Bolt Size for Rear Bumper to Frame?

So had my son helping me adjust my Pelfreybilt rear bumper today and found out 14 year olds and impact drivers are a dangerous combination! He somehow managed to strip one of the 5 bolts that attach the bumper to the frame and I've been unable to figure what size they are? Anybody happen to know what size I need to get???

 

You can remove one of the others and match it up at a store . Edit , you need to go there anyway.

 

That's the plan but figured I'd check here in case it's some size that I'll need to special order or a Toyota specific situation...

 

I would not worry about it . It is already an aftermarket bumper . Even if you have to , you can get the closest size from mm to SAE , (often close ) and get same grade bolt . Most likely grade 5 or 8. Someone in Home depot will be able to tell from looking at the removed one .