I am going to build a safe , looking to buy sheet steel cut to size and then weld it . I’m leaning towards .25” but could go bigger. This isn’t a gun safe, basically a square box for other types of valuables . I prefer heavy, hard to move .

What size do you recommend? Thickness

Are you just building a box or a safe?

If a safe you will need an inner box and an outer box and fill in between with concrete. Metal thickness won't matter much.

If it's just a box no matter what thickness you use, someone is just a cutting torch away from getting in there.

If you know someone that can get you a sheet of A36, that should be plenty. Its the same plate they use to cover roads while they are working on it. Those plates are 1" thick and are fairly heavy. (41lb /ft) I had a guy that was going to give me one for a welding table but it was not as flat as I would have liked.

You need it to be fire proof or just a big old metal box?

After going through this more than once I suggest finding a gun safe that’s on sale and the size and weight you’re looking for. The locking mechanism on your diy project will be very difficult.

Just a thick metal box that won’t be moved , I was thinking 3/8” but I’ll check the one inch out

I’d go 3/16” and I would rig it up where I could bolt it to the floor from the inside of the box

Our floor safes have 1/4” walls & doors. I’m not sure how much weight the fireproofing adds but they’re heavy. Bolting them to the floor adds additional protection from honest thieves.

Have considered looking at a job box?

I haven’t mostly because I’ve always thought they were too big but I will check them out