How to Find the Right Fit for Running Clothes | No Chafing, No Sagging

The right fit for running clothes is snug but not restrictive, with moisture-wicking fabric, flat seams, and zero ride-up during your stride.

Pull on a pair of shorts that looks fine in the mirror, and by mile three the waistband is digging in and the inner seam is rubbing raw. The problem isn’t your body—it’s the fit. Running clothes have a specific job: move with you, wick sweat, and disappear from your awareness. One wrong measurement or fabric choice turns a good run into a chafing session. Here is how to get it right the first time, from chest measurements to the five-second stretch test.

What Makes Running Clothes Fit Right?

Running gear should feel tight but never restrictive. Think of it as a second skin with 10 percent give—when you’re soaked with sweat, the fabric will stretch, and it needs to stay comfortable at that expanded size. The key signals are minimal flapping fabric, flat or welded seams that stay away from your inner thighs and underarms, and moisture-wicking materials like polyester, nylon, or merino wool. Cotton is the enemy; it holds sweat, gets heavy, and is the single biggest cause of chafing mid-run.

Consummate Athlete’s guide to running fit confirms that the garment must allow easy breathing and free arm movement without riding up or stretching uncomfortably when saturated. If a top feels perfect in the store but pulls at the shoulders when you reach forward, it will only get worse on the road.

How to Measure Yourself for Running Apparel

Brands do not share a universal size chart. New Balance’s medium sits at a 38–41 inch chest for men, while On Running’s medium starts at 38.2 inches and tops out at 40.2. That half-inch difference matters if you’re on the edge. The only way to get the right size is to measure your body first, then check the brand’s chart every time. Adidas and New Balance recommend these five steps:

1. Chest: Measure horizontally around the widest part of your bust or chest, just under the arms.
2. Waist: Measure around the narrowest point—your natural waist, not where your pants sit.
3. Hips or Seat: Measure around the fullest part of your hips while standing straight.
4. Inseam: For tops that need a specific length, measure from your crotch to the floor.
5. Height: Stand straight and measure from the top of your head to the floor.

Write those numbers down. Then pull up the size chart for whatever brand you are buying. Brooks Running says their clothing runs true to size, so start with the size you normally wear—but verify it against your own measurements before clicking “buy.”

Brand Size Chart Comparison: New Balance vs On Running

The table below shows how two major brands define a medium and large. Notice the overlap—and the gaps. A 40-inch chest fits both brands’ medium, but a 41-inch chest pushes you to a large in New Balance while still sitting inside On Running’s medium range.

Women’s sizing follows the same logic but different ranges. Always check the gender-specific chart on the brand’s site.

The Five Fit Tests You Should Run Before Buying

Once you have the garment in hand, do not rely on the size tag. Run through these checks from REI and Consummate Athlete. They take about thirty seconds and can save you from a post-return headache.

The stretch test. Pull the fabric at the chest and waist. It should stretch and snap back without resistance. If it feels tight even when dry, it will be uncomfortable when wet and stretched 10 percent from sweat absorption.

The seam check. Run your hand along the inner seams of shorts and tops. Flatlock or welded seams are ideal. If a seam sits directly on your inner thigh or underarm, expect chafing by mile five.

The ride-up test. Take a few steps, then jog in place. Shorts and shirts should stay put. If they climb immediately, they are too loose in the waist or too short in the inseam.

The breathing test. Inhale deeply. The fabric should expand with your ribs. This matters most for sports bras and tight-fitting singlets—restriction here affects your stride and your oxygen intake.

The layering check. For jackets or long sleeves, make a fist and raise your arm overhead. The sleeve should not pull your shirt up. If it does, size up for the outer layer.

How Body Shape Changes Your Fit

Standard size charts assume an average proportion, but real runners come in every build. Shorter runners often find that standard tank tops sag under the armpits, while runners with broader shoulders need a wider cut through the chest to avoid restricted arm movement. For runners with a smaller frame, more fitted options prevent the fabric from flapping and catching wind.

Inclusive sizing is becoming more common, with brands expanding their ranges to fit diverse body shapes. The principle stays the same: prioritize how it feels over how it looks on the hanger. If you are ready to buy gear that fits well and won’t break the bank, check out our roundup of affordable running clothes that actually perform.

Common Fit Mistakes Most Runners Make

The biggest error is buying cotton. It feels soft in the store, but once you start sweating, it turns into a heavy, abrasive towel against your skin. The second mistake is ignoring the seasonal stretch factor—buying clothes tight at mile one without accounting for that 10 percent expansion when wet. The third is choosing style over feel. A shirt that looks sharp but restricts your shoulders or sits on the wrong seam will ruin a long run. And never assume “true to size” applies across brands. New Balance and On Running have different calibrations, so always check the specific chart before checkout.

Runners who wear orthotic inserts also need to size up half a size in shoes, a point echoed by Runners Need’s shoe guide, but that same logic applies to apparel: if you wear a base layer under a jacket, the jacket needs room for that extra fabric without becoming restrictive.

Temperature and Safety: What to Wear When

Warm weather calls for lightweight, airy fabrics that wick moisture and dry fast. Look for a UPF rating on sunny days—the higher the number, the more UV protection. For cold weather, layer with a lightly insulated, water-resistant softshell and cover your hands and ears. Chafing risk is higher in the cold if a layer is too loose, because fabric rubs against skin rather than moving with it.

Putting It All Together: Your Fit Checklist

Before you buy or hit the road in new gear, run through this short list. It captures everything from measurement to final check.

• Measure your chest, waist, hips, and inseam with a soft tape.
• Check the brand’s specific size chart—never assume “true to size.”
• Choose moisture-wicking fabric (polyester, nylon, merino wool). Skip cotton.
• Look for flatlock or welded seams away from high-friction zones.
• Perform the five fit tests: stretch, seam, ride-up, breathing, layering.
• If you use orthotics, size up half a size in shoes and check for room in layered apparel.

Getting the fit right transforms your run. No tugging, no chafing, no stopping to adjust a waistband. Just you and the road.