Big Frenchies Size & Fit Guide: How to Measure for the Perfect Fit

Size and fit guide banner with a measuring tape and folded organic cotton baby onesies on a wooden surface

Nothing ruins a cute matching-family moment like a onesie that will not snap or a tee that swims on you. Here is how to measure, how to read our size charts for babies, women, and men, and what to do when you are between sizes.

Key Takeaways

All Big Frenchies tees and onesies are unisex and fit true to size.
The two measurements that matter are chest flat and front body length.
Measure a garment you already love, laid flat, and compare it to the chart.
For babies between sizes, size up; they grow fast.
Wash cold and tumble dry low to keep cotton from shrinking.

Do Big Frenchies Clothes Run True to Size?

Yes. All Big Frenchies t-shirts and onesies are unisex and fit true to size. That said, every style has its own size chart, so it is worth a quick check before you check out, especially if you are between sizes.

How Do You Measure for the Right Fit?

Two measurements matter most:

Chest flat: measured about 1" below the armhole (½" below the armhole for baby onesies), with the garment laid flat.
Front body length: for adult tees, measured from the high point of the shoulder down to the hem; for baby onesies, from the neck seam down to the bottom hem at the snaps.

Sizes vary among manufacturers and brands, according to the University of Tennessee Extension's clothing guide, which also says it is important to know your measurements to ensure a proper fit. The easiest method: lay a similar shirt you already own flat on a table, measure it the same way with a soft tape measure, and compare it to our chart before you buy.

What Size Onesie Should Your Baby Wear?

Babies grow fast in the first year, and the CDC recommends WHO growth charts, which plot size as a percentile, for tracking children from birth to age 2. Because babies of the same age can land at very different points on those charts, our sizing runs in tight two-to-three-month bands, and measurements matter more than the age on the label.

Size	Chest Flat	Body Length
0-3 months	7¼"	14¼"
3-6 months	8"	15¼"
6-12 months	8¾"	16¼"
12-18 months	9½"	17¼"
18-24 months	10¼"	17¾"

If your little one is right on the edge between two sizes, it is usually smarter to size up. Cloth diapers also take up more room, so size up if you use them.

What Are the Women's T-Shirt Sizes?

Use the women's chart below for a more fitted cut; sizes run from XS up, with chest and body length listed for each.

Size	Chest Flat	Body Length
XS	16.5"	24.75"
S	17.5"	25.38"
M	18.5"	26"
L	19.5"	26.63"
XL	21"	27.25"
XXL	22.5"	27.88"

What Are the Men's T-Shirt Sizes?

Use the men's chart below for a roomier fit, with chest and body length listed for each size.

Size	Chest Flat	Body Length
S	18"	27½"
M	20"	28½"
L	22"	29½"
XL	24"	30½"
XXL	26"	31½"

You will find these charts, kept up to date, on our Size Guide page. Product pages may also list a style-specific chart.

What If You Are Still Between Sizes?

Size up for babies and choose by fit preference for adults. For adults, pick the smaller size for a fitted look or the larger for a relaxed fit. Cotton can shrink slightly in hot water or a hot dryer, so wash cold and tumble dry low (or hang dry) to keep your fit consistent. Sizing up is for daytime wear: the U.S. Consumer Product Safety Commission says children's sleepwear sized larger than nine months must be flame-resistant or snug-fitting, and that loose-fitting cotton garments such as T-shirts should not be used as children's sleepwear. Ready to shop with confidence? Browse our organic cotton baby onesies or family organic cotton clothing.

Which Organic Cotton Picks Fit Every Budget?

Want more options? Here are more in-stock picks from our catalog, from everyday basics to premium upgrades, so you can compare across price points.