Sizing a stretch tank is less straightforward than the numbers suggest, because the fabric, the cut, the length and whatever you wear underneath all change how one size feels. I've spent a lot of time on fit sessions with knit tops, and the same rule always holds: the chest measurement gets you close, and movement tells you the rest. Below I walk through sizes 0 to 20, when to stay with your chart size and when going up is kinder, plus how Align and Swiftly racerback tanks differ in cut and length. Start with your own measurements, then think about how much coverage you want and what you'll be doing in it.
Same Number, Different Knit, Different Fit
The reason one size can feel snug in one tank and easy in another is mostly the fabric. A brushed, high-stretch knit like Nulu gives a lot and recovers softly, so it sinks into the body and feels gentle even when it's tight. A circular knit such as the Swiftly is made on a circular machine that knits the body as a tube, with tighter and looser zones built into the stitch. There are no side seams to let out, and the fit is set by the knit itself, so it feels firmer and more structured.
Two practical consequences follow. First, a tight-fit style in a soft fabric can usually stay at your chart size, while a structured knit is the one where people more often reach for the next size up. Second, knitted-to-shape tops don't stretch much wider with wear, so if one feels tight on day one, expect it to feel the same on day thirty.
1. Lululemon Racerback Tank Top Fit Guide
The first thing I look at is the fit description, because racerbacks in this range come in two intentions. The Align Waist-Length Racerback is cut tight, meant to sit right against the skin, while the Swiftly Tech Racerback 2.0 is a slim athletic fit with a little more ease. Both run in numeric sizes 0 through 20, though colors come and go. Your chart size should sit flat without pulling at the armholes or neckline. Go up a size if you have a fuller chest, want breathing room or hate fabric clinging to your middle; stay put if you want the close, sleek line the cut was designed for.
2. How To Measure For A Lululemon Racerback Tank
Measure yourself before you compare any tanks. Wear a light or unpadded bra similar to what you'll train in, stand relaxed with your arms down, and wrap a soft tape around the fullest part of the chest. Keep the tape level across the back; it tends to drop between the shoulder blades, which gives a smaller number than you really are. The tape should touch without squeezing. Then compare the figure with the official women’s tops chart. If you land between two ranges, your shoulders, chest shape and how loose you like things should decide. Laying a favorite fitted tank flat and measuring armpit to armpit is a useful cross-check, but your body measurement is the better baseline.
3. Lululemon Racerback Tank Size 0
Size 0 is drafted for a 28.5-inch chest, roughly an XXXS, and it's the smallest numeric size in these racerbacks. Choose it when your chest sits right on that figure and you want the intended close fit. In the mirror, check that the neckline lies flat and the armholes don't drag forward toward the bust. If you measure nearer 30 inches, or the tank strains across your upper back when you hug your arms forward, size 2 will move better. One thing I'd never do is pick size 0 just to get more compression; a stretch knit pulled past its design point thins out and shows everything underneath.
4. Lululemon Racerback Tank Size 2
Size 2 is roughly an XXS, built around a 30-inch chest. In the Align it fits close; in the Swiftly it reads as a neat athletic shape. During a fitting, raise both arms and roll your shoulders. The tank should follow you without cutting into the front of the underarm, then settle back into place without you tugging it down. Wear it with matching high-rise leggings and streamlined sneakers for yoga, walks or light training. If you're at the top of 30 inches, have broad shoulders or want to layer it over a sports bra, try size 4 next to it before deciding.
5. Lululemon Racerback Tank Size 4
Size 4 is comparable to an XS and covers a 31 to 32 inch chest. It should feel fitted while still letting you take a full breath and swing your arms. Nulu is soft and very stretchy, so a size 4 Align can feel almost like a second skin without any stiffness. The circular-knit Swiftly feels more athletic because it's knitted to shape and holds its own form. When deciding between 2 and 4, look at the fabric across the upper chest. Horizontal pull lines mean too small; armholes that gape mean the cut, the bra or the size is off. Comfort matters more than the number on the tag.
6. Lululemon Racerback Tank Size 6
Size 6 is a small, drafted for a 33 to 34 inch chest, and it's the size you'll often see on models in product photos. I treat that only as visual context, because height and torso length change how the same tank hangs. A good size 6 follows your shape without bunching under the arms or pulling tight across the bust. Pair it with flared leggings, clean sneakers and a belt bag for a studio-to-errands day. If you're close to 35 inches, try 6 and 8 together; the larger one often lies smoother while keeping the racerback's narrow back panel.
