Use a non-stretch string or ribbon, mark where the ends meet and measure that length against a ruler. This can provide a rough underbust and bust measurement. It is less reliable than a flexible tape, so repeat the readings and confirm them before an expensive or difficult-to-return purchase.

Choose material that does not stretch

Thin string, flat ribbon, a non-elastic shoelace or securely joined paper strips can work. Avoid elastic cord, knitted yarn and anything that changes length when pulled.

You also need a ruler, yardstick or another verified straight scale. Do not estimate the length from memory.

Method 1, using string and a ruler

  1. Wrap the string directly under the bust for the underbust measurement.
  2. Keep it level around the torso and comfortably snug.
  3. Mark the exact meeting point without cutting the string too early.
  4. Lay it flat against the ruler and record the length.
  5. Repeat around the fullest part of the bust without compression.
  6. Take each measurement twice.

Turn the rough measurements into a starting range

Enter the exact values and select the correct unit and sizing system.

Calculate My Size

Keep the position consistent

The underbust string should sit directly beneath the breast root, not around a narrower part of the waist. The bust string should remain level and pass around the fullest area.

Use a mirror or another person to check the back. A sloping string can add or remove enough length to change the result.

Do not compress the breast tissue

The string should touch the body without flattening the bust. Measuring over a thick padded bra can also distort the result. Use an unpadded bra or follow the calculator’s measuring method.

Why random objects are less reliable

Phone dimensions vary by model and case. Banknotes and paper sizes work only when their exact dimensions are known. Joining several short objects creates more opportunities for error.

A marked string transfers the complete circumference to one straight measurement and is usually simpler.

Know the limits of the method

Error can come from stretch, an imprecise mark, a tilted string, inconsistent tension or an inaccurate ruler. If repeated measurements differ noticeably, wait for a flexible tape.

Do not round the measurement before entering it. Let the calculator apply the sizing rules.

Use a known bra only as a secondary clue

A current bra can help when the band clearly rides up or the cups clearly overflow, but its label is not a body measurement. The bra may be stretched, use another country system or have the wrong shape.

The useful conclusion

A non-stretch string and ruler can give a useful estimate. Keep the string level, repeat the measurement and treat the result as a starting range. Confirm it with a flexible tape when accuracy matters.