Vintage NFL sizing:

Why the tag size is only a starting point

US clothing sizes have drifted over the decades. Labeled sizes have tended to get more generous, so the same letter on a modern shirt usually describes a bigger garment than it did in the 1970s or 80s. One often-cited example: a garment labeled medium in the 1970s can be closer in size to a 2010s extra-small. Men's sizing is not immune either: a 2010 study found pants labeled with a 36-inch waist actually measuring anywhere from 37 to 41 inches across retailers.

Sizes also vary from maker to maker, as that pants study shows, so two large crewnecks from different licensed makers in the same year will not necessarily match. And the tag records the size the garment was sewn at, not the size it is after decades of laundry. Treat a tag size as a filter for your eBay search, then decide on measurements.

Cotton, blends and shrinkage

Sweatshirt fleece is knit from cotton or a cotton blend, with a brushed (napped) inside. Natural fibers shrink more than synthetics because they absorb more water, and washing lets the tension built into the fabric during manufacturing relax. Blends of cotton and polyester are more stable than pure cotton, though not immune. Modern fabric sold as preshrunk is processed to keep shrinkage under about 3 percent in either direction, which tells you how much an untreated cotton garment can move.

Shrinkage in sweatshirts was a big enough problem that Champion built a product around it. In the 1930s the company began cutting its Reverse Weave fleece on the cross-grain to cut down lengthwise shrinkage, and a 1952 patent added side panels to address shrinkage in width. The practical lesson for a buyer: a heavily washed 100 percent cotton crewneck can come up shorter in the body and sleeves than its tag suggests, while a cotton-poly blend from the same decade is more likely to have kept its original shape. Check the fiber content on the care tag, and ask for length as well as chest.

Raglan vs set-in sleeves, and era fit

Look at where the sleeve joins the body. A raglan sleeve runs in one piece all the way up to the collar, leaving a diagonal seam from the underarm to the neckline, and it gives a larger, looser armhole. A set-in sleeve is sewn on at the shoulder with a rounded armhole. On a raglan there is no shoulder seam to measure from, so a shoulder-width number means little; sleeve length from the center back of the neck is the useful measurement. Raglans tend to feel roomier through the shoulders than a set-in sweatshirt with the same chest width.

Fashion matters too. In the mid-to-late 1990s, sportswear worn oversized became everyday streetwear, including oversized sweatshirts and T-shirts. Many 90s pieces were bought and worn big on purpose, so an old photo or a seller's 'oversized fit' note may describe how it was worn rather than how it was cut. Go by the tape measure, not the vibe.

How to measure a sweatshirt or jersey

All of these are taken with the garment laid flat on a table, smoothed but not stretched. Ask the seller for the same numbers, in inches, taken the same way.

• Pit to pit (chest width): straight across from one armpit seam to the other. This is the most important number for tops, because letter and numeric sizes track chest width more closely than anything else. Double it for an approximate chest circumference.
• Length: from the highest point of the shoulder, next to the collar, straight down to the bottom edge, including the rib waistband.
• Sleeve: from the center back of the neck, over the point of the shoulder, down to the cuff. On a set-in sleeve some sellers measure from the shoulder seam instead; ask which method they used, because the two can differ by several inches.
• Waistband and cuffs: note whether the ribbing is still snug or has stretched out, since that changes how the bottom hem sits.

Jersey sizes: replica letters, authentic numbers

Fan jerseys come in two broad grades. Replicas are the cheaper fan versions and are generally sold in letter sizes. Authentics are built to follow the on-field uniform and, for football, are sold in numbered sizes such as 40, 44, 48, 52 and 56. The number tracks chest size in inches, but there is no single conversion to letters. Mitchell & Ness, a major maker of throwback jerseys, has changed its own conversion over the years.

So a seller's note that a 48 'fits like a large' is a guess, and the same is true of any chart that claims one fixed answer. Older authentics from Wilson, Champion, Russell or Reebok may not match either column below. Ask for pit to pit and length.

Length is the other surprise. On-field football jerseys are built to stay tucked in, with an extension at the bottom for that purpose, and authentic retail versions often follow the game cut. Expect some authentics to run long in the body compared with a replica of the same chest width. Fabric matters as well: a jersey that is mostly polyester or nylon will not shrink the way an old cotton sweatshirt can, so its tag is a better guide to its original size, though not to how it fits you.

Youth and women's sizes listed as adult

A common reason a 'large' jersey arrives tiny: it is a youth large. Youth sizes are often printed with a number range, for example youth XL marked 18/20, and a careless title can drop the word youth. If the tag shows a size range in parentheses, a 'Y', or 'boys', it is a kids' garment. Whatever the letter, ask for pit to pit before you bid.

Women's cuts are the other trap. Women's team apparel is generally cut slimmer and often shorter; Mitchell & Ness, for example, describes its women's sizing as designed for a slim fit and suggests sizing up for a relaxed fit. A women's large can measure closer to a men's small across the chest. Check the tag for a women's or ladies' line and, again, compare measurements rather than letters.

What to ask a seller, and how to compare

Before you search, lay your best-fitting sweatshirt or jersey flat and measure it the same way: pit to pit, length from the high point of the shoulder, and sleeve from the center back of the neck. Write those three numbers down. If you prefer a roomier fit, add an inch or two to the chest number; if you like it trim, stay close to your own measurements. Then compare listings against your numbers, not against the tag.

When the listing lacks measurements, a short, polite message usually gets them. Our tag dating guide covers what the tag itself tells you about era, and our reproductions guide covers newer pieces made to look old.

• Can you measure pit to pit and length, with the item laid flat, in inches?
• How did you measure the sleeve: from the center back of the neck or from the shoulder seam?
• Is the tag an adult, youth or women's size? Could you photograph the full size and fiber-content tag?
• Is the waistband or cuff ribbing stretched out?
• Has it been washed and dried by you, and do you know if it has shrunk?