What's your method of calculating fast in your head instead of using a calculator/paper & pen?

Trying to break it down and find a part of the number I can calculate easily, then adding that to the rest of it. (Did that make sense?) Like if I want to multiply 24×8 I’ll do 8×20 + 8×4.

Here are 3 cool tips I just found:

1. Round Off to a Quick Approximation

First determine a level of accuracy you are happy with, then round the numbers appropriately before making the calculation. This is also useful to determine a quick ballpark figure before doing the actual calculation.

2. Break Up Into Parts

Say you have to multiply 613 by three. Try multiplying first 600 by 3, then 10 by 3 then 3 by 3 and adding the results.

4. Quickly Find Three Common Percentages

20% -> Divide the number by 5
33.3% -> Divide the number by 3
50% -> Divide the number by 2

Source

If I want to be correct, I use a calculator.

If I only want to be in the right ballpark, I’ll round all the numbers to the nearest power of 10 and then do the math. In Allie’s example, I’d round 24×8 to 10×10 and say the answer is “over a hundred.” This is TheBox193’s method #1 with a level of accuracy of “not very.”

If I want someone else to figure it out, I’ll declare some arbitrary number to be the answer until someone gets annoyed enough to prove me wrong.

jason: Wouldn’t it be more accurate to call 8 a 10 and 24 a 20? Then do 20×10 and say it’s around 200? Cause using 10×10 leaves you nearly 100 off.

…but I get what you’re saying.

Yeah, the order-of-magnitude thing works better with bigger numbers.

I agree, break the number up, do the tens first, then bust up the rest of it. This will give you the exact answer quickly without ballparking it.

24 times 12

4times 2 gives 8 (colum multiply)
cross multiply both numbers and add them 2times 2 + 4times 1 = 8
finally 2times 1 gives 2 (colum multiply)

thus 288

from speed math

Base10 is my friend.

12+4 = 10 + 6 to me.
2347×7 = 2000×7 + 347

Like everyone here, I just break it down into simple chunks and then mush them back together.

@squirbel, wouldn’t that be
2000×7 plus
300×7 plus
40×7 plus
7×7.
Just checking LOL

I am all about the 10s.

9×35 gets converted to
10×35 = 350 then
350 – 35 = 315