I don't know the number, but here's a "weigh" to find out. Also, when you say "standard", ... the size might be standard, but paper comes in different grades or weights. Two envelopes made out of different paper might have quite different weights even if they are the same size. One envelope doesn't weigh very much, maybe 1/4 or 1/10 ounce ?. So it would be hard to weigh just one envelope without some high precision equipment. But if you took 10 or 20 or 50 of them, then they would weigh an amount which you could measure with household-type equipment. So, get a bunch of envelopes, weigh them, then divide by their number, and you'll have your answer.