7. Lululemon Racerback Tank Size 8
Size 8 is about a medium, for a 35 to 36 inch chest. It's often the right answer for someone who fits a 6 at the waist but needs more room through the bust or upper back. Try it with the exact sports bra you plan to wear, since bra straps, band width and neckline height interact differently as sizes change. A good fit should skim or hug according to the cut without twisting the side seams. If the waist feels looser than you'd like, remember that dropping to a 6 also shrinks the chest and armholes, so I'd put free movement first.
8. Lululemon Racerback Tank Size 10
Size 10 sits roughly at a large and covers 37 to 38.5 inches. That range is more than an inch wide, so two people wearing a 10 can have quite different experiences: at 37 inches it has some ease, at 38.5 it's firmly fitted. I check this size from the front, the side and the back. The neckline should stay flat and the racer panel should sit centered between the shoulder blades, not pulled to one side. If a 10 presses on your chest, makes deep breaths feel short or cuts into the armholes when you move, size 12 is the better choice.
9. Lululemon Racerback Tank Size 12
Size 12 is about an XL, drafted for a 40-inch chest. In the Align expect a close silhouette, since the cut is deliberately tight; in the Swiftly expect a slim training shape. Neither should feel painful or restrictive. During a fitting, bend, stretch and reach overhead, and watch the neckline, hem and armholes. Joggers, running shoes and a light jacket make a good test outfit. If you measure anywhere from 38.5 to 40 inches, 12 usually reduces pulling. If you prefer a looser look, compare it with 14 before you take the tags off.
10. Lululemon Racerback Tank Size 14
Size 14 is listed as XL too, but it's drafted for a 42-inch chest, which is why I always tell people to read the numeric chart rather than the letter. It should leave enough room for your chosen sports bra and full shoulder movement while keeping the tank's shape. Look at the upper back: smooth fabric is right, while tight horizontal creases mean you need more width. If you measure between 40 and 42 inches, try both sizes and choose the one that covers you comfortably without oversized armholes or a neckline that won't stay put.
11. Lululemon Racerback Tank Size 16
Size 16 is roughly a 1X, built around a 45-inch chest. That's a three-inch jump from size 14, which is a big step in a close-fitting knit, so the change across the chest, shoulders and armholes is noticeable. The right size sits over a sports bra without rolling at the hem, pinching at the side or pulling the bra straps out of line. Check the hem both standing and seated. If what you really want is more length rather than more width, switching to a hip-length version will do more than sizing up.
12. Lululemon Racerback Tank Size 18
Size 18 is also roughly a 1X, but it follows a 47-inch chest, so it's roomier than 16 even though the letter label is the same. Go by the number and your measurement. In the fitting room, check coverage at the sides of the sports bra, since that's where a racerback armhole tends to show the bra wing, and move your arms through a full circle. The racer straps shouldn't tug sharply on your shoulders. If 18 fits your chest but feels loose at the waist, that's a proportion issue the size chart can't solve; a close fit at the bust with ease below is normal for many figures.
13. Lululemon Racerback Tank Size 20
Size 20 is approximately a 2X and covers 49 to 51 inches. A two-inch range means the final fit depends a lot on chest shape, shoulder width and torso length. Choose it when your measurement falls inside that range and you want the tank to fit as designed. Wear it over a racerback sports bra so the straps line up under the back panel. During fitting, confirm the armholes cover the sides comfortably and the hem stays down when you lift your arms. Size 20 is the top of the range in these styles, and not every color is stocked in it.
14. Lululemon Align Racerback Tank Size Guide
The Align Waist-Length Racerback is cut for yoga: tight, short and made in soft Nulu. Because it hugs the body, your chart size gives a sleek line rather than a relaxed one. Stay with your measured size if you like a fitted feel over a separate sports bra. Go one size up for more room at the chest, less definition across the stomach or a little more space at the armhole. Complete the yoga outfit with high-rise Align leggings, a low or medium support racerback bra and grip socks. From a pattern point of view, sizing up adds more width than length, so if torso coverage is your concern, change the length style instead.
