Bra Sister Sizes Explained: How to Finally Find a Bra That Fits
Bra Sister Sizes Explained: How to Finally Find a Bra That Fits
Here is a secret the lingerie industry doesn't tell you clearly enough: Cup sizes are not static.
A "C cup" on a size 32 band is smaller than a "C cup" on a size 38 band. The letter represents the difference between your band and bust, not a fixed volume of fruit (like the old "lemon vs. melon" analogy).
This is why you might be a 34C in one brand and a 32D in another. These are Sister Sizes.
If you've ever tried on a bra that fits in the cup but chokes you in the band, you don't need a random new size—you need a Sister Size.
Table of Contents
- ●What is a Sister Size?
- ●The Golden Rule of Adjusting
- ●Sister Size Chart
- ●Signs You Are Wearing the Wrong Size
1. What is a Sister Size?
Sister sizes are a group of bra sizes that share the same cup volume but have different band lengths.
Imagine the physical plastic wire (underwire) of a bra.
- ●The wire for a 34C is the exact same size as the wire for a 32D and a 36B.
- ●They hold the exact same amount of tissue.
- ●Only the length of the stretchy fabric band changes.
👙 Interactive Tool: Find Your Sister Size
Don't do the mental math. Enter your current size to see your Sisters instantly:
Sister Sizes (Same Volume)
Visualizer
Cup C Projection
2. The Golden Rule of Adjusting
When you change the Band size, you must change the Cup letter in the opposite direction to keep the same volume.
- ●If you go DOWN a band size ⬇️ ... you must go UP a cup letter ⬆️.
- ●Example: 34C band is too loose? Try a 32D.
- ●If you go UP a band size ⬆️ ... you must go DOWN a cup letter ⬇️.
- ●Example: 34C band is too tight? Try a 36B.
🧠 The Logic: If you make the band smaller, you are taking length away from the bra. To keep the same room for your bust, you have to add that space back into the cup.
3. Sister Size Chart
Here are the common families. Read across the row—all these sizes hold the roughly same volume.
| Small Band / Large Cup | Medium Band / Medium Cup | Large Band / Small Cup |
|---|---|---|
| 30D | 32C | 34B |
| 32D | 34C | 36B |
| 34D | 36C | 38B |
| 32DD (E) | 34D | 36C |
| 34DD (E) | 36D | 38C |
Note: While the volume is the same, the fit isn't. A 30D might hold the same amount as a 36A, but the 36A band will likely be way too wide for the person who needs a 30. Sister sizes usually only work well within one jump (up or down).
4. Signs You Are Wearing the Wrong Size
How do you know you need to switch?
1. The Band Rides Up If the back of your bra is pulled up towards your shoulder blades, the band is too big.
- ●Fix: Go Down a Band, Up a Cup. (e.g., 36B → 34C).
2. The "Quad-Boob" If your tissue is spilling over the top of the cup, the cup is too small.
- ●Fix: Keep the band, go Up a Cup. (e.g., 34C → 34D).
3. Gaping Cups If there is empty space at the top of the cup, but the wire fits well, the cup might be too tall or the band too loose.
- ●Fix: Try the Sister Size down. (e.g., 34C → 32D) or simply go down a cup (34B).
Finding the perfect fit is often trial and error, but understanding the math of Sister Sizes cuts that time in half.