If the room is square, you could take the square root, though that wouldn't give you an exact measure if the room is not exactly square.

Yes, square root.

Use the square roo--- oh.

What they said.

I always try to "back in" to the number (so, 400 square feet is ..uh.. ..hm.. 20x20!). Hell if I remember how to do square roots.

My high school physics teacher was an ex-Navy pilot, and he was short-ish and bald and had very thick aviator-style glasses and walked like a wrestler. He would tell us stories about doing recon in Cambodia during the Vietnam War, which was way, way, way not allowed by the govt. at the time.

He also used to love to go outside and stare intently at the roof of our school, simply to see how many students and teachers would start doing the same.

Anytime he explained an equation involving a square root, he would say, "square ROOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOTTTTTTTTT!!!!" and draw the symbol such that it extended way beyond the prescribed place.

So I think of Mr. Byrd every time I think of square roots, and I am grateful he showed up in my math/science education, because I think he was the only math or science teacher I ever had who actually got me interested in the real-world application of the concepts he was teaching, even if some of his test questions were on the realm of "Will your gun shoot a bullet fast enough to stop a tiger jumping at you?" Because, seriously, that's a cool physics question.

I'm completely hopeless at maths. If faced with that question, I'd start making guesses until something seemed about right. Or use a calculator, more likely.

When the square root is not obvious, I always just make it 10 by X and picture what a long room would look like and then reconfigure that space in my head to a square shape.

A nun taught me to do square roots by hand in 7th grade. It's a pretty useless skill. She probably knew I was an atheist and was punishing me for it (joke). Memorizing the multiplication tables from 1-20 (same nun) was more useful.

The function is the square root, usually abbreviated to sqrt in computery shit. You can type "square root of 300" or "sqrt(300)" right into Google. (Or "300^0.5".)

If it's not square, you just have to measure the walls, or there's no way to know. Unless you just want the perimeter of the walls (as in, you're painting) but then you'd want to know the ceiling height too.

Yes, the way to derive square roots is a big pain. Better to memorize the easy ones and use a calculator for the rest.