This old thread but applies now with 2017 eclipse...

Someone says the numbers DO add up so I figure using two number 10s from cheap Harbor Freight welding goggles will be okay. I wouldn't plan on staring at it for extended periods in any case. Would be cool to put it on front end of telescope and view, but that's pushing it. Long ago I used 6 polarized filters (crossing two at 90 degrees, then three sets at 120 degrees) which made it almost black (easy to see sun) but don't know if it blocks UV and IR. I was stupid (younger..) and used it to look at the sun through a telescope... didn't take long and image got blurry (I don't remember seeing the sun) so looked at filter and it was MELTING... dumb bass... remember burning leaves with sun using mag glass and a telescope is much more powerful... glad I didn't burn my eyeballs (am 60 now so damage from 40 years ago would be apparent).