How to do the calendar square trick

Ask someone to draw a three-by-three square around nine dates on any monthly calendar. If they tell you the smallest date, add 8 and multiply by 9, and you know the total of all nine dates before they have finished adding. It works because the total of any such square is always nine times the date in the middle. No sleight of hand, no setup, and any wall calendar or phone calendar will do.

What does the audience see?

You hand a friend a calendar and look away. They draw a box around any three-by-three block of dates, nine numbers in all, and tell you just one of them: the smallest, in the top-left corner. Straight away you write a number on a slip of paper, fold it and put it in an envelope. Then they add up all nine dates, slowly and carefully, perhaps with a phone calculator. When they open the envelope, your number matches their total.

What do you need?

• A calendar laid out in the usual way, with seven days to a row. A printed wall calendar, a desk calendar or a calendar app on a phone all work.
• A pen, a slip of paper and, if you like, an envelope.

The square must sit inside one month, so all nine dates are real dates.

How do you do the calendar square trick?

1. Let them choose freely. Ask your friend to draw a box around any three-by-three block of dates, three across and three down.
2. Ask for the smallest date. This is the number in the top-left corner of their square.
3. Add 8. This gives you the date in the middle of the square.
4. Multiply by 9. This is the total of all nine dates. Write it down and seal it away.
5. Let them add. Have them total all nine dates themselves, then reveal your prediction.

Here is an example from November 2026. If your friend boxes the square whose top-left date is the 9th, the nine dates are 9, 10, 11, 16, 17, 18, 23, 24 and 25. You add 8 to get 17, then multiply by 9 to get 153. Add the nine dates yourself and you get 153.

Why does it always work?

On a calendar, each date is one more than the date to its left and seven more than the date above it. Call the top-left date n. The nine dates in the square are then:

• n, n + 1, n + 2
• n + 7, n + 8, n + 9
• n + 14, n + 15, n + 16

Add them up and you get 9n + 72, which is the same as 9 × (n + 8). Since n + 8 is the middle date, the total is always nine times the middle.

There is another way to see it. Each date in the square has a partner on the opposite side of the middle, such as the top-left and bottom-right corners, and every pair averages out to the middle number. So all nine dates together are worth nine middles.

Can you do it the other way round?

Yes, and it makes a good follow-up. This time, let your friend add up their square in secret and tell you only the total. Divide it by 9 to get the middle date, then subtract 8 to get the top-left date. From there you can name all nine dates in their square, as if you could see through the paper. With the example above, a total of 153 divided by 9 gives 17, and 17 minus 8 gives 9.

How do you make it look like mind reading?

• Do the sum before they do. The trick lands because you write your answer while they still face nine numbers to add. Don’t rush, but don’t pause to calculate either. Practise multiplying by 9 in your head: multiply by 10, then subtract the number once. For a middle date of 17, that is 170 − 17 = 153.
• Seal the prediction. An envelope, a folded slip under a glass or a note left in plain view makes it clear you committed to the number first.
• Use their calendar. A calendar app on their own phone shows you didn’t prepare anything.
• Don’t repeat it on the same calendar. If you do it twice, someone may notice every total divides neatly by 9. Move on to a different number trick instead.

For more self-working number magic, try the 1089 number trick, the magic square trick or the age cards trick.

What helps you take it further?

• Mathematics, Magic & Mystery by Martin Gardner. A 174-page softbound book that links card tricks, mental calculation feats and stage “mind-reading” to probability, sets, number theory and topology. Its 115 sections include demonstrations with pure numbers, and its description says none of the more than 500 tricks needs sleight of hand.
• Mathematical Recreations and Essays by W. W. Rouse Ball. A free, complete 394-page PDF of the 1922 tenth edition. Its contents include arithmetical recreations, magic squares and calendar problems.

For tricks that look like real mind reading, browse our easy mentalism collection.

Frequently asked questions