping 10 coins?
Five on average, but exactly 5 heads comes up only 252 times in 1,024, about 24.6%. Four, five or six heads together cover about 65.6%, and all ten heads is 1 in 1,024. Set Coins per flip to 10 to try it; the tally percentages move toward 50% only over many throws.
Why does the page say secure random numbers are not available?
The browser has no crypto.getRandomValues. The page then shows that message instead of flipping with a weaker generator, so no result appears until you use a browser that has it.

Heads or tails for a quick decision

Press Flip Coin and the page shows HEADS or TAILS, with the flip number for this visit under it. Say which side stands for which option before you press, for example heads you serve first. Every press is a new, independent flip, so a run of tails does not make heads due.

Coins per flip throws 1 to 10 coins at once, shows each one and sums them up, as in 3 heads, 2 tails. The tally under the button counts every coin thrown this visit with its heads and tails percentages, so a best of three keeps score; Reset tally starts again, and the tally is not saved. A link cannot flip the coin: only the button does. If the browser has no secure random source, the page says so and does not fall back to a weaker generator.

Where the result comes from

The browser's crypto.getRandomValues fills one 32-bit number. An even number means HEADS and an odd number means TAILS. A draw of 0 gives HEADS and a draw of 7 gives TAILS.

The code that turns a draw into a choice discards any draw that would favor some outcomes (rejection sampling), so it stays fair for any number of outcomes. With two outcomes nothing is ever discarded, because 2 divides 2^32 exactly: half of all 32-bit numbers are even and half are odd. The coin is fair to the limits of the browser's random source. Math.random is not used.

Each coin in a multi-coin flip is its own draw. Independent flips multiply: three heads in a row has probability 1/8 (12.5%) and ten in a row 1/1,024. With 10 coins, exactly 5 heads comes up 252 times in 1,024 (about 24.6%).

Fun Fact: A 2023 preprint on arXiv (Bartoš et al., 2310.04153, not peer reviewed) reported that in 350,757 real coin tosses the coin landed on the side it started on 50.8% of the time. This page has no starting side, so that effect does not apply to it.

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