How do you work out a percentage difference between two prices without a calculator?

Answer 1

When estimating percentages mentally, I usually don't worry about accuracy.

As I practice mentally more and more, I get closer to the exact number.

(...science tells that accuracy depends on the measurement tool on hand: in this case, our brain). Or, if today I'm lazy to think, I just use a calculator, or fingers to count. It doesn't matter...

The key here is to first get the difference between the two numbers (DELTA).

...and to CORRECTLY report the percentage change.

The percentage change is RELATIVE to the moment being analysed.

Here are 3 examples:

1) If gas price increases from $1.50 to $2.00, then

the percentage represents 33% (0.50/1.50 = 0.33)

(the percentage change, or growth, is relative to the FIRST number)

2) If gas price has increased from $1.50 to $2.00, then

it is NOW 25% more expensive (0.50/2.00 = 0.25) than before.

(It means, if you'd like to tell how much higher percentually now you've became, you'll have to divide the DELTA from the LAST number)

3) And like the previous example:

If you buy a merchandise at $7, and sell at $8, then your percentual gross margin is 12.5%. ($8-$7)/$8.