Swimwear needs four numbers: underbust, bust, torso length, and hip. Knowing how to measure for swimwear starts there, because a bra guide gives you the first two and stops. The underbust sets the band that carries your support. Bust minus underbust gives your cup volume. Torso length decides whether a one-piece fits or drags at the crotch. Hip decides the bottom. Measure all four before you order.
Every measuring page in this category measures for a bra. That is the gap. A bra needs an underbust and a bust. A swimsuit needs those plus two more: shoulder to crotch for torso length, and hip at the widest point for the bottom rise. No single cited page covers all four, so people buy one-pieces that fit the chest and pull at the crotch, or bottoms sized off a bra chart that never measured the hip. This page fixes that. Each measurement below is written to stand alone, with a number attached.
You need a soft tape measure, a mirror, and either no bra or an unpadded one. Read each number in inches. Do not round until the instruction tells you to.
How to measure your underbust
Run the tape directly under your bust, where a bra band sits, and read the number with your arms down after a normal exhale. Keep the tape level all the way round, including across your back. Pull it snug enough that it does not slip when you drop your arms, then read it. This is your band, and the band is the measurement that carries the support.
Good Housekeeping’s measuring guidance describes the same step: run the tape all the way around your back where the band sits, keeping it level. Its guidance allows room for two fingers under the tape, which is a fit check for a finished bra rather than a raw body measurement. For a swim size chart, take the raw body number first, then compare it to the chart.
The underbust matters more than any other number because the band does the work. The University of Portsmouth’s Research Group in Breast Health teaches that the underband provides most of a bra’s support rather than the straps, and its fitting guidance puts the band first: it should sit level all the way around the ribs and fit firmly, with no more than about a five centimetre gap when pulled away from the body. In swimwear with a shelf bra or an encased wire, the same physics holds. If the band rides up, the support is gone regardless of cup.
Best for: anyone starting from a failed order, because a wrong band is the most common reason a supportive swim top stops supporting.
How to measure your bust
Run the tape around the fullest part of your bust, level all the way round, wearing an unpadded bra or none. The tape should skim the front without compressing. Read the number after a normal breath. Your cup is the difference between this number and your underbust, not the bust figure on its own.
Subtract your underbust from your bust. The difference in inches gives the cup: zero is AA, one inch is A, two is B, three is C, four is D, five is DD. So a 37-inch bust over a 34-inch underbust is a three-inch difference, which reads as a C cup, and the size is 34C.
| Bust minus underbust (inches) | Cup (US) |
|---|---|
| 0 | AA |
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | DD |
The table above uses US lettering. Beyond DD the systems diverge and the letters stop matching across countries: UK E is US DDD, UK J is US M, UK JJ is US N, and UK K is US O. A chart that prints a single letter tells you nothing until you know which country’s system it uses, so confirm the system before you trust a cup letter anywhere. Our swim size conversion chart carries the full mapping.
Best for: anyone whose bust and band suggest a fuller cup, where the difference, not the raw bust, decides whether a top holds.
How to measure your torso length
Run the tape from the top of one shoulder, straight down the front of your body, and through to the crotch in a single pass. Read that number. Under 26 inches is a short torso, 26 to 28 inches is standard, and above 28 inches is long. These thresholds are our own working guidance, not an industry standard, because no industry standard for torso length exists.
Torso length is the number bra guides never take, and it is the one that decides whether a one-piece fits. Too short for your torso and the suit drags the shoulder seams down and rides up into the hip crease. Too long and the fabric bags at the waist and sags in the seat. The single-pass shoulder-to-crotch measurement above is the method to use, because it maps to how a suit sits on the front of your body.
Some brands publish a full-loop figure instead: shoulder, down the front, through the crotch, up the back, and back to the same shoulder. That method runs roughly double the single pass. Monday Swimwear’s fit guide, for example, carries a torso length column across the nine sizes on its one-piece chart: PP 57 to 58 inches, P 58 to 59, S 59 to 60, S/L 59.5 to 60.5, M 60 to 61, L 61 to 62, V 68 to 69, VV 69 to 70, and VVV 70 to 71. A 57-inch figure cannot be a single shoulder-to-crotch pass on an adult body, so it reads as a full-loop measurement. Monday does not state its method on that chart, so treat this as an inference from the numbers, not a brand statement.
Note the jump in that column. It steps about an inch per size up to L, then rises roughly six inches to V. If your full loop measures between 62 and 68 inches, no Monday one-piece size is cut for your torso, and you are choosing which direction to compromise in. That is worth knowing before you order rather than after.
The practical rule: a torso number near 26 and a torso number near 57 can describe the same body by different methods. Before you compare your measurement to any chart, confirm whether it uses a single pass or a full loop. A single-pass number checked against a full-loop chart will send you badly wrong.
