Originally Posted by

**davidedric**
The key to international paper sizes is the ratio between their edges. It's always the square root of 2, 1.414 give or take.

That's why you can always get the next size down by folding in half, and why actual dimensions are such odd looking numbers.

It's also why I have 1:1.414 in my Lightroom preset crop ratios!

**What is a bit odd with photo paper in the UK is that, while larger sizes are generally A3 or A4, smaller sizes are still sold as 6" by 4", or 150mm by 100mm. No idea why.**

Dave