Which Pizza Is Bigger?

weirdly accurate.

    Which Pizza Is Bigger?

    What is the square vs round pizza test?

    This square vs round pizza test lets you find out how well your own eye compares pizza sizes when the shape changes: a square on its side, the same square on its corner, a triangle, and one big pizza against two small ones. You stretch one pizza until it holds as much as the other, and only after the sixth round does the page show which way your eye leaned and why. It is built on a 2001 study in which Krider, Raghubir and Krishna showed 90 students a round and a square pizza of exactly the same area.

    This test measures how your own eye compares pizza sizes, using the method of adjustment. Two pizzas are drawn from straight above with no perspective, so their areas on screen are exactly proportional to their real areas. Pizza A stays fixed; you drag, scroll or use the bar to stretch pizza B until both hold the same amount, then lock it in. There are six rounds: a square on its side, the same square on its corner, a triangle, the two square rounds again with the roles swapped, and one big pizza against two small ones. The bar shows no numbers, A changes size every round, and B starts a quarter or more off in balanced directions, so habits like over-stretching or anchoring on the start cancel out. Your main result is how much the 45° turn changed the size of the same square in your eyes, set next to the 2001 study’s group averages.

    It is not a vision test and says nothing about eyesight or intelligence. Six rounds is a small sample, and a single slip moves your numbers by a few percent. The studies used paper shapes, small groups of university students and a different task, so your result is compared with their group averages by direction, never as a percentile.

    What the test measures

    One thing: how your eye judges area when a pizza’s shape changes.

    The task

    • Method of adjustment: stretch pizza B until it holds exactly as much as pizza A, then lock it in
    • Six rounds: a square on its side, a square on its corner (the comparison in Krider, Raghubir & Krishna, 2001), a triangle, both square rounds with the roles swapped, and one large pizza against two small ones
    • A changes size every round and B starts 24–30% off, in directions balanced across the swapped pairs, so you can’t match from memory or from the starting point
    • Seen from straight above with no perspective, so on-screen area is exactly proportional; the number of toppings is random and has nothing to do with size

    Your result

    • How much the 45° turn changed the size of the same square in your eyes, next to the group averages from the 2001 study
    • How many of the six rounds were spot on (within 5%) and your average miss
    • What each round shows: the square on its side and on its corner, the triangle, and two small pizzas against one, in inches and centimetres
    • Why the illusion happens and how shops use it, with the sources
    • A shareable card and a link that challenges a friend to get more rounds spot on

    Before you start

    • About two minutes; sound is optional
    • On a phone, drag up or down anywhere; on a computer, drag, scroll or use the arrow keys and Enter
    • The size bar deliberately shows no numbers: trust your eye

    Pizza size and area illusion FAQ

    Is this test free? Do I need to sign up to see my result?

    Yes, it’s free, and there’s nothing to sign up for: no account, no email, no payment. Your result appears on this page the moment you finish, and you can share it or play again straight away.

    Does turning a square pizza on its corner change how big it looks?

    That is exactly what rounds 2 and 5 measure, so the page lets your eye answer before it explains anything. After the sixth round, the result sheet shows how much the turn changed the square for you, how the students in the 2001 study did, and the reason, which Krider, Raghubir and Krishna call the area estimation model. Knowing it first would change how you stretch the pizza.

    How is the pizza test scored?

    Each round records pizza B’s area divided by pizza A’s at the moment you lock it in, without rounding. A round within 5% of equal counts as spot on. The headline number combines rounds 1, 2, 4 and 5 into how much the same square changed for you between its side and its corner, and the result sheet sets it next to the effect implied by the two group averages in Krider et al.’s 2001 Study 2.

    Is one 18-inch pizza bigger than two 12-inch pizzas?

    Round 6 asks exactly this, one big pizza against two small ones, so the page lets your eye answer first. After the last round, the result sheet works out one 18-inch pizza against two 12-inch ones (and 45 cm against two 30 cm), then turns your own round-6 setting into the size of pizza you would have ordered to match. You can check the arithmetic yourself with a calculator.

    Where do the pizza test’s comparison numbers come from?

    The comparison numbers come from Krider, Raghubir and Krishna’s 2001 paper: Study 2, in which 38 students picked which of six squares matched a cut-out circle, and Study 3, in which 90 students judged pizzas. The result sheet also draws on Chandon and Ordabayeva (2009) for how perceived size grows with real size. All are group results with paper shapes, so you are compared by direction and never ranked.

    Do restaurants use the square pizza illusion?

    The 2001 study was written with a real restaurant in mind, and its authors ended with concrete advice on which shape to make pizzas and how to display them. The result sheet gives that advice, what the students offered to pay, and a cafeteria study of cream-cheese tubs, but only after you finish, because each of them gives away which way the illusion runs.

    Is the pizza test a vision test?

    No. The pizza test is a game about an area illusion, not a check of eyesight, and it doesn’t give a percentile because no study has published how individuals spread on a task like this. Your answers stay on your device; a challenge link carries only which rounds were spot on.

    Where the numbers come from

    Study figures are group results from Krider et al. (2001): Study 2 (38 students choosing among six squares), Study 3 (90 students judging pizzas), Studies 4 and 6 and footnote 12. The turn effect shown on the result sheet is our calculation from Study 2’s two group averages. The triangle comparison comes from older studies as reviewed in the same paper. The 18-inch and 45 cm comparisons are plain arithmetic. You are compared by direction with group averages, never ranked; the site does not publish its own players’ data.

    How it is scored

    Method of adjustment

    You change one shape until it looks equal to a fixed one. The setting you stop at, B’s area divided by A’s, is your point of subjective equality: 1.00 is perfect, 1.06 means B had to be 6% bigger before it looked as big.

    The 45° turn

    Rounds 1 and 4 combined give the square-to-round area at which a square on its side looked equal to you; rounds 2 and 5 do the same for a square on its corner. Side divided by corner is how much the turn changed the square for you. A change of 3% or more counts as fooled by the turn, and the result sheet says which way yours went and which way most people go.

    Spot on

    A round within ±5% of equal area.