Bra sizing systems vary significantly across countries:

• US & UK: Band size is measured in inches (e.g., 32, 34, 36). Cup letters differ after DD — US uses DDD/F, G, H, I while UK uses E, F, FF, G, GG.
• EU & FR: Band size is in centimeters (e.g., 70, 75, 80). The cup progression is similar to US but uses E instead of DD.
• AUS: Uses a unique numbered band system (8, 10, 12, 14) that corresponds to UK/US band sizes minus 22. Cup letters generally follow the UK pattern.

The key difference between US and UK cup sizing starts after DD:

• US DD = UK DD (same)
• US DDD/F = UK E
• US G = UK F
• US H = UK FF
• US I = UK G
• US J = UK GG

UK sizing adds double letters (FF, GG, HH) while US typically uses single letters with DDD as a triple. This is why a US 34H equals a UK 34FF — always check the brand's origin when shopping!

Sister sizes are bra sizes that hold the same cup volume but have different band sizes. The rule is:

• Go up one band size → go down one cup letter (e.g., 34B → 36A)
• Go down one band size → go up one cup letter (e.g., 34B → 32C)

Sister sizes are incredibly useful when your exact size isn't available, or when a particular brand's band runs tight or loose. Trying a sister size can often give you a better fit without changing the overall cup volume.

Australian bra sizes use a numbered band system. To convert AUS to US/UK band size, simply add 22:

• AUS 8 = US/UK 30
• AUS 10 = US/UK 32
• AUS 12 = US/UK 34
• AUS 14 = US/UK 36
• AUS 16 = US/UK 38

The cup letters in AUS generally follow the UK system (DD, E, F, FF, G, GG). So an AUS 12DD is equivalent to a UK 34DD and a US 34DD.

EU (European) and FR (French) bra sizes are very similar but not identical. Both use centimeters for band size (e.g., 75, 80, 85), and their cup letter progressions are nearly the same. However, some French brands may have slight variations in cup grading. For most practical purposes and common sizes (AA through G), EU and FR sizes are interchangeable. Always refer to the specific brand's size chart when shopping from French lingerie brands like Chantelle or Simone Pérèle.

Follow these steps for accurate measurements:

1. Band Measurement: Wrap a soft measuring tape snugly around your ribcage, directly under your bust. Keep the tape horizontal and parallel to the floor. Round to the nearest whole number. This is your band size (in inches for US/UK, or cm for EU).
2. Bust Measurement: Measure around the fullest part of your bust, keeping the tape loose enough that it doesn't compress breast tissue. Round to the nearest whole number.
3. Calculate Cup Size: Subtract band measurement from bust measurement. Each inch of difference equals one cup letter: 1" = A, 2" = B, 3" = C, 4" = D, 5" = DD/E, 6" = DDD/F, etc.

For best results, measure while wearing a well-fitting, non-padded bra and stand in front of a mirror to ensure the tape is level.

Even within the same country's sizing system, bra fit can vary due to:

• Brand-specific grading: Each brand has its own fit model and grading rules, leading to slight variations.
• Fabric & construction: Stretch lace, molded cups, and underwire design all affect how a bra fits.
• Style differences: A balconette, plunge, and full-coverage bra in the same size can fit very differently.
• Manufacturing tolerances: Small variations occur during production.

Always check the brand's specific size guide and read customer reviews about fit before purchasing online.

If you're between band sizes (e.g., measuring 33 inches), try both the smaller and larger band sizes with corresponding sister sizes:

• Try the smaller band with a larger cup (e.g., 32D instead of 34C) — this provides more support.
• Try the larger band with a smaller cup (e.g., 34B instead of 32C) — this offers more comfort.

Always fasten a new bra on the loosest hook set; as the band stretches over time, you can move to tighter hooks for continued support.