How to Choose a Backpacking Sleeping Bag

A sleeping bag is the one piece of gear whose headline number is routinely misread, and the misreading is expensive in both money and cold nights. “20°F” printed on a label can mean the temperature at which a standardised test subject survives, the temperature at which a different test subject is comfortable, or a figure the maker arrived at by looking at the loft. Those are not the same number, and the difference can be thirty degrees.

The five decisions, in order

Rating, fill type, fill power, fit, weight. Work in that order and each answer shrinks the field. Work backwards from weight, which is how most people shop, and you end up with a light bag that does not keep you warm or does not fit.

1. The rating, read honestly

Find your coldest realistic night — look up overnight lows for the elevations and months you actually hike — and subtract about 10°F. Buy the rating that covers the result. For most of the lower 48 across three seasons, that lands on 20°F.

None of those makers is lying. They are using different conventions, and only EN 13537 or ISO 23537 testing makes two numbers directly comparable. Our temperature ratings guide walks through the standards in full; the practical rule is to treat an untested rating as a survival figure and add margin.

2. Down or synthetic

Buy down unless you are consistently camping somewhere damp with no way to dry gear, or your budget rules it out. A hydrophobic-treated down, like the 800 FP goose down in the Eolus 15, narrows the wet-weather gap without closing it. The full argument is in down vs synthetic insulation.

3. Fill power, and why it is not warmth

Fill power measures loft per ounce: how many cubic inches one ounce of that down occupies. It is a measure of efficiency, not of warmth. A 550 fill bag and an 800 fill bag can be equally warm; the 800 fill one gets there with less down, so it weighs less and packs smaller, and costs more.

What makes a bag warm is total fill weight at a given loft. Kelty lists 16.4 oz of 550 fill down in the Regular Cosmic Down 20. Hyke & Byke lists 620 g of 800 FP goose down in the Regular Eolus 15. Paria lists 25.8 oz of 700 fill duck down in the Regular Thermodown 15. Those three figures tell you more than the fill-power number alone ever will. Detail in down fill power explained.

4. Length and girth

A bag that is too long has dead air at your feet that you have to heat; a bag that is too tight compresses the insulation against you, which is exactly where insulation stops working. Both cost you real degrees.

• Length. Kelty lists the Regular Cosmic Down 20 at 78 in, fitting to 6 ft. Paria lists the Regular Thermodown 15 at 82.5 in, also sized to 6 ft. Teton sells the TrailHead in Short (75 in) and Large (87 in) only, with no Regular.
• Girth. Kelty publishes 62 in at the shoulder, 58 in at the hip and 46 in at the foot. Hyke & Byke publishes a 59 in shoulder girth on a 28 in wide bag. A narrower cut is lighter and warmer, right up to the point where you cannot turn over.

If you sleep on your side and move at night, read the side-sleeper bag ranking before anything else: girth matters more for you than rating does.

5. The weight you should expect

For a three-season bag in the 15-20°F band, a realistic target is roughly 1,100-1,200 g in down and 1,500 g or more in synthetic. The Paria Thermodown 15 is 1,134 g (2 lb 8.0 oz), the Kelty Cosmic Down 20 is 1,106 g (2 lb 7.0 oz), the Hyke & Byke Eolus 15 is 1,152 g (2 lb 8.6 oz), and the synthetic Teton TrailHead 20 in Large is 1,579 g (3 lb 7.7 oz).

Under 900 g for a genuine 20°F bag means premium down, a premium price, or a quilt. Our sleeping bag weight guide sets the targets by rating and fill type.

Four bags across the spread

One bag per approach: budget down, mainstream down, high-fill-power down, and the cheapest synthetic. Every weight, rating and fill figure is the maker's own, linked in the sources.

The pad decides more than the bag

The insulation underneath you is crushed flat by your body weight and does almost nothing. What keeps you off the cold ground is the pad's R-value, and ground pulls heat out of you roughly twenty times faster than still air does. A 20°F bag on an R 1.3 pad is a cold night, every time, and no amount of down on top fixes it.

Pair a 20°F bag with a pad of R 4.5 or better. Hyke & Byke says outright that the Eolus is designed with differential fill — more insulation on top, less underneath — and is intended for use with a sleeping pad. Read the R-value guide next, and what size pad you need after that.

Two numbers you can ignore

Shell denier, mostly. Nearly every three-season bag uses 20D to 50D nylon or polyester. The difference between 20D and 30D on a bag that lives inside a tent is not the thing that will fail.

Packed size on the maker's page. Paria lists 10 x 6 in for the Thermodown 15 and Hyke & Byke 7 x 11 in for the Eolus 15, but both are measured in the stuff sack the bag ships with, and those sacks differ wildly in how hard they compress. Judge packed size by fill weight and fill power instead.