Bra Size Calculator

Find Your True Bra Size

Studies consistently show that 70–80% of women wear the wrong bra size — most commonly a band that's too large and a cup that's too small. An ill-fitting bra causes discomfort, poor support, shoulder grooves, back pain, and an unflattering silhouette. The problem isn't that women don't care about fit — it's that bra sizing is confusing, inconsistent across brands, and rarely taught properly.

Our calculator determines your correct size from two simple measurements: underbust (band) and fullest bust circumference. It then converts between US, UK, and EU sizing systems and provides your sister sizes for alternative fit options.

How to measure: Use a soft tape measure while wearing a non-padded bra or no bra. Band measurement: wrap the tape snugly around your ribcage directly under your bust — firm but you should be able to slide a finger underneath. Bust measurement: wrap the tape around the fullest part without compressing the tissue, tape level and parallel to the floor. Stand naturally — don't puff out your chest or hunch.

How Bra Sizing Works

Band size equals your underbust measurement rounded to the nearest even number. If your underbust measures 31 inches, your band size is 32. If it measures 33, your band size is 34. Some fitting methods add 4–5 inches to the underbust measurement — this is outdated (from an era when bras had no elastic) and consistently produces bands that are too loose. Modern bras with elastic bands should fit snugly without the addition.

Cup size is determined by the difference between your bust measurement and your band size. Each inch of difference corresponds to one cup size: 1 inch = A, 2 inches = B, 3 inches = C, 4 inches = D, 5 inches = DD (or E), 6 inches = DDD (or F), 7 inches = G (or DDDD), and so on.

Example: Underbust measures 33 inches → band size 34. Bust measures 38 inches. Difference: 38 - 34 = 4 inches = D cup. Your calculated size: 34D.

This calculation provides a starting point. Breast shape, tissue density, personal comfort preferences, and brand variation mean the calculated size may need adjustment up or down in band or cup — which is why understanding sister sizes and fit indicators matters as much as the initial measurement.

International Sizing Systems

US sizing uses even numbers for bands (30, 32, 34, 36, 38, 40, 42) and letters for cups (A, B, C, D, DD, DDD/F, G, H, I, J). After DD, sizing becomes inconsistent across brands — some use DDD, others jump to F, others use E. This lack of standardization is one of the most frustrating aspects of US bra shopping, particularly for larger cup sizes.

UK sizing uses the same band numbers but a more logical and consistent cup progression: A, B, C, D, DD, E, F, FF, G, GG, H, HH, J, JJ, K. The double-letter system continues predictably into very large sizes. This consistency is why UK brands (Panache, Freya, Elomi, Curvy Kate) are often recommended for anyone above a US DD — their sizing is more reliable and their size range is broader.

EU/European sizing uses centimeter-based bands (65, 70, 75, 80, 85, 90, 95, corresponding roughly to US 30, 32, 34, 36, 38, 40, 42) and lettered cups similar to US but with slight variations in the upper range. French and Spanish sizing adds 15 to the EU band number (EU 75 = FR/ES 90).

Our calculator converts between all three systems so you can shop confidently from any international brand or retailer.

Sister Sizes: Your Secret Weapon

Sister sizes have the same cup volume but different band sizes. Understanding sister sizes is essential for navigating inconsistent brand sizing, shopping sales racks, and finding alternatives when your exact size is sold out.

How it works: Moving up one band size → go down one cup size (same volume, looser band). Moving down one band size → go up one cup size (same volume, tighter band). The cup letter changes but the actual breast space remains identical.

Example sister size chains: 30DD → 32D → 34C → 36B (all same cup volume). 32G → 34FF → 36F → 38E (UK sizing, all same cup volume). 28F → 30E → 32DD → 34D (all same cup volume).

When to use sister sizes: When a brand's 34 band fits like a 32 in your usual brand, try 34C instead of 34D (if 34D is your calculated size). When your exact size is unavailable, the sister size one band away is usually a workable alternative. When switching between countries' sizing systems — our calculator shows sister sizes in each system.

Limitations: Sister sizes maintain cup volume but change band tightness. If the band is the problem (too loose or too tight), sister sizing solves it. If the cup shape or coverage doesn't suit your breast shape, sister sizing won't help — you need a different style.

Signs of a Poor Fit and How to Fix Them

Band rides up in back: The band is too loose — it should sit level or slightly lower in the back than the front. The band provides 80% of support, so a loose band forces straps to compensate, causing shoulder pain. Fix: go down a band size (and up a cup to maintain volume). A properly fitting band should feel snug on the loosest hook when new.

Cups gap or wrinkle at the top: The cups are too large, or the cup shape doesn't match your breast shape. Full-coverage cups can gap on shallow breasts; plunge or demi styles may fit better. Fix: try a smaller cup or different style. If the cup fits at the bottom but gaps at the top, the shape is wrong rather than the size.

Spillage over the top (quad-boob) or sides: The cups are too small. Breast tissue should be fully contained within the cup — any overflow means the cup isn't large enough. Fix: go up one or two cup sizes. This is the most common fit issue because most women underestimate their cup size.

Underwire sits on breast tissue: The underwire should follow your breast's natural crease (inframammary fold) and lie flat against your ribcage, with all breast tissue inside the cup. If the wire presses on tissue at the sides or bottom, the cup is too small or too narrow. Fix: increase cup size, try a wider wire style, or try a brand that cuts for wider roots.

Center gore (between the cups) floats away from the chest: The cups are too small to fully contain the breasts, pushing the center gore outward. Fix: increase cup size until the gore tacks flat against your sternum. For very close-set breasts, a plunge style with a lower, narrower gore may be more comfortable.

Straps dig into shoulders: If straps leave red marks or grooves, the band isn't supporting the weight — forcing the straps to bear it. Fix: go down a band size (tighter band = more support from band, less from straps). Straps should be snug but not weight-bearing. Wide or padded straps can also help distribute the small amount of weight they should carry.

Straps fall off: The band may be too large (causing the whole bra to sit too low, shortening the strap path), or the cup style doesn't suit your shoulder width. Fix: tighter band, and try racerback or U-back styles if you have narrow-set shoulders.

How Bra Size Changes Over Time

Your bra size isn't a permanent number — it shifts with your body. Factors that change bra size include weight fluctuation (5+ lbs can shift band and/or cup), hormonal changes during the menstrual cycle (many women go up half a cup in the luteal phase), pregnancy and breastfeeding (significant and sometimes permanent changes in both band and cup), menopause (tissue density changes, often affecting cup size), and aging (tissue migration and loss of elasticity).

Remeasure every 6–12 months or whenever you notice fit changes. A professional fitting at a specialty bra store (not a department store) establishes a reliable baseline. After that, home measurements with our calculator are sufficient for tracking changes.

Bras themselves change over time. Elastic stretches with wear and washing — a new bra should fit correctly on the loosest hook, giving you two tighter hooks to use as the band stretches over months. When the tightest hook no longer provides adequate support, replace the bra. With regular wear (2–3 times per week), most bras last 6–12 months. Hand washing and air drying significantly extends lifespan compared to machine washing and drying.

Building a rotation of 3–4 everyday bras means each gets worn every 3–4 days, allowing elastic to recover between wears. This extends each bra's functional life to 12–18 months.