Size Calculation
2009/11/18 (updated: 2026/07/15)
iSwim Exclusive Quick Size Calculation
Find your iSwim underwear and swimwear size more easily with our low-rise waist measurement guide, two-stage stretch system, and quick size calculation method.
Underwear and Swimwear Size Chart
- iSwim underwear is made with our own two-stage stretch construction. The first stage is the standard comfort range shown in the size chart, while the second stage is a tighter stretch range.
- If you prefer underwear that feels comfortable and not too tight, we recommend choosing your size based on the standard stretch range. For lower-rise styles, such as Ultra Low or Extreme Low, or fabrics with slightly less stretch, you may consider choosing one size larger. If you are between two sizes, we recommend using our Minor Custom Tweaks service.
- For customers with non-Asian body proportions, we may recommend choosing one size larger for the first order.
How to Choose Your iSwim Size
Measure Your Low-Rise Waist
Measure around the low-rise position at the hip bones, approximately one palm width or 10–12 cm below the navel.
Check the Size Chart
If you prefer a comfortable fit that is not too tight, choose your size according to the standard stretch range shown in the iSwim size chart.
Use the Quick Size Calculation
Subtract your weight in kilograms from your height in centimeters to obtain value G, then use the iSwim quick size calculation chart to estimate your size.
Adjust for Your Body Shape and Fit Preference
If you are taller, have fuller hips or thighs, have non-Asian body proportions, or do not prefer a tight fit, consider choosing one size larger.
Choose Your Swimwear Size Separately
iSwim swimwear waistbands are generally tighter than underwear waistbands, so follow the swimwear sizing recommendations instead of automatically choosing the same size as your underwear.
Use a Minor Custom Tweak If You Are Between Sizes
If you are between two sizes, you may use a minor custom tweak to add approximately half a size, or contact us with your height, weight, low-rise waist measurement, and preferred tightness.
Quick Size Calculation
- For iSwim underwear and swimwear, we have developed a quick calculation method to help you estimate a suitable size more easily.
- If your calculated size is M, but your height is 172 cm or above, or your hips and thighs are fuller and you are concerned about a tight fit, we recommend choosing size L for your first order. You may also use our Minor Custom Tweaks service to slightly adjust the waist size.
- Sizing recommendations for swimwear: iSwim swimwear comes in 5 sizes. The waistband of our swimwear is tighter than our underwear to help prevent loosening during swimming. Size M swimwear has the same waist size as size M underwear, while size XL swimwear has the same waist size as size L underwear.
- If your underwear size is M and you are used to a slightly tighter fit, you may also choose size M for swimwear. If you are not used to a tighter fit, we recommend choosing at least size L.
- If your underwear size is L, the corresponding swimwear size is XL. If you are not used to a tight fit, you may choose size XXL.
- iSwim bikinis and swimwear use different construction methods from ordinary commercial products to improve the overall fit. When the cut, material, color, or stretch level is different, the flat waistband width may vary slightly. Therefore, we do not recommend using the flat-width measuring method commonly used for other brands, and there is no need to ask us for the flat waistband width in centimeters.
- iSwim underwear and swimwear are made to order, and returns or exchanges are not available. If you have any questions about size selection, please contact customer service first and provide your height, weight, low-rise waist measurement, and preferred tightness.
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