This simple jean-sizing hack is the only guide you'll ever need for your denims.

We’ve sent a human to the moon, invented gluten-free Vegemite and figured out how to refresh white shoes with denture cleaner, yet shopping for jeans is still one of life’s greatest mysteries.

Between length, rise, style and fit, it can be a total minefield.

To further muddle the equation, jean sizes sometimes go by a numerical digit which measures the size of your waist in inches. For example, a size 10, a medium, and a waist size 28 are all the same size… The mind boggles.

Watch: We went to Sydney Fashion Week and asked some punters what their most expensive fashion items were? This is what they said.

But we’ve discovered a handy hack you’ll want to commit to memory. An equation to find your jeans' waist size from your standard clothing size (e.g. a size 12).

Prepare to have your mind blown.

Shopping for women’s jeans sizes for sizes AU 6-11.

Your AU size is simply the two numbers of your denim waist size added together. This applies to waist sizes 24-29.

So, if you’re a size 24 waist, you’ll be an AUS size 6 because 2+4=6.

For example:

Size 25 waist – AUS size 7 (2+5=7)

Size 26 waist – AUS size 8 (2+6=8)

Size 27 waist – AUS size 9 (2+7=9)

Size 28 waist – AUS size 10 (2+8=10)

Size 29 waist – AUS size 11 (2+9=11)

Read more: ROADTEST: I'm a size 24 and went searching for the perfect pair of jeans.

Shopping for women’s jeans sizes for sizes AU 12-19.

To work out your denim waist size to your standard AU size, take the last number of your denim waist size and add 12.

Size 30 waist – AUS size 12 (0+12=12)

Size 31 waist – AUS size 13 (1+12=13)

Size 32 waist – AUS size 14 (2+12=14)

Size 33 waist – AUS size 15 (3+12=15)

Size 34 waist – AUS size 16 (4+12=16)

Size 35 waist – AUS size 17 (5+12=17)

Size 36 waist – AUS size 18 (6+12=18)

Size 37 waist – AUS size 19 (7+12=19)

Read more: 'I’m a short size 14-16. This is my quest for finding the right jeans.'

Shopping for women’s jeans sizes for sizes AU 20 upwards.

While this applies to all the previous sizes we’ve just listed, a really, really simple way to convert your jeans waist size to your standard pant size is simply to minus 18. Conversely, if you were going from your clothing size to your denim waist size, you would just add 18.

Size 38 waist – AUS size 20 (20+18=38)

Size 39 waist – AUS size 21 (21+18=39)

Size 40 waist – AUS size 22 (22+18=40)

Size 41 waist – AUS size 23 (23+18=41)

Size 42 waist – AUS size 24 (24+18=42)

Size 43 waist – AUS size 25 (25+18=43)

Size 44 waist – AUS size 26 (26+18=44)

Now if only denim brands could figure out a metric to make all women’s jean sizes fit consistently across the board… That would be nice, wouldn’t it?

Did you find this hack helpful? Where do you buy your jeans from? Tell us in a comment below.

Feature Image: Canva/Mamamia.