A bikini size calculator can only do one honest thing: read your measurements against a size chart that somebody actually published, and tell you where they land. This page does that against a named, linked chart, reports the four measurements separately rather than blending them into a single confident answer, and says plainly when they disagree — because when they disagree, that disagreement is the useful information.
Arb Digital builds free calculators that source their reference data instead of inventing it. There is no universal swimwear sizing standard, so no general chart can be authoritative; what a tool can do is show you the arithmetic against one published chart, name it, and be clear that the brand you are buying from governs.
What This Bikini Size Calculator Does
You enter four measurements: bust, underbust, waist and hips. The calculator looks each one up independently on the size chart and reports the size that measurement suggests. It then combines them into a top suggestion, driven by bust and underbust, and a bottom suggestion, driven by hips and waist.
Where a measurement falls in a gap between two published ranges, the calculator says so rather than hiding it, and applies whichever direction you chose in the between-sizes dropdown. Where the top and bottom suggestions differ, it says that too, and points out that most swimwear brands sell separates precisely because bodies do not come in one size.
What it does not do is tell you what will suit you or how anything will look. It is a fitting aid: a starting point for what to try on or which size to order first, and nothing more.
The Size Chart This Uses, and Why Only One
The chart is the women's swimwear size guide published by Roxy, which gives bust, underbust, waist and hip ranges in inches for XS through XXL. Its bust ranges run 32–33 for XS, 33–35 for S, 35–37 for M, 37–39 for L, 39–41 for XL and 42–44 for XXL, with underbust, waist and hip ranges alongside each.
Only one chart is offered here, and that is deliberate. Swimwear sizing is not standardised, so combining several brands' charts into an "average" would produce a table that matches no actual garment. Compare it with the swimwear size guide published by Seafolly, which runs XXS to XXL and publishes cup-specific bust measurements as well as body measurements, and the differences are immediately obvious. Neither is wrong. They are simply different brands cutting to different blocks.
The practical rule follows from that: use this page to understand where your measurements sit and how far apart your top and bottom sizes are, then buy against the specific brand's own published chart. If the brand publishes one, it beats anything general, including this.
How to Use It
- Measure over bare skin or a thin unpadded bra. A padded or push-up bra inflates the bust figure and will size you up.
- Keep the tape level with the floor all the way round. A tape that rides up at the back is the single most common measuring error and it always reads small.
- Take the underbust snug, the bust relaxed, the waist at its narrowest point and the hips at their fullest.
- Read the four grid results before the headline. If they all agree, the chart fits your proportions well; if they scatter, that is the real answer.
- Choose a between-sizes preference and read the note, which explains any gaps and any disagreement between top and bottom.
How the Lookup Works
There is no formula here, and pretending otherwise would be dishonest. The calculator performs a table lookup and reports what it finds.
For each measurement it scans the chart from the smallest size upward and returns the first size whose published range contains the value. If the value sits below the smallest published range, it reports that the chart does not cover it. If it sits above the largest, likewise. If it falls in a gap between two ranges — and the published chart does have gaps, such as hips between 39 and 40 inches — it returns the size on the side you chose and marks the result as a between-sizes case.
Work the defaults through. A 36 inch bust falls inside the M range of 35–37. A 30 inch underbust falls inside M's 29–31. A 29 inch waist falls inside M's 28–29. But a 40.5 inch hip measurement falls inside L's 40–41, not M's 38–39. The result is therefore a medium top and a large bottom — a common and entirely normal outcome, and exactly the case that separates exist for.
Why Buying a Set in One Size Often Fails
A matched set assumes your bust, waist and hips sit at the same point on one size chart. For a substantial share of people they simply do not, and no amount of choosing carefully changes that.
Two measurements a size apart is normal. Three sizes apart is not unusual. When that is the case, a single-size set will fit one half properly and the other badly, and the half that fits badly is the one that ruins the garment: a top that gapes at the band, or bottoms that dig at the hip.
The answer is separates, which almost every swimwear brand now sells as mix-and-match. It costs nothing extra in most ranges and it removes the compromise entirely. If the range you want is only sold as a set, the usual advice is to size to the larger of the two and accept that the other half will be looser, because a garment that is slightly loose can often be worn and one that is too tight cannot.
Band-and-Cup Tops Are a Different System
Some swimwear, particularly in larger bust sizes, is sold in bra sizing — 34C, 36D and so on — rather than in letter sizes. That is a genuinely different system and it is not interchangeable with S, M and L.
In bra sizing, the number is the band, derived from the underbust measurement, and the letter is the cup, derived from the difference between bust and band. Roughly one inch of difference corresponds to one cup letter, though the exact convention varies between countries. Our bra size calculator works that system properly, including US, UK, EU and Australian notation and sister sizes, and this page deliberately does not duplicate it — if the swimwear you are buying is labelled with a band and a cup, that tool is the right one.
