How to Do a Number Trick to Guess Someone's Age

Co-authored by Kevin Wang

Last Updated: September 23, 2024 Fact Checked

There are lots of number tricks you can do by using math. You can give someone a particular set of instructions and calculations and their current age can be deduced with a bit of mental math. To them it appears as though you are performing a magic trick, but all you need to know are the instructions. The math will work every time. Another set of instructions can be given to get the month and day of their birth. You can also use math to appear to guess the age of a stranger.

Method 1

Method 1 of 3:

Deducing Someone’s Age Using a Selected Number

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1
Ask your friend to choose a number between two and ten . To make the game more fun, you might ask them how many times per week they would like to have ice cream, go out to eat, or something similar. Once they have a number in mind, ask them to announce it. ^{[1]
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- Say that they choose the number 6. This will be the example used throughout this method.
2
Multiply the given number by two . You can do this math by hand or your friend can use a calculator for the rest of the trick. Explain to your friend that they must hit the equal (=) button after each instruction you give.
- Example: 6 x 2 = 12.
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3
Add five to the answer.
- Example: 12 + 5 = 17.
4
Multiply that number by 50 .
- Example: 17 x 50 = 850.
5
Ask your friend if they’ve already celebrated their birthday for this year. If their birthday has passed already, then tell them to add 1767 to the prior result. If their birthday is coming up, then ask them to add 1766 to the prior result.
- Example 1: (with prior birthday): 850 + 1767 = 2617.
- Example 2: (with upcoming birthday): 850 + 1766 = 2616.
- Be aware that the numbers are for the year 2017. You need to add a number for each year that passes. For 2022 it is 1771 and 1772 if they have had their birthday.
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- Example 1: 2617 - 1981 (year of birth) = 636
- Example 2: 2616 – 1981 (year of birth) = 635
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Look at your final answer. The first digit, the one in the hundred’s place, will be the original one chosen. The remaining two numbers will reveal your friend’s age.
- Example 1: The answer is 636. 6 was the original number chosen. 36 is the current age.
- Example 2: The answer is 635. 6 was the original number chosen. 35 is the current age.

Method 2

Method 2 of 3:

Using a Calculator to Find Someone’s Age

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1
Ask the person to multiply the first number of their age by five . For the sake of this example, let’s say the person is 35 years old. They can use a calculator or scratch paper for this. Let the person know that they must hit the equal (=) button after each instruction you give. ^{[2]
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- Example: 5 x 3 = 15.
2
Tell them to add three .
- Example: 15 + 3 = 18.
3
Tell them to double the resulting figure.
- Example: 18 x 2 = 36.
4
Have them add the second number of their age to that number.
- Example: 36 + 5 = 41.
5
Tell them to subtract six . The answer will be their current age. ^{[3]
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- Example: 41 – 6 = 35.

Method 3

Method 3 of 3:

Using a Calculator to Deduce the Month and Date of Birth

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1
Enter the number seven . Then, multiply it by the month of their birth . For this example, let’s say their date of birth is May 28, 1981. ^{[4]
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- Example: 7 x 5 (birth month: May) = 35.
2
Subtract one . Then, multiply by 13 . ^{[5]
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- Example: 35 – 1 = 34
- Then: 34 x 13 = 442.
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Add the day of their birth . ^{[6]
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- Example: 442 + 28 = 470.
4
Add three . Then, multiply by 11 . ^{[7]
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- Example: 470 + 3 = 473.
- Then: 473 x 11 = 5,203.
5
Subtract the month of their birth . Then, subtract the day of their birth. ^{[8]
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- Example: 5,203 - 5 (May) = 5,198.
- Then: 5,198 – 28 = 5,170.
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Divide by 10 . Then, add 11 . ^{[9]
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- Example: 5,170 ÷ 10 = 517.
- Then: 517 + 11 = 528.
7
Divide by 100 . The first digit, before the decimal, is their birth month (May). The digits after the decimal denote the day of birth (28). ^{[10]
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- Example: 528 ÷ 100 = 5.28.

Community Q&A

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Question

This doesn't make sense. How does 17x50 equal 850? I thought it was 400.

Community Answer

Check your multiplication again. 17x50 does equal 850.

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Question

Could I also subtract their birth year from the current year to determine their age?

Alexis Styles-Rainies-Thompson

Community Answer

No. It’s either current year if their birthday has passed, or last year if their birthday hasn’t passed yet.

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Question

Is this just a trick or does it really help you guess someone's age?

Community Answer

If done correctly, this should actually help you determine a person's age.

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