Nursing Bra Sizing Guide
Research-backed, with the measuring method and an honest account of where sizing stops being reliable.
| # | Pick | Best for | Score | Price |
|---|---|---|---|---|
| 1 | Cake Maternity Tutti Frutti Wire Free Nursing Bra for B-H CupsB-H cups, properly graded | When letter sizing has never fitted | 8.8/10 | |
| 2 | Kindred Bravely Essentials Crossover Nursing Bra Wire Free 2 PackStretchy, spans a size or two | When you cannot predict your size | 8.6/10 | |
| 3 | Gratlin Cotton Nursing Bra for BreastfeedingTraditional band-and-cup, cotton | Conventional sizing at a lower price | 7.9/10 |
Rows link to the full write-up below. Scores are ours, from published specs and what buyers consistently report — we have not worn these.
By Selena H. · Last reviewed: October 2026
As an Amazon Associate, we earn from qualifying purchases. We do not set prices and we do not show them here — every link goes to Amazon, where the current price lives. How this site is funded.
The short answer
Measure your band under the bust and buy on that number. Expect your cup size to change twice — when milk comes in, and again as things settle around six weeks. Buy stretchy styles before the birth so they span a size or two, and buy fitted bras at six weeks postpartum. Above a D cup, buy band-and-cup sizing rather than S/M/L.
How to measure
Two numbers, both easier than people expect, and you want to do this standing in front of a mirror.
Band
Measure around your rib cage directly under your bust, with the tape level and snug but not pulled tight. That number, rounded to the nearest even number in US sizing, is your band.
This is the measurement that matters most. The band does the supporting; if it is loose, the straps take the weight and your shoulders hurt by lunchtime.
Cup
Measure around the fullest part of your bust, tape level, not pulled tight. Subtract your band measurement from this number: each inch of difference is roughly one cup size.
One inch is an A, two a B, three a C, four a D, and so on. It is approximate, and it is a starting point rather than an answer.
When to measure
At 36 weeks for the bras you buy before the birth. Your rib cage has done most of its expanding by then, so the band is close to what it will be afterwards.
At six weeks postpartum for fitted bras. By then your size has largely settled and it is worth measuring again properly.
Not between days two and fourteen after the birth. Milk coming in changes size dramatically and temporarily, and anything you measure then is a size you will not be in a fortnight later. More in when to buy nursing bras.
What changes, and when
| When | What happens |
|---|---|
| First trimester | Tenderness arrives; some growth; band largely unchanged |
| Second and third trimester | Rib cage expands, so band increases, often by one or two sizes |
| Days 2-5 postpartum | Milk comes in; size can change dramatically and temporarily |
| Weeks 2-6 postpartum | Settles; rib cage contracts somewhat; cup stabilizes |
| From 6 weeks | Size relatively stable; worth buying fitted bras |
Why letter sizing fails above a D
An S-to-XL bra is one pattern graded four times, and the grading increases band and cup together in a fixed ratio. That works through a middle range and fails at the edges.
If you have a narrow rib cage and a large cup, an XL gives you a band that gapes. If you have a wide rib cage and a large cup, the XL cup is still too shallow. Either way you get a bra that is simultaneously too big and too small, which is a pattern-grading limitation rather than anything you are measuring wrong.
Band-and-cup sizing varies the two independently. It costs manufacturers more, which is why those bras are more expensive, and above a D cup it is the difference between a bra that works and one that does not. See nursing bras for large breasts.
Checking the fit
- Band: snug on the loosest hook when the bra is new, so you have tightening room as the elastic relaxes. You should be able to get two fingers under it and no more.
- Cup: no spillage at the top or the side, and no gaping. At a larger size, check the side specifically — side support is what stops the whole thing shifting when you move.
- Straps: should not be taking the weight. If your shoulders ache, the band is too loose, and tightening the straps makes it worse rather than better.
- Clips: practice opening them one-handed before you need to. Sturdier clips on larger-cup bras are more durable and harder to work.
- Nothing should press persistently in one spot. This is the one that matters beyond comfort.
Where the sizing is genuinely unreliable
Said plainly rather than pretending to a precision nobody has:
- Letter sizing is brand-specific. An L from one brand is not an L from another, and there is no standard to appeal to.
- The cup calculation is approximate. The inch-per-cup rule is a rough guide and breaks down at the extremes.
- Nobody can predict your postpartum size. Not us, not a size calculator, not a shop. This is why stretchy styles before the birth and a proper measure at six weeks is the right approach.
- Stretch content is rarely specified usefully, and it determines how much a size error matters.
Where reviews consistently report a brand running small or large, that is real signal. Where we do not know, we say so — which is the standard in how we pick.
When fit is a medical question rather than a sizing one
A bra pressing persistently on one area of breast tissue is the thing to avoid, which is why fit matters here beyond comfort.
