Sister Size Matrix Playbook
Decode the logic behind band-and-cup swaps so you can fine-tune tension, adjust for different fabrics, and stay comfortable all day.
Core principles
Equal cup volume
Every time you move a band size, adjust the cup in the opposite direction. The overall cup volume stays the same, the band tension changes.
Two-number band increments
Bands change in steps of two (e.g., 32 → 34). Sister swaps only make sense across those increments.
Cup letters shift by one
When the band goes up, the cup letter goes down one step (D → C). When the band goes down, the cup letter goes up (D → DD/E).
Quick lookup examples
Base size
32D
When to use
Need firmer support or prefer longlines
Base size
36C
When to use
Trying different brands with firmer/looser bands
Base size
40DD
When to use
Transitioning between everyday and lounge bras
Scenario planner
Band feels too tight
Go up one band, down one cup (e.g., 32DD → 34D).
Ideal when breathing feels restricted or hooks sit on the loosest setting. Verify the straps after swapping; they may need tightening.
Band rides up the back
Go down one band, up one cup (e.g., 36C → 34D).
Ensures the band anchors under the shoulder blades. Check the gore: if it sits flat, you have the right tension.
Cup volume is perfect but straps dig
Reduce band size with a cup increase (e.g., 34E → 32F).
A snugger band carries more weight, relieving the shoulders without reducing cup depth.
Switching styles or fabrics
Test both tighter and looser sisters to accommodate stretch lace or rigid mesh.
Keep a note of your preferred band tension for each fabric type. Stretch lace often tolerates a tighter band, rigid mesh may require a looser one.
Action checklist
- Generate your baseline in the main calculator and note the US size.
- Open the Sister Size Matrix tool from the calculator page to visualize alternatives.
- Planned a tighter band? Increase the cup by one index before you buy or try.
- Need more comfort? Loosen the band and confirm strap adjustments after switching.
- Keep track of the brands or styles where each sister size excels using a simple fit journal.
Frequently asked questions
Do sister sizes always fit the same as the original size?→
They maintain cup volume, but strap placement, wire width, and brand patterns still vary. Always try on or order multiple options to confirm.
How many sister size steps should I consider?→
Most people stay within one band step up or down. Two steps can work for specialty fits, but monitor comfort closely.
Is the cup letter the same in every system?→
Cup notation changes internationally. Use the calculator’s conversion table to map your US sister size to UK, EU, or JP equivalents.