You might be able to get away it. C5 envelopes (6.4"x9") are very close in dimensions to the envelopes Americans usually use (6"x9"). The problem is that the cutoff point for letter size is 6.1", but unless the postal service is really strict about it, it could just slip through. Good luck anyhow.

Thanks both of you, I better hope that some of the celbs Ive written to will be kind enough to add an extra 26cents to my envelopes then

A4 and C5 are both counted as large envelope size.

I'm not familiar with C5 and A4 sizes, but generally here in the States, letter size is considered a #10 envelope (10.4 cm by 24.1 cm) or smaller.

You can go to the USPS site and check postage prices, but you'd have to do the math to translate from our weight system to yours so that you can figure out the postage for your SASE. Yeah, it's a headache. The good ol' USA is the only country that I know of that refused to convert to the metric system.

Could someone help me regarding envelope sizes please?

Can anyone tell me is an c5 envelope counted as letter size and an a4 envelope counted as a large letter size for the US postal prices?

The reason I ask is that I have been putting in c5 return envelopes with my 5x7 photos and they have weighed under 1oz so I have put on 98cents postage but that just seems really low price and Im worried Im putting the wrong amount on and will not end up getting my photos back.

Thanks