Okay so I'm 13 and the bra's that I'm currently wearing are a 36C, but they are too small on the cup and too big on the back. I've measured up and I know that I need a 34D but my mom won't believe me :/ how can I convince her to let me get a bigger size?

You can show her how the bra fits you. Most department stores have salespeople who will correctly measure you. Ask her to take you.

You can show her how the bra fits you. Most department stores have salespeople who will correctly measure you. Ask her to take you. She has taken me before but they tried to give me a smaller size even though they could see my current one was crushing me. Even my mom knew that wasn't right, Its more the fact that she won't accept that I'm growing up and need a bigger size

She has taken me before but they tried to give me a smaller size even though they could see my current one was crushing me. Even my mom knew that wasn't right, Its more the fact that she won't accept that I'm growing up and need a bigger size
Maybe you can try another brand. Bras differ with each brand.

Maybe you can try another brand. Bras differ with each brand. yeah I thought that :)

If you are 13 and wearing a C or D cup, you may need an evaluation in a lingerie shop, not just a department store that sells bras.

If you are 13 and wearing a C or D cup, you may need an evaluation in a lingerie shop, not just a department store that sells bras.

The nearest lingerie shop shut down so we would need to travel further than necessary :/

As a mom, I can't imagine seeing my daughter in a bra that is too small and not getting one that fits.
This sounds a bit odd.

As a mom, I can't imagine seeing my daughter in a bra that is too small and not getting one that fits.
This sounds a bit odd.
I don't think she's realising when I say its too small, how badly it fits, she thinks its just 'a bit tight'

I don't know what to tell you. Is it uncomfortable or does it just not fit?
Again, I can't imagine a mom wanting you to go around with a bra that is too small.

I don't know what to tell you. Is it uncomfortable or does it just not fit?
Again, I can't imagine a mom wanting you to go around with a bra that is too small.
Because it's the cup that's too small, its effectively pushing my chest inwards and upwards so it can get pretty uncomfortable, especially when playing sports or running. It can also cause some awkward situations if I have a lower cut top on because I have a lot of guy-friends (so yeah, you get the picture with that part)

Do all of your bras fit that way? Show her what you mean.

Yes, have her come to your room, and have the bra on and show her where it is not fitting properly