15. Lululemon Align Hip-Length Racerback Tank Size Guide
The Align Hip-Length Racerback uses the same chest chart from 0 to 20 but carries the body further down the torso. I suggest it to anyone whose waist-length tops ride up or leave a gap above lower-rise bottoms. Choose your size by chest first, then check where the hem lands on you. On a petite frame the hip length can come down quite low, while a long torso will welcome the extra inches. Flare leggings and low sneakers suit it. Don't size down to shorten the body; you'll lose chest and armhole room and the length barely changes.
16. Lululemon Swiftly Racerback Waist-Length Size Guide
The Swiftly Tech Racerback 2.0 in waist length is cut slim for running and training, and its shorter body meets high-rise bottoms close to the natural waist. Stay with your chest size for the intended athletic fit, or go up if you prefer less contact or wear a structured bra underneath. During the fitting, jog in place and swing your arms to check the armholes and shoulders. Your arms should move freely, the neckline shouldn't shift, and nothing should rub at the inner arm, which is where chafing starts on a long run.
17. Lululemon Swiftly Racerback Hip-Length Size Guide
The hip-length Swiftly keeps the same slim cut but extends further, which helps with mid-rise shorts, covering a waistband or keeping skin covered during overhead moves. Pick the size that matches your chest, then judge the hem on its own. If the chest fits and the body feels too long, try the waist-length version rather than a smaller size. Test it with your running shorts and the sports bra you'll actually use. It should feel secure without limiting your stride or your shoulder swing.
18. Lululemon Racerback Tank Between Sizes Guide
Falling between sizes doesn't automatically mean the bigger or the smaller one is right. First work out where you sit: if your chest is above one range but your waist is smaller, fit the chest, because a racerback has to let you breathe and move your arms. Go smaller for a close performance fit, but only if the neckline, armholes and seams stay smooth. Go larger if you want easier layering, gentler compression or more coverage at the bust. Try both with high-rise leggings and your usual bra, then sit, stretch and raise your arms. The size that stays put through all of that is your size.
19. Sports Bra Fit Under A Lululemon Racerback Tank
Your sports bra can change how the tank fits even when the tank size stays the same. A lightly made bra adds almost no bulk, while molded cups, thick straps or a firm underband can make the chest and armholes feel a size tighter. Racerback bras usually sit neatly under racerback tanks and hide their straps, but strap placement varies by style, so test the actual pair together. If the tank fits alone but pulls once layered, try the next size. And unless the description says the tank has built-in support, don't count on the tank to hold you during impact.
20. Lululemon Racerback Tank Length And Coverage Guide
Length affects comfort as much as width. A waist-length racerback gives a shorter, modern proportion and works best with high-rise leggings, bike shorts or running shorts. Hip length gives more coverage and suits mid-rise bottoms, long torsos or workouts with a lot of reaching overhead. Before choosing, measure from the top of your shoulder down to where you'd like the hem to stop, then compare that with the length category. The bottoms should meet or overlap the hem. If the chest and armholes are right but the hem feels wrong, change the length style, not your numeric size.
The Five Moves I Make Before Keeping a Tank
Standing still in front of a mirror tells you very little about an activewear top. This is the short routine I run on every sample, and it takes less than five minutes.
- Arms straight overhead: the hem should stay at or below your waistband.
- Hug yourself tightly: if the back pulls across the shoulder blades, you need more width.
- Swing your arms as if running: watch for rubbing at the inner armhole.
- Bend forward to touch your shins: check whether the neckline gapes and shows the bra.
- Sit down and slouch: the hem shouldn't roll up into a band at the stomach.
Heat Is What Changes the Fit Later
Synthetic knits rarely shrink in the way cotton does, so a tank that fits in the store generally keeps its size. What changes over time is recovery. Elastane, the fiber that lets the fabric spring back, breaks down with heat, which is why an old tank starts to feel looser at the armholes and wavy at the hem.
Wash it cold, skip fabric softener and hang it to dry. A tank you treat that way will keep the fit you chose for far longer than one that goes through a hot dryer every week.
Measure First, Then Move in It
A good racerback fit starts with an honest chest measurement and finishes with how the tank feels when you move. Read the numeric chart rather than the letter, since several sizes share the same label. Then weigh the cut, your bra, your torso length and your activity. The Align is the closer, yoga-minded fit; the Swiftly is the slimmer performance shape for running and training. Waist length suits high-rise bottoms, and hip length adds coverage. Whenever you can, try two neighboring sizes and move naturally in both before deciding.
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