How do I measure my blanket?

I am making a baby blanket. The bottom (c/o edge) is about 30"…so I thought. I just re-measured and it’s like 27". Then I measured the center of the blanket and it was like 25" then 26"!!!:hair:

How do I measure it to make an accurate measurement? I’m knitting in garter stitch and it’s stretchy. So the measurements change depending on how I place/hold the blanket! I ask because I wanted to make it into a square. So I have to figure out when to stop knitting!:eyes:

Lay it out on a table or bed, pull it flat so it’s not bunchy, but don’t stretch it out. Then measure. It can be an inch or so off, doesn’t need to be an exact square.