You’re scrolling through sleeping bags for sale online and see a 600-fill down bag for half the price of similar options.
The specs look identical to the expensive ones. Same fill power, same weight, same temperature rating.
But here’s what most buyers don’t know: about 30% of down-insulated products on the market have inaccurate fill power ratings, according to testing done by independent outdoor gear labs. Some bags labeled as 600-fill actually test closer to 400-fill.
What Does Fill Power Actually Mean?
Fill power measures how much space one ounce of down takes up when it’s fully expanded. Higher numbers mean the down is fluffier and traps more air, which gives you better insulation with less weight.
A 600-fill down will expand to 600 cubic inches per ounce. An 800-fill expands to 800 cubic inches. The higher the number, the warmer and lighter your sleeping bag should be.
But here’s the problem: fill power gets tested in labs under controlled conditions. The number on the label is what the manufacturer claims, not necessarily what you’re getting.
And when you’re buying discounted bags from unknown sellers or budget brands, those numbers can be completely made up.
The International Down and Feather Testing Laboratory uses a standardized test, but it’s voluntary. Plenty of manufacturers skip the official testing and just print whatever number sounds good on their tags.
How Can You Test Down Quality Yourself?
You don’t need lab equipment to check if a sleeping bag has the insulation it claims. The loft test works because down insulation follows basic physics—real high-quality down rebounds and expands predictably.
Here’s what you do: Take the sleeping bag out of its storage sack and let it sit for at least 30 minutes. Down needs time to fully expand after being compressed. Shake it a few times to distribute the fill evenly.
Now lay the bag flat on a table or floor. Using a ruler or measuring tape, measure how thick the insulated sections are when the bag is lying naturally without any pressure on it. The sides of the bag should puff up noticeably.
For a 600-fill bag rated to 20°F, you should see about 2.5 to 3 inches of loft in the main body sections. An 800-fill bag rated to the same temperature will achieve that warmth with only 2 to 2.5 inches because the down is more efficient.
If your “600-fill” bag barely reaches 1.5 inches of loft, something’s wrong. Either the fill power rating is inflated, or there’s not enough down in the bag to match the temperature rating.
What’s the Relationship Between Loft and Temperature Ratings?
Temperature ratings on sleeping bags are supposed to follow EN or ISO testing standards, but lots of manufacturers ignore these standards completely, especially on budget bags.
The EN 13537 standard tests bags in lab conditions and provides three ratings: comfort (for cold sleepers), lower limit (for warm sleepers), and extreme (survival only). But only bags that voluntarily submit to this testing can display EN ratings.
Many discount bags just make up their temperature ratings based on nothing. A study by outdoor gear reviewers found that temperature ratings on budget sleeping bags were off by an average of 10-15 degrees compared to tested bags with similar loft.
| Loft Thickness | Approximate Fill Needed (600-fill) | Realistic Temperature Rating |
| 1.5 inches | 12-14 oz | 35-40°F |
| 2 inches | 16-18 oz | 25-30°F |
| 2.5 inches | 20-24 oz | 15-20°F |
| 3 inches | 26-30 oz | 5-10°F |
These are rough estimates for average-sized bags. Your actual warmth depends on your metabolism, what you wear inside the bag, your sleeping pad’s insulation, and how well the bag fits your body.
What Should You Look for When Shopping Sleeping Bags for Sale?
Before you even get to the loft test, there are warning signs you can spot online or in stores that tell you a bag might not be legit.
Check if the listing specifies the total weight of down fill. A genuine 600-fill bag rated to 20°F should have at least 20-24 ounces of down in it.
If the product description doesn’t mention fill weight at all, that’s suspicious. Legitimate manufacturers always list this spec.
Look at the shell fabric weight. Quality sleeping bags use fabric that weighs around 10-30 denier for the shell.
If a bag claims to be lightweight but doesn’t specify the fabric weight, or if it feels unusually heavy for its size, the extra weight might be coming from low-quality down that needs more material to achieve warmth.
Read the fine print about down quality. Some bags use “duck down” which typically has lower fill power than goose down.
Others mix down with feathers, which are heavier and less insulating. A bag labeled “down blend” or “down alternative” isn’t pure down at all.
Check where the down comes from. European down generally has stricter quality standards than down from other regions.
If a bag is suspiciously cheap and doesn’t specify the down source, it might contain low-grade down or even synthetic fill mislabeled as down.
Can You Trust Third-Party Testing Claims?
Some manufacturers claim their bags are “independently tested” or “laboratory certified.” These phrases sound official but often mean nothing.
Real certifications you should look for include Responsible Down Standard (RDS) certification, which verifies animal welfare and traceability, and IDFL testing, which is the actual International Down and Feather Laboratory standard for fill power.
But even these certifications just verify what was sent to the lab. They don’t guarantee every single bag rolling off the production line has the same quality. Manufacturing inconsistencies happen, especially with budget brands trying to hit low price points.
That’s exactly where your personal loft test comes in. If you’re buying in a store, ask to see the bag fully expanded. If you’re buying online, plan to test it immediately when it arrives and return it if the loft doesn’t match what it should be.
What Happens When Down Gets Wet or Old?
Here’s something else retailers don’t advertise: down insulation degrades over time, and it completely loses its insulating ability when wet.
A sleeping bag that had genuine 600-fill down when it was new might only perform like 500-fill after a few years of use. The down clusters break down, oils from your body contaminate the fill, and compression from storage damages the structure.
Most manufacturers apply a water-resistant treatment called DWR to their down, but this coating wears off. After 50-60 nights of use, even treated down will start absorbing moisture more easily.
When you’re looking at used or heavily discounted sleeping bags, factor in that the actual performance is probably lower than the original specs. A three-year-old bag that saw regular use is not the same as a new bag, even if they have identical tags.
What’s Your Best Protection as a Buyer?
Honestly, the safest approach is buying from established retailers with solid return policies.
If you can test the bag at home for a few days and return it if the loft doesn’t check out, you’re protected.
For online purchases, save screenshots of all the specs and claims. If the bag arrives and clearly doesn’t match what was advertised, you’ll have documentation for your return or dispute.
Consider that if a deal seems too good to be true on sleeping bags for sale, it probably is.
Real high-fill-power down costs money to source and process. A bag selling for 50% less than comparable options is cutting costs somewhere, and that somewhere is usually the down quality.
The loft test isn’t perfect, but it gives you actual physical evidence of what you’re getting.
Retailers can print whatever numbers they want on tags, but they can’t fake the basic physics of how insulation works. Trust what you can measure, not just what you can read.
Frequently Asked Questions
Can I trust the fill power listed on a discounted sleeping bag?
Answer: Not always. About 30% of down bags have inflated fill power ratings, especially from unknown brands. Always verify with a loft test at home.
What is fill power, and why does it matter?
Answer: Fill power measures how much space one ounce of down occupies. Higher fill power means better warmth-to-weight ratio. A 600-fill traps less air than an 800-fill, so loft affects insulation.
How do I check if a sleeping bag actually matches its temperature rating?
Answer: Let the bag expand for 30 minutes, then measure loft. For example, a 600-fill 20°F bag should have ~2.5–3 inches of loft. Less loft usually means lower warmth than advertised.
How does moisture affect down insulation?
Answer: Down loses almost all insulation when wet. Synthetic retains some warmth, but down clumps and fails. Treated down resists light moisture, but heavy wetting still reduces effectiveness.
What should I look for when buying sleeping bags online?
Answer: Check fill weight, fill type (duck vs. goose), fabric weight, down source, and certifications like RDS or IDFL. If specs are missing or the price is too low, the quality may be questionable.