Best for: long-torso and short-torso shoppers, and anyone buying a one-piece who has never measured this number before.
If this measurement is the one that keeps breaking the fit, continue with our guide to the best swimwear for long torsos.
How to measure your hips
Run the tape around the widest point of your hips and seat, feet together, level all the way round. That widest point usually sits 7 to 9 inches below your natural waist. Read the number without pulling the tape tight. This decides your bottom, not your top.
Good swimwear sizes the top and the bottom separately, because most bodies are not the same size top and bottom. A bust and a hip that fall in different columns of a size chart are common, and a suit sold as separates lets you pick each half. Sizing the bottom off a bust measurement, or off a bra chart that never measured the hip, is how bottoms end up tight at the seat or gaping at the waist.
The hip governs the bottom’s rise and seat coverage. The natural waist, higher up, governs where a high-waisted bottom sits and is a separate number again. If you take only one lower-body measurement, take the hip at the widest point, because that is the one most bottom charts key on.
Best for: anyone whose top and bottom are different sizes, and anyone buying bottoms separately from a top.
The three mistakes that make every measurement wrong
Most measuring errors are the same three. Naming them is more useful than repeating the method, because the method is simple and the mistakes are where the number goes wrong.
Measuring over a padded bra is the first. Padding changes the bust profile and can inflate the fullest-point measurement, so the cup calculation becomes unreliable. Measure in an unlined, non-padded bra or none at all, not over clothes.
Holding the tape too tight on the underbust is the second. The band should be snug, but a tape hauled in hard reads one to two inches small, which pushes you into a band you cannot wear all day and a cup that reads too large by comparison. Snug means it stays level when you drop your arms, not that it compresses your ribs. Take the reading after a normal exhale, not a held breath.
Measuring the waist at the navel instead of the natural waist is the third. The natural waist is the narrowest part of your torso, usually an inch or two above the navel, not at it. Measuring at the navel inflates the waist number and misplaces where a high-waisted bottom will sit. Find the crease when you bend to one side, and measure there.
Measurement reference
Every value below traces to the method described in its section. Cup figures use US lettering.
| Measurement | Where to take it | What it decides | Common error |
|---|---|---|---|
| Underbust | Directly under the bust, level, arms down, after exhale | The band, which carries the support | Pulling the tape too tight, reading small |
| Bust | Fullest point, level, unpadded bra or none | Cup volume (bust minus underbust) | Measuring over padding |
| Torso length | Shoulder to crotch, single pass | One-piece length: under 26 short, 26-28 standard, above 28 long | Comparing a single-pass number to a full-loop chart |
| Hip | Widest point, 7-9 inches below the natural waist, feet together | The bottom’s rise and seat coverage | Measuring at the navel, not the natural waist |
Sources: the underbust and bust methods follow published measuring guidance from Good Housekeeping and Woman & Home, cited by name. The torso-length thresholds are our own working editorial guidance, because no industry standard exists. The full-loop example is Monday Swimwear’s own published fit guide, with the measuring method inferred from the figures rather than stated by the brand.
Who this is not for
This page is for taking accurate body measurements before you order. It is not a size calculator and it will not tell you your size in a specific brand. Every brand’s chart differs, and the same four numbers land you in different sizes at different labels. Take your numbers here, then read them against the chart on the product you want, confirming which torso method that chart uses.
Measurements narrow the size; shape still changes how a cut sits. Our swimwear-by-body-shape guide covers that second decision.
Frequently asked questions
01 How do I measure myself for a swimsuit?
Take four measurements: underbust, bust, torso length, and hip. Measure underbust directly below the bust with the tape level and snug; bust at the fullest point; torso from shoulder to crotch in a single pass; and hip at the widest point. Compare each number with the specific product's chart.
02 What is my cup size if my bust is 4 inches bigger than my underbust?
A four-inch difference is a D cup in US lettering. The basic sequence is zero for AA, one inch for A, two for B, three for C, four for D, and five for DD. Beyond DD, cup letters diverge by country.
03 How do I know if I have a long torso?
Using this guide's single-pass shoulder-to-crotch method, under 26 inches is short, 26 to 28 inches is standard, and above 28 inches is long. These are working editorial thresholds, not an industry standard. Confirm whether a brand uses a single pass or a full torso loop before comparing numbers.
04 Should I measure in a bra or without one?
Measure in an unpadded bra or with none at all. Padding can distort the bust measurement and therefore the difference used to estimate cup size. Take raw body measurements without clothing wherever possible.
Editorial buying guide. Prices, sizing, and availability change over time; confirm current details on each brand's site before purchasing.
