How to tell a girl's bra size?

A cup=lemons B cup=oranges C cup=grapefruit D cup=cantaloupe

No way I think to tell, and we all just can't make assumptions depending how old she is.

I am 16 and I am a 36e.

Most bikinis come in tops in Small, Medium and Large (and in your age group I highly doubt she is anything bigger than a "D") so it's kind of obvious. If she has a small chest get her a small...most women or "girls" in your case wear medium! :)

Well, you can text 5454 to find out how to tell. This video helps to: link

There is no way to tell just by looking. Eaither guess and save the recipt or ask her friends.

Don't guess, look at the tag on her bra or ask her friend to find out.

It's fine to buy a swim suit for her, but give her the receipt and tell her she can exchange it if she wants.

You can't tell just by looking, but I agree with those who said you should just get a gift card. Even if you do get a bathing suit that's the right size, that doesn't mean it's the right style for her. You're better off having her try them on before buying one.

Ask her friends, maybe they can help you out by checking her bra or just asking her if you can't do it yourself... But it's better to know exact size she wears!

There really isn't a way to tell just by looking. Companies all make different bra/bathing suit sizes anyway, and the style has a lot to do with the way it fits, too. I'd suggest a gift card.

You don't need to know bra size to buy a bathing suit.

why not take a peak at the tag in one of her bras? but bathing suits are usually not sorted by bra size (34B, 38C, 36A, etc.) bathing suit tops are just small, medium, and large and their very adjustable, so it won't be so hard to guess.

like it says on that ad. Base her bra size by melons, apples, etc

If indeed it is a bikini or tankini that is based on bra size and you aren't willing to settle on one that doesn't base itself on bra size then the easiest option is to ask her mother (awkward but explain why and that its a surprise gift) or her friends, most girls will discuss what bra size they are with each other when discussing clothes, if the friend doesn't know think of a way she could find out subtly in conversation about clothes.
It is nigh on impossible to find out a bra size just looking because there is 2 things to take into account, circumference of the back under the breast and around the rip cage and then the cup size.
Which is why it is known like 36 D, it means 36 inch circumference around the rib cage under the breast and a D sized cup (at least that is the method here, I know size calculations vary wherever you go).

Some places go by 3 measurements. link

You have to keep in mind even girls need to be measured in shop often to take into account there breasts changing shape and there weight altering and being young and still developing she may be a different bra size in 6 months time, not so easy to just look and guess I'm afraid.

Again if all else fails just buy a bikini that doesn't rely on bra size.

ok don't do that. if you wanna buy her a bathing suit then take her out and go bathing suit shopping but even if you do get the right size she might not like the style or the cut. or it may be uncomfortable. there are a lot of things that unless she's with you you just won't know. now how long have you two been dating and how much do you have to spend? if you have between 50-100 then buy her a little necklace. something simple but tells her you car. and that she can wear and remind herself of you.
now I'm not saying a bathing suit is a bad idea and who knows she might need one. but there are soo many different little things that can make it a great present or a horrible one and tossed to the side of the closet. so if you can get her to come with you on her bday or after and get it then she will get one that she likes and you like. but also remmber to get her something else even if its small. so she doesn't think you completely forgot about her and getting her a gift after the fact. buy her a dozen roses a card and a coupon book of stuff to do for her. and then a little I owe you swim suit or lingerie shopping.

I don't think a girl would like a bathing suit she hasn't at least tried on first. Maybe it's better to get a gift certificate first and then that way you can go looking for one together. Could be fun.