Before you start
What you'll need
A soft tape measure
A flexible fabric or vinyl tape — the kind used for sewing — rather than a rigid ruler or metal tape.
Thin, unpadded bra or none
Padding and moulded cups add false volume to your bust reading, so measure without them if possible.
A mirror, standing naturally
A mirror helps confirm the tape stays level all the way around, front and back.
Any consistent time works
There's no single correct time of day, but avoid measuring right before or during your period if you want the most stable reading.
The process
Three measurements, two minutes
Wrap the tape under your bust
Pull it snug — not painfully tight — directly beneath the bust, and check in the mirror that it sits level all the way round, not riding up at the back. Round the reading to the nearest whole number; this is your underbust measurement.
Wrap the tape across the fullest point
Standing naturally, loop the tape loosely across the fullest part of your bust, usually around nipple level. It should rest against you without pulling tight or flattening anything. Round this reading too.
Subtract to find the cup
Take your bust number and subtract your underbust number. The result, in inches, is your cup index — feed both numbers into the bra size calculator to get your full size instantly, or use the difference to read across the cup size chart yourself.
cup difference = round(bust, in) − band
0=AA · 1=A · 2=B · 3=C · 4=D · 5=DD/E · 6=F · 7=FF · 8=G
Common mistakes
What throws the reading off
Measuring over a padded bra
Foam and moulding inflate the bust number specifically — the underbust reading is less affected, but the bust reading rarely is accurate over padding.
Pulling the underbust too tight
The band reading should be snug, not compressed. You should still be able to slide two fingers underneath comfortably.
Letting the tape tilt
A tape that rides up at the back gives a smaller underbust reading than reality — always check the mirror before reading the number.
Pulling the bust tape tight
The bust tape should rest, not compress — pulling it tight will understate your actual bust measurement and cup size.
After measuring
What to do with your two numbers
Once you have both readings, the fastest path is straight to the bra size calculator, which handles the rounding and subtraction automatically and shows your size across every region we support. If you'd rather look things up manually, the bra size chart and cup size chart both let you cross-reference your two numbers by hand.
Keep in mind that this measurement is a strong starting point, not a locked-in answer — re-measure every 6 to 12 months, and sooner after any significant weight change, pregnancy, nursing or surgery.
Questions
Measuring FAQs
No — both measurements can be taken solo in front of a mirror. A second pair of hands can help keep the tape level, but it isn't required.
A piece of string or ribbon measured afterward against a ruler works as a reasonable substitute if you don't have a flexible tape measure on hand.
Either is fine for most people, though measuring around the same time of day you'd normally wear the bra keeps things consistent.
It's best to measure over a thin, non-padded bra or bare skin — thick clothing adds inches that throw off both readings.
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