[Heaven's Night]

Albeit not perfect, this calculator is the best out there. Works to some people, messes up others. For me it always gives four or five sizes too big cups, but knowing to compensate for that, it is fairly accurate.

https://www.abrathatfits.org/calculator.php

[Mel87]

The best website I know of for measuring accurately is https://www.abrathatfits.org/calculator.php it will ask for slightly more information like leaning forward to be more accurate reading.

For your second question some brands can have up to 3 levels https://www.shyaway.com/wardrobe/complet...bra-guide/ usually you want to follow this with the current size you are this can very still between brands, cheaper off the rack stuff might all be level 1 at best and not give desired lift. I'd suggest not going down a size and it might be too tight and hurt. 

Have fun <3

[francesca]

ok, i understand the most important thing for me is to have breasts.
ivana

[9westgirlfla]

The more I read the bra size measurement methods and look at the sizing charts I think they either all have no clue or want to sell bras online and hope they do not get returned, so I stopped looking at them :-) 

The strangest one said to measure the chest size and ADD 4 inches (Good housekeeping) to get your band size. This method would give me a negative boob and I'm clearly a 36B+ (36 chest 40 boob) maybe even pushing a smaller C cup. Goodhousekeeping makes me a 40AA

The most reliable method that worked for me is:

1. Wear a comfortable unlined bra to position boobs not lying flat on the chest.
2. Measure snugly (but not tight) around the rib cage under the boobs, ADD 1 inch if it falls on an odd number (to avoid getting a band too small but they NEVER say this) 
3. Measure at the fullest part of the bust.
4. Record the difference.

Then it's the lookup chart that makes or break this method and all but one stink. 

Some (at my 4" difference) make me a D (Victoria's Secret) No way Jose!!! I SWIM in my GF's "C" bras. 

I can't find the chart I used but it I committed it to memory and it was like this

Subtract chest from bust

1" = AAA (when I was dead flat)
2" = AA   (I was Still flat)
3" = A     (Where I budded and my GP ordered a mammogram) 
4" = B     (my current measurement)
5" = C     If I hit 5" or more I'll report back :-)

With this lookup table I can (and have) been able to buy a 36B from just about every maker and be highly confident it will fit. The only issues I've had is some may be too tight around the rib cage, and if there was padding, push up or thick padding aside from feeling silly they were not my intention for wearing a bra but they DID fit.

[Heaven's Night]

Bra sizing is really peculiar art... And I say art because it has no decent rules and system that would always work as its a huge mess. What I have noticed is that it keep on getting more accurate. Closer to maturity I get and bigger I grow, more accurately calculators and traditional methods work. But nothing beats measuring volume of each breast, adding them together and comparing to a sister size chart which lists approximate volumes for each size.