To measure for a nursing bra, you need two measurements: your underbust (ribcage) and your fullest bust. The difference between them determines your cup size, while the underbust gives you your band size. The whole process takes about two minutes with a soft measuring tape.
When to Measure
Your breasts change significantly throughout pregnancy and into the postpartum period, so timing matters. Around 36 to 38 weeks of pregnancy is the sweet spot for measuring. By that point, your breasts have done most of their growing, and the size you measure will be close to what you’ll need during breastfeeding. If you measure too early in pregnancy, you’ll likely end up with a bra that’s too small by the time you actually need it.
That said, your size can still fluctuate once your milk comes in, typically a few days after delivery. Many people find their breasts are noticeably larger during that initial engorgement phase before settling into a more stable size over the following weeks. Measuring at 36 to 38 weeks gives you the best balance between accuracy and having a bra ready when you need it.
Step 1: Measure Your Underbust
Put on a non-padded bra. Wrap the measuring tape around your ribcage, right where the band of your bra sits (just below your breasts). Keep the tape level all the way around and pull it snug, but not tight enough to dig in. Round to the nearest whole number.
Your underbust measurement in inches converts to a band size using standard groupings:
- 26 to 27 inches: band size 30
- 28 to 30 inches: band size 32
- 31 to 33 inches: band size 34
- 34 to 36 inches: band size 36
- 37 to 38 inches: band size 38
- 39 to 40 inches: band size 40
- 41 to 42 inches: band size 42
- 43 to 44 inches: band size 44
So if your ribcage measures 33 inches, your band size is 34.
Step 2: Measure Your Full Bust
With the same non-padded bra on, wrap the measuring tape around the fullest part of your bust. This is usually right at nipple level. Keep the tape parallel to the floor and don’t compress the tissue. You want an accurate reflection of your actual fullest point, not a flattened one. Round to the nearest whole number.
Step 3: Calculate Your Cup Size
Subtract your band size (not your raw underbust measurement) from your full bust measurement. Each inch of difference equals one cup size, starting from A.
- 1-inch difference: A cup
- 2-inch difference: B cup
- 3-inch difference: C cup
- 4-inch difference: D cup
- 5-inch difference: DD/E cup
- 6-inch difference: F cup
- 7-inch difference: G cup
Here’s a full example: your underbust measures 37 inches, which rounds up to a band size of 38. Your full bust measures 44 inches. Subtract 38 from 44 to get 6, which means your cup size is F. Your nursing bra size is 38F.
How a Nursing Bra Should Fit
A well-fitting nursing bra feels supportive without restricting. The band should sit level around your torso and feel firm on the loosest hook when brand new. This gives you room to tighten it later as the elastic stretches over time. If you’re buying during late pregnancy, starting on the loosest hook also leaves room for your ribcage to shrink slightly after delivery as your uterus contracts and your organs shift back into place.
The cups should fully contain your breast tissue without gapping at the top or spilling over the edges. A small amount of extra room in the cup is normal and even expected, since your breasts will fluctuate in size between feedings. Some people go up a cup size from their measured size to accommodate that fullness right before a feeding session.
The nursing clips (the hardware that lets you drop the cup for feeding) should be easy to open with one hand, since your other arm will be holding a baby. Test this in the store or check reviews if you’re buying online. Clips that require two hands or excessive force quickly become frustrating multiple times a day.
Sizing Between Brands
Nursing bra sizing is not perfectly standardized across brands. Some brands use traditional bra sizing (like 36D), while others use simplified S/M/L systems that correspond to a range of bra sizes. If a brand uses S/M/L, check their specific size chart, because the ranges vary. A medium in one brand might cover 36B to 36D, while another brand’s medium covers 34C to 36B.
If you fall between two sizes, go with the larger one. A slightly roomy nursing bra is far more comfortable than one that’s too snug, especially during the early weeks of breastfeeding when your supply is regulating and your breast size shifts throughout the day.
Does Fit Affect Milk Supply?
You may have heard that wearing a too-tight bra can cause clogged ducts or mastitis. There is actually zero scientific evidence supporting this. A bra cannot cause internal breast pathology any more than a tight belt can cause an intestinal blockage. That said, a bra that’s too tight is simply uncomfortable, and comfort matters a lot when you’re wearing something around the clock during the postpartum period. Fit your nursing bra for comfort and support, not out of fear that the wrong size will interfere with breastfeeding.