The reason cup-sized swim tops exist at all is the same reason the grid on this page shows bust and underbust separately. Two people with an identical 36 inch bust can have very different underbust measurements, and a letter-sized top cut for one will not support the other. If your bust and underbust results here point at different sizes, that is a strong hint that a cup-sized top will fit you better than a letter-sized one.
What a Measurement Cannot Tell You
Three things sit outside any size chart, and they matter more than the last half inch of a tape measure.
Cut is the first. A high-leg brief, a full-coverage brief and a tie-side all sit differently on the same hips, and the same nominal size in two cuts is not the same garment. Tie-sides in particular are far more forgiving of a between-sizes measurement than fixed-side briefs.
Fabric is the second. Swim fabrics vary in stretch and recovery, and a heavier compression fabric behaves nothing like a light jersey at the same measurement. This is why brands so often advise sizing up when between sizes: the fabric will accommodate, but only in one direction.
Construction is the third, and it is the reason a bikini top can fit at the band and still fail. Underwire, moulded cups, adjustable straps and a hook-and-eye closure all change how a top behaves on the same body. A tie-neck triangle top is nearly measurement-agnostic; a moulded underwired top is not remotely so.
Getting the Measurements Right
Most sizing disappointment traces back to the tape, not the chart. A few habits fix nearly all of it.
Measure in front of a mirror, or have someone else do it, so you can see whether the tape is level at the back. Breathe normally and take the reading on a relaxed exhale, not held in. Take each measurement twice, and if the two differ by more than about half an inch, take a third.
Record the numbers rather than the sizes. Body measurements are stable and comparable across every brand; a size letter is only meaningful next to the chart it came from. If you are shopping across markets, our clothing size converter and dress size calculator cover general apparel sizing, the length converter handles unit mismatches, and the waist to hip ratio calculator and body frame size calculator work with the same measurements for entirely different purposes.
Arb Digital designs and builds free interactive calculators that name and link their source data, state their limits, and earn links because they are genuinely useful. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Measuring in a padded bra. The padding is added to your bust figure and reliably sizes you up a whole step.
- Letting the tape ride up at the back. It always reads small, and it is the most common measuring error there is. Use a mirror.
- Assuming a size letter travels between brands. There is no swimwear sizing standard. Record measurements, not sizes.
- Forcing a matched set. If your top and bottom results differ, separates solve it at no extra cost in most ranges.
- Treating a chart as a fit guarantee. Cut, fabric stretch and construction change how the same nominal size behaves on the same body.
Related Free Tools From Arb Digital
For band-and-cup swimwear and bras, use the bra size calculator, which handles US, UK, EU and Australian notation and sister sizes. For general apparel, the clothing size converter and dress size calculator cover cross-market sizing, and the shoe size converter does the same for footwear. The same measurements feed the waist to hip ratio calculator and the body frame size calculator, and the length converter handles inches and centimetres. Everything else is on the free online tools hub.
Frequently Asked Questions
Take four measurements with the tape level with the floor: bust at the fullest point, underbust directly beneath the bust, waist at its narrowest and hips at their fullest. Measure over bare skin or a thin unpadded bra, breathe normally, and take each measurement twice to check it.
Because bodies are not proportioned to a single size chart. Two measurements landing a size apart is entirely normal. Most swimwear brands sell separates for exactly this reason, and buying a top and bottom in different sizes usually costs nothing extra.
The women's swimwear size guide published by Roxy, which gives bust, underbust, waist and hip ranges in inches from XS to XXL. Only one chart is used, because swimwear sizing is not standardised and averaging several brands together would produce a table matching no actual garment.
It is a genuine judgement call, which is why the calculator asks rather than deciding for you. Swim fabric stretches, so a slightly loose garment is usually wearable and a slightly tight one often is not, which is why many brands advise sizing up. Cut matters too: tie-sides forgive a between-sizes measurement far better than fixed-side briefs.
That is a different system and it is not interchangeable with S, M and L. The number is the band, from your underbust, and the letter is the cup, from the difference between bust and band. Use our bra size calculator for that, which handles US, UK, EU and Australian notation.
Not necessarily, and you should not assume it will. There is no swimwear sizing standard, so ranges differ between makers. Use this page to see where your measurements sit and how far apart your top and bottom sizes are, then buy against the specific brand's own published chart.
The calculator says so rather than guessing, because extrapolating beyond a published range invents data. Look for a brand whose own chart covers your measurements, including extended-size and cup-sized ranges, and buy against that chart directly.
This page is a fitting aid, not a fit guarantee. It reports where your measurements fall on one named, published size chart; swimwear sizing is not standardised, cut and fabric change how any given size behaves, and the manufacturer's own chart and returns policy govern what you actually buy.
