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Bikini Size Calculator

Convert hip measurements to accurate bikini sizes across US, UK, EU, and Japanese sizing systems with one simple calculation.

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Inputs

Hip Measurement

Sizing Region

Recommended Bikini Bottom Size

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Recommended Bikini Bottom Size—

The formula

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result is
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How the Bikini Size Calculator Works

The bikini size calculator converts a single hip measurement into the correct swimwear size across four major regional sizing systems: US, UK, EU, and Japanese (JP). Because no universal size standard exists globally, the same body measurement translates to a different numerical label depending on the country of purchase. This tool applies the official conversion formulas used by leading swimwear retailers and international size guides to eliminate confusion when shopping across borders.

The Sizing Formulas Explained

Each regional system anchors to hip circumference as its base measurement. The four formulas below define the exact relationships between body measurement and labeled size:

US Bikini Size Formula

The US system uses hip measurement in inches:

S_US = 2 x (H_in - 34)

Where H_in is the hip circumference in inches. A 38-inch hip yields: 2 x (38 - 34) = US size 8. A 40-inch hip gives: 2 x (40 - 34) = US size 12. US women's swimwear sizes follow even numbers, typically ranging from 0 to 20.

UK Bikini Size Formula

The UK system adds a constant offset of 4 to the US size:

S_UK = S_US + 4

A US size 8 therefore equals UK size 12, and a US size 12 equals UK size 16. This fixed offset reflects the historical divergence between American and British clothing size numbering, as documented in the Women's Size Conversion Chart.

EU Bikini Size Formula

The European system uses centimeters and anchors at a different baseline:

S_EU = 36 + (H_cm - 88) / 2

A hip measurement of 96 cm gives: 36 + (96 - 88) / 2 = 36 + 4 = EU size 40. The baseline is 88 cm at EU size 36, and every additional 2 cm of hip circumference increases the EU size by 1. European swimwear sizes typically follow even numbers (36, 38, 40, 42, 44, 46) and are rounded to the nearest whole number when the formula yields a decimal.

Japanese Bikini Size Formula

The Japanese system also uses centimeters but anchors at its own baseline:

S_JP = 7 + (H_cm - 87) / 2

For a 96 cm hip: 7 + (96 - 87) / 2 = 7 + 4.5 = approximately JP size 11. Japanese swimwear sizes round to the nearest whole number and typically use odd numbers (7, 9, 11, 13). The baseline of 87 cm at size 7 reflects the sizing conventions established by Japanese swimwear manufacturers.

How to Take an Accurate Hip Measurement

Accurate results depend entirely on correct measurement technique. Follow these steps for a reliable reading:

Stand with feet together and wear minimal, form-fitting undergarments.
Wrap a soft measuring tape around the fullest part of the hips and buttocks.
Keep the tape perfectly parallel to the floor — any tilt produces a smaller, inaccurate reading.
Pull the tape snug against the body without compressing the skin.
Take the measurement twice and average the results if they differ.
Record in both inches and centimeters to use all four regional size formulas.

Variables Reference

H_in: Hip circumference in inches — used for US and UK calculations.
H_cm: Hip circumference in centimeters — used for EU and JP calculations. (1 in = 2.54 cm)
S_US: US women's swimwear size — even numbers from 0 to 20.
S_UK: UK women's swimwear size — even numbers from 4 to 24.
S_EU: European Continental size — typically even numbers starting at 34.
S_JP: Japanese swimwear size — typically odd numbers starting at 5.

Size Conversion Examples

The following examples show four common hip measurements and their equivalents across all regional systems (results rounded to nearest whole number):

36 in / 91 cm: US 4, UK 8, EU 38, JP 9
38 in / 97 cm: US 8, UK 12, EU 40, JP 12
40 in / 102 cm: US 12, UK 16, EU 43, JP 15
42 in / 107 cm: US 16, UK 20, EU 46, JP 17

Why Sizes Differ by Country

Clothing and swimwear sizing standards developed independently in each major market throughout the 20th century, producing persistent numerical gaps for identical body measurements. A shopper with a 38-inch hip is US size 8, UK size 12, EU size 40, and JP size 12 — four different numbers for the same body. Leading international retailers maintain their own conversion tools to address this complexity, as demonstrated by the International Size Conversion Guide used by global fashion retailers. Always consult a brand-specific size chart when available, as individual manufacturers may deviate slightly from the standard formulas.

Reference

Frequently asked questions

How do I measure my hips correctly for a bikini size calculator?

Stand with feet together and wrap a soft measuring tape around the fullest part of the hips and buttocks, keeping the tape parallel to the floor. Pull the tape snug but not tight and avoid compressing the skin. Measure twice and average the two readings if they differ. Record in both inches for US and UK sizes and centimeters for EU and Japanese sizes. Even a 1 cm error in hip measurement can shift the result by one full EU or JP size.

What is the difference between US and UK bikini sizes?

UK bikini sizes are exactly 4 numbers higher than the equivalent US size for the same body measurement. A US size 8 equals a UK size 12, a US size 10 equals a UK size 14, and a US size 14 equals a UK size 18. This fixed offset of 4 applies consistently across the entire size range and reflects the historical divergence between American and British clothing size numbering conventions, which developed independently during the 20th century with different baseline measurements as reference points.

How does EU bikini sizing work and how is it calculated?

EU bikini sizing uses the formula S_EU = 36 + (H_cm minus 88) divided by 2, where H_cm is hip circumference in centimeters. The baseline is 88 cm at EU size 36, and the size increases by 1 for every 2 cm of additional hip circumference. For example, an 88 cm hip gives EU size 36, a 92 cm hip gives EU size 38, and a 96 cm hip gives EU size 40. EU swimwear sizes are typically even numbers ranging from 34 to 52, and decimal results are rounded to the nearest whole number.

What is Japanese bikini sizing and how does it compare to US sizing?

Japanese bikini sizing uses the formula S_JP = 7 + (H_cm minus 87) divided by 2 and produces much smaller numbers than US or EU sizing. A 38-inch (97 cm) hip that is US size 8 and EU size 40 translates to approximately Japanese size 12. Japanese swimwear sizes are typically odd numbers (7, 9, 11, 13) based on the sizing conventions of Japanese textile associations. Always verify with a brand-specific JP size chart, as some manufacturers anchor their scales at slightly different baseline measurements.

What should I do if my hip measurement falls between two bikini sizes?

When a hip measurement falls exactly between two sizes, select the larger size for a more comfortable and secure fit, particularly for swimwear where the fabric must stretch and move freely during activity. For example, a hip measurement yielding a decimal EU result such as 40.5 should be rounded up to EU 42 for comfort rather than down to EU 40. Bikini bottoms with adjustable ties or drawstring waists can also accommodate measurements that fall between standard sizes without sacrificing fit or support.

Is the bikini size calculator accurate for all swimwear brands?

The bikini size calculator provides a reliable starting point based on standardized regional size formulas, but individual brands may deviate from these standards. Luxury, designer, and fast-fashion swimwear brands sometimes use vanity sizing or proprietary sizing scales that differ by half a size or a full size from the standard formula. Specialty swimwear retailers maintain their own published size charts that should always be consulted before purchasing online. Cross-referencing the calculator result with the brand's specific chart gives the highest accuracy and reduces the need for returns or exchanges.