Along with your sleeping bag’s EN Temperature Ratings, a proper fit is critical to your bag’s warmth, comfort and overall performance. A bag that’s too big will allow to move too freely around you and out of your bag, wasting heat; too small and you’ll compress the insulation, reducing it’s effectiveness. Most models will be presented in different sizes, but sleeping bag size is not all-encompassing of fit. You’ll need to do a little more investigation to get the full scope of a sleeping bag’s fit.

We use our onsite cold chamber to perfect our sleeping bag shape. Testing in the chamber allows us to analyze and optimize the balance between spacious luxury and lightweight warmth.  Some of our designs focus more on comfort and feature internal space at the expense of weight. Others aim for being as light and compact as possible and cut down on that wiggle room. But, no matter which model sleeping bag you climb into at night, if it doesn’t fit properly it won’t perform properly. Fit is critical.

There are right and wrong ways to fit a sleeping bag, and there are more subjective, preference-based dimensions of fitting a bag. How then, do you know if a bag fits and if it is the right bag for you? Here we dive into sleeping bag size, shape, and fit in order to answer those questions.

relaxing in sleeping bags

1. Sleeping Bag Length

Sleeping bags are sized by their length, and though this may seem obvious, the correct length for your bag should correlate with your height. For example, if you’re 6 feet tall, you should be sleeping in roughly the same length bag.

However, don’t think that a 6-foot sleeping bag should fit like a latex glove. A 6-foot sleeping bag will actually measure roughly 6-foot 8-inches long, giving a 6-foot tall person the extra room needed for a correct fit. Why the extra room? When we lay flat, our spines decompress, our feet fall forward and our bodies spread out. As a result, sleeping bag designers compensate for this by adding a few extra inches to the length.

The goal is to minimize excess space because excess space requires more energy to heat. So, choose a sleeping bag size with an appropriate length for you. Therm-a-Rest sleeping bags come in three sizes: small (5 ft. 6 in.), regular (6 ft.), and long (6 ft. 6 in.). Pick the sleeping bag size closest to your measured height.

shaking on sleeping bag

2. Sleeping Bag Shape

The shape of your sleeping bag should reflect the style of camping you’re planning on doing. There are three basic shapes of bags: Mummy, Rectangular, and Semi-Rectangular. We’ll break it down to help you decide which shape best fits your style.

The most popular shape, the mummy bag has a snug fit that tapers toward the feet, making it the warmest option. This shape allows the least amount of air between you and the insulating wall of the bag, meaning there is less air to heat. Due to its efficient shape, it’s also lightweight and packs easily, making it the ideal choice for backpacking or technical adventures that require packability but also a high degree of warmth. The downside? Room. A mummy bag can be restrictive compared to other shapes.

As the name implies, this is a non-tapered design, which is less efficient at heating, but also less restrictive while sleeping. Warm weather car campers often opt for rectangular-shaped sleeping bags due to the freedom of movement they provide. If comfort, not warmth-to-weight efficiency, is your main priority, consider this style of bag.

A hybrid between the mummy and rectangular shape, this is the go-between for fair-weather campers that require warmth but also desire the extra room for added comfort. While not as warm as mummy bags that can withstand 3 to 4 season camping, a semi-rectangular bag can work for fair-weather trips, and also be efficient enough for shorter backpacking adventures.

A special exception is our semi-rectangular ultralight Ohm™ 32F/0C Sleeping Bag. The bag is a perennial staff favorite at Therm-a-Rest HQ on account of its clever versatility. Its premium 900-fill Nikwax Hydrophobic Down™ delivers warmth while helping cut down on bulk. Fully unzipped, it lays flat and can be used as a quilt on nights that need extra ventilation. Or, if you and your partner have two Ohms, they can be fully unzipped, stacked and zipped together for a two-person bag. The Ohm is ultralight, ultra-versatile and still spaciously comfortable due to its design. Truly a unicorn piece of gear.

Thermarest sleeping bag fit chart

3. Sleeping Bag Fit

Our sleeping bags’ fits are carefully crafted to fit the needs of your adventure. Most of our Fast & Light bags fall under Performance Fit. This fit is tapered to maximize thermal efficiency and minimize pack size, while still providing some girth for comfort.  Our Ultralight Fit is even further tapered and cuts down on internal space for those counting every single ounce. For more comfort-minded backpackers we offer W.A.R.M. Fit. This fit is what you’ll find in our Trek & Travel line of sleeping bags.  We dove into our cold chamber to create the a balance of roominess and thermal efficiency. While slightly heavier and bulkier than Performance Fit, more internal space means you can sleep in whatever position you want.  Welcome to your most comfortable nights under the backcountry stars yet.

We also realize that some campers are uncomfortable when a bag feels too tight, so to combat this feeling, we’ve added girth to our mummy bags between the hips and shoulders where most manufacturers don’t. This gives sleepers the freedom to move and wear a jacket in extra cold conditions. The bottom line, however, is that room is a personal preference, but the right balance will increase your bag’s performance.

relaxing in sleeping bag with food

Finding Your Fit

Much like a jacket, the best way to get the right fit is to try them on if you can. It’s important to realize that each brand and model of sleeping bag will fit differently. but with the information above, you’ll be able to dial in the perfect fit.

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Updated. Originally Published October 25, 2019.