Bikini First Line - Next Gen Iron-Underwear-TomboyX
Bikini First Line - Next Gen Iron-Underwear-TomboyX
First Line Leakproof Bikini - Next Gen Iron-Underwear-TomboyX

First Line Leakproof Bikini

Color Next Gen Iron

It's all in the hips

For the best fit, pick your size based on your hip measurement.
Body measurement in inches

Size Waist Hips
3XS 24-25" 28.5-31"
2XS 25-26" 31-33"
XS 26-27.5" 33-34"
S 28-29.5" 35-37"
M 30-31.5" 38-40"
L 32.5-34" 41-43"
XL 35-37.5" 44.5-46.5"
2X 38.5-42" 48-50"
3X 43-45" 51.5-53.5"
4X 46-48" 55-56"
5X 49-51" 57.5-59.5"
6X 53-55" 61-63"

How to measure your hips

Stand on a level surface, feet together without shoes.
Measure around the fullest part of your hips and bottom - this is usually 7-9" below your natural waistline.
Keep the tape measure parallel to the floor, and don't pull too tight.

Size chart

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Like the arid high plateaus of the Gobi desert, First Line keeps you dry and comfortable while you're on the move. With built in leak protection for light days and incontinence you can say goodbye to pads, tampons, and menstrual cups.

First Line underwear is made with OEKO-TEX® 100 Certified fabrics which are free from chemicals that are toxic to humans including PFOS and PFOA chemicals that have been shown to cause infertility or cancer. Learn more.

  • Looks and fits just like regular TomboyX Underwear
  • 95% OEKO-TEX®-certified cotton, 5% spandex
  • Holds 8 tsps of liquid
  • Odor-resistant
  • Smooth, durable flatlock seams won’t irritate or chafe
  • Machine wash cold in a mesh bag, line dries overnight. Avoid machine drying.


      Living Wage factories

      Ethically produced in women-owned factories that pay living wages.


      fit-tested for every body

      Obsessively fit-tested on real people, sizes XS-4X.


      Love at first wear

      If you don't love them, your first pair is on us.


      eco-friendly process

      Sustainably harvested materials and earth-conscious production.

      Fabrics & Technology


      Our classic soft jersey knit holds its shape after many wear and care cycles.

      Shop Cotton


      The luxurious companion to cotton. Supersoft, breathable and stretches to meet you.

      Shop Micromodal


      Quick-drying, durable and ultralight performance fabric. Anti-bacterial and anti-microbial.

      Shop Traveler

      First Line Leakproof

      Your first line of defense — our classic cotton armed with leak protection for light period days and incontinence.

      Shop First Line