Speak to your midwife, GP or a lactation consultant promptly about a tender lump, a red or hot patch, a painful area that does not settle, flu-like symptoms with breast pain, or cracked and bleeding skin. Those are common and treatable, and none of them is solved by a different size. ACOG and the NHS both publish guidance on breastfeeding problems worth reading in advance.
Every pick, in detail
Scores below are ours. They come from the published specification sheets and from the patterns in what buyers consistently report — not from wearing these ourselves.
Top pick · When letter sizing has never fitted
8.8/10Cake Maternity Tutti Frutti Wire Free Nursing Bra for B-H Cups
The pick for anyone whose band and cup are not in the ratio letter sizing assumes. Cups B to H graded independently of the band, from a brand that only makes maternity bras, which is where sizing consistency comes from.
- Cups
- B to H
- Wires
- Wire-free
- Access
- Clip-down
- Designed for
- Larger cups without an underwire
What works
- Band and cup sized independently
- Cups to an H with real engineering
- Specialist brand, consistent across the range
- Wire free
What it gives up
- Higher price
- Fewer colors
- Firmer fabric
- Clips at night
Skip it ifletter sizing already fits you well.
#ad · affiliate link · prices live on Amazon, checked October 13, 2026
Pick 2 · When you cannot predict your size
8.6/10Kindred Bravely Essentials Crossover Nursing Bra Wire Free 2 Pack
The practical answer to a moving target: stretchy enough to fit across a size or two, which is what you need in the weeks when your size changes twice. Buy these before you know, and fitted bras after.
- Wires
- Wireless
- Access
- Pull-aside crossover, no clips
- Pack
- 2
- Use
- Sleep and lounge
What works
- Spans a size or two, which removes the prediction problem
- No clips
- Two pack
- Comfortable round the clock
What it gives up
- Minimal support
- No shaping
- Not enough above a D cup
- Stretches out
Skip it ifyou need support more than flexibility.
#ad · affiliate link · prices live on Amazon, checked October 13, 2026
Pick 3 · Conventional sizing at a lower price
7.9/10Gratlin Cotton Nursing Bra for Breastfeeding
Band-and-cup sizing without the specialist price. Straightforward construction, cotton, and conventional sizing you can reason about using your normal bra size as a starting point.
- Fabric
- Cotton
- Wires
- Wireless
- Access
- Clip-down
- Sizing
- Traditional band-and-cup sizing
What works
- Band-and-cup sizing at a mid price
- Cotton against skin
- Clip-down access
- Simple to size using your usual bra size
What it gives up
- Plain styling
- Cotton loses shape through a day
- Light support
- Fewer sizes stocked
Skip it ifyou need engineered support at a larger cup.
#ad · affiliate link · prices live on Amazon, checked October 13, 2026
Questions people actually ask
How do I measure for a nursing bra?
Measure your band around your rib cage directly under the bust, level and snug but not tight — that number rounded to the nearest even number is your band size, and it matters most.
Then measure around the fullest part of your bust and subtract the band measurement. Each inch of difference is roughly one cup size.
When should I measure for nursing bras?
At 36 weeks for bras you buy before the birth, because your rib cage has mostly finished expanding by then, and again at around six weeks postpartum for fitted bras.
Not between days two and fourteen after the birth — milk coming in changes size dramatically and temporarily.
Why does my nursing bra fit badly even in the biggest size?
Because letter sizing grades band and cup together in a fixed ratio. If your band and cup are not in that ratio, you get a bra that is too big in one dimension and too small in the other.
That is a pattern-grading limitation rather than a measuring mistake. Band-and-cup sizing varies the two independently and fixes it.
How do I know if a nursing bra fits?
The band should be snug on the loosest hook when new, with room for two fingers and no more. The cup should have no spillage at the top or side and no gaping. The straps should not be taking the weight.
If your shoulders ache, the band is too loose — tightening the straps makes it worse, not better.
What size will I be after the birth?
Nobody can predict it: not us, not a size calculator, not a shop. Your size changes when milk comes in and again as things settle over about six weeks.
That is why the right approach is stretchy styles bought at 36 weeks and a proper measure at six weeks postpartum.
Can a badly fitting bra cause problems?
A bra pressing persistently on one area of breast tissue is worth avoiding, which is why fit matters beyond comfort here.
Speak to your midwife, GP or a lactation consultant promptly about a tender lump, a red or hot patch, a painful area that does not settle, or flu-like symptoms with breast pain.
Sources
- American College of Obstetricians and Gynecologists. Breastfeeding Your Baby
- NHS Start for Life. Breastfeeding your baby
- MedlinePlus, US National Library of Medicine. Breastfeeding
Who wrote this
Selena H.
Selena owns Bump & Bolster and researches and writes every page herself. She built the site around the problems pregnancy throws at you in the order they arrive: sleep first, then pain, then nothing fitting, then recovery.