ThirdLove Sizing Guide: Half Cups, Sister Sizes and How to Pick Your Size
- ThirdLove’s published chart lists bands 32 to 44 and cups AA to H, with more than 60 size combinations.
- Half cups sit between full cup letters, so you can adjust cup fit without changing the band.
- Sister sizes keep the same cup volume: change band and cup in opposite directions.
- A new bra should start on the loosest hooks so you can tighten it as it stretches.
- Order your best-guess size plus one neighbor, then return the one that fits worse.
What makes ThirdLove sizing different
Most bra brands sell sizes in whole cup letters: a B, then a C, then a D. The jump between two cup letters is a fairly large step in volume, and many people live in the gap between them, a C that is slightly too small and a D that is slightly too large. ThirdLove’s answer is the half cup, an extra size between each pair of letters. That gives you a finer ladder to climb and a better chance of finding a cup that fits your shape without compromise.
That is useful, but it also means your usual size is a starting point, not an answer. Three features of their system matter when you shop:
Half cups
Sizes that fall between full cup letters. If a full cup is too small and the next is too big, a half cup can land in the middle.
A wide range
The published chart covers bands 32 to 44 and cups from AA to H, with more than 60 combinations, so there is room above and below the common middle sizes.
A fit quiz
Their online Fit Finder asks about your current bras and preferences and recommends a size, including half cups. It is a good cross-check against a measured size.
One thing to keep in mind: ThirdLove’s page explains how to read the chart but does not document how its half cups are calculated. That is normal for brand sizing, and it is the reason to test rather than assume. Use the helper above as a map, and trust how the bra actually feels on you.
Measure first, in inches
ThirdLove’s calculator takes two numbers in inches, an underbust measurement and a bust measurement. The brand’s own guidance describes the band as a snug measurement around the ribcage right under the bust, and the cup as a measurement around the fullest part of the bust. Do both yourself before you order.
- Wear a thin, unpadded bra or none at all. Padding adds inches that are not yours.
- Measure the underbust. Wrap a soft tape snugly around your ribcage just below the bust, level all the way around, and exhale.
- Measure the bust. Wrap the tape loosely around the fullest part of your bust, level at the back and not squeezing.
- Run both numbers through a calculator. Try our bra size calculator, then compare with ThirdLove’s own tool and Fit Finder.
- Note the cup letter and the difference. If you sit between two cups, that is exactly where a half cup helps.
How half cups work
Picture the cup letters as a ladder. In a normal system, each rung is one letter: B, C, D. With half cups there is a rung between each pair. If you wear a C and find the cups slightly too big, you can move down half a rung instead of a whole one. If the C is slightly too small, you can move up half a rung. The band stays the same, so the rest of the bra keeps fitting.
The practical rule is simple. Gaping or wrinkling in the cup means the cup is too big, so go down. Spilling over the top or sides means the cup is too small, so go up. Half cups let you make that move in a smaller step.
Sister sizes: change the band, keep the cup volume
Half cups fix cup problems. Band problems need a different tool. A sister size changes band and cup together so the cup volume stays about the same. ThirdLove’s chart uses the same idea: sizes in the same row share cup volume, for example 36C, 38B and 40A. Going down a band means going up a cup letter, and going up a band means going down a cup letter.
| Your size | Tighter band (cup goes up) | Looser band (cup goes down) |
|---|---|---|
| 32B | 30C | 34A |
| 32C | 30D | 34B |
| 32D | 30DD | 34C |
| 34B | 32C | 36A |
| 34C | 32D | 36B |
| 34D | 32DD | 36C |
| 36B | 34C | 38A |
| 36C | 34D | 38B |
| 36D | 34DD | 38C |
Note that a 30 band is below ThirdLove’s published range, which starts at 32. If your sister-size option falls outside their bands, use the other direction or a different solution. The sister size calculator shows every nearby combination, and the plus-size bra calculator covers larger bands and cups.
A two-size ordering plan that avoids a return spiral
The biggest source of frustration with any new brand is ordering one size, getting it slightly wrong and starting over. A simple plan reduces that to one round, at most.
- Pick your anchor size. Use your measured size, cross-checked against ThirdLove’s calculator or Fit Finder. If they agree, that is your anchor.
- Pick one neighbor. Choose the single most likely adjustment: a half cup up if you usually spill, a half cup down if you usually gape, or a sister size if the band is your usual problem.
- Order both, in the same style. Comparing in one style removes the noise of different cuts.
- Try both with the loosest hooks. Fasten on the loosest hooks first, as the brand suggests for a new bra, so there is room to tighten later.
- Keep the better fit, return the other. Check the return terms before you order so you know the window and the process.
When you shop, a soft tape makes the measuring step much easier. If you need one, browse soft body measuring tapes.
The five-point fit check
- Band: level around the ribs, snug, and it stays in place when you raise your arms.
- Cups: your breast tissue sits fully inside, with no spilling at the top or sides.
- Wrinkles: the cup fabric lies smooth, with no gaping or collapse at the top.
- Center: the center panel sits flat against your breastbone.
- Straps: they support without digging into your shoulders. They should not carry the load.
Symptom, cause, and the size move that fixes it
| What you notice | Likely cause | Try this |
|---|---|---|
| Cup wrinkles or gapes at the top | Cup too big or wrong shape. See the cup gaping guide | Half cup down, or a shallower style |
| Tissue spills over the cup | Cup too small | Half cup up, or a full cup up if it persists |
| Band rides up the back | Band too loose. See why bands ride up | Smaller band with a sister-size cup |
| Band digs in or leaves grooves | Band too tight, or cups too small pulling it tight | Larger band with a sister-size cup, or check cup size |
| Wire digs in | Cup too small or wire sitting on tissue. See underwire digging in | Half or full cup up |
| Bulge at the side or underarm | Band too loose or cups too small. See the side bulge fix | Band down with sister cup, or cup up |
| Fits on one side but not the other | Breasts differ in size | Fit the larger side and use a removable insert on the smaller |
If more than one row applies, fix the band first, then the cup. And if it is not obvious which problem you have, the bra fit problem solver lets you work back from the symptom.
Half-cup sizing vs standard sizing
| Feature | Standard full-cup sizing | Half-cup sizing |
|---|---|---|
| Steps between cup letters | None; jump a full letter | A half step between letters |
| Best for | Shoppers who fit a whole letter well | Shoppers between two cup letters |
| Size choices to compare | Fewer | More, so a two-size order is more useful |
| Risk | Settling for a near fit | Decision fatigue from more options |
| How to simplify | Use the sister-size ladder | Anchor size plus one neighbor |
If you already fit a whole letter comfortably, you may never need a half cup. They help most when you are the person who always seems to be between sizes. For a visual check on how cup volumes compare, the breast size comparison is a useful reference, and you can find every size guide in the bra size guides index.
Four sizing myths worth retiring
Brands differ in cut, stretch and cup shape. Treat any size as a starting point.
For people between letters they give a smaller step. For people who fit a whole letter, they may not matter.
Straps cannot replace band grip. Change the band size with a sister cup.
The brand’s guidance is to start on the loosest so you can tighten as the elastic relaxes.
A calculated starting size makes any brand’s size chart easier to read.
Calculate my sizeSister size calculator
