> ## Content Index > Fetch the complete content index at: https://www.unknowndivide.com/llms.txt > Use this file to discover other available public pages before exploring further. # Down Fill Power Explained: What 600, 800 and 900 Fill Really Mean - URL: https://www.unknowndivide.com/down-fill-power-explained/ - Published: 2024-02-09T15:00:00.000Z - Updated: 2026-10-31T06:14:12.000Z - Description: Fill power is the number on every down jacket and sleeping bag. Here is what it measures, how it changes weight and packed size, how down compares to synthetic insulation and how to keep down lofty. - Author: unknowndivide staff - Tags: Gear *Unknowndivide may earn a commission from purchases made through links in this post.* Shop for a down jacket or sleeping bag and you will see the same numbers again and again: 600 fill, 650 fill, 800 fill, 900 fill. Those numbers tell you a lot about how warm, light and packable a piece of gear will be, but they are rarely explained. This guide breaks down what fill power means, how it affects weight and packed size, how down compares to synthetic insulation and how to care for down so it lasts. ## What fill power means Fill power is a measure of loft, or how fluffy the down is. Therm-a-Rest's maker, Cascade Designs, [defines it](https://support.cascadedesigns.com/hc/en-us/articles/35377027786003?ref=unknowndivide.com) as the number of cubic inches that one ounce of down will displace. In the brand's guide to [down vs synthetic insulation](https://cascadedesigns.com/blogs/thermarest-gear/synthetic-vs-down-sleeping-bag?ref=unknowndivide.com), it adds that the test is done inside a calibrated container. So a down with a fill power of 800 fills 800 cubic inches per ounce, while 600 fill down fills 600\. The higher the number, the more space each ounce takes up, and the more air it traps. Trapped air is what keeps you warm. ## Why it matters: weight and warmth Here is the key point. Fill power does not tell you how warm a bag or jacket is on its own. Warmth comes from how much loft there is in total. What fill power tells you is how much down it takes to get there. Cascade Designs explains that the higher the fill power, the less down is needed to achieve a given temperature rating. Its example: a 10 degree sleeping bag made with 800 fill down will weigh less and use less down than a 10 degree bag made with 600 fill down. Both are rated for the same temperature; the higher fill bag simply gets there with less weight. That is why ultralight gear leans on high fill numbers. Therm-a-Rest's Parsec sleeping bag, for example, is [filled with 900 fill hydrophobic down](https://cascadedesigns.com/products/parsec-20f-6c-sleeping-bag?ref=unknowndivide.com) and aimed at three season, fast and light backcountry trips. ## Fill power and packed size Higher fill power also means a smaller packed size. Cascade Designs says that because less down is needed to fill the space, there is more air in the bag, and that air can be squeezed out when the bag is compressed. The result is a smaller stuff sack and more room in your pack. If you are working out how much space you have, our guide to [backpack sizes](https://www.unknowndivide.com/backpack-sizes-explained-how-many-liters/) can help. ## How to read the numbers When comparing two products, keep these points in mind: - **Compare fill power and temperature rating together.** A 600 fill bag and an 800 fill bag can carry the same rating. The difference shows up in weight and packed size. - **Look at fill weight, too.** Spec sheets often list how many ounces or grams of down are inside; the Parsec, for example, lists 17 ounces in the regular size. More down of the same fill power means more loft and more warmth. - **Use the temperature rating to pick warmth.** For sleeping bags, Therm-a-Rest points to the EN or ISO rating, an industry wide test that helps you compare bags and quilts. ## Down vs synthetic insulation Down is not the only option. Synthetic insulation is made from polyester fibers. In its [insulation guide](https://cascadedesigns.com/blogs/thermarest-gear/synthetic-vs-down-sleeping-bag?ref=unknowndivide.com), Therm-a-Rest lays out the tradeoffs: - **Warmth to weight:** down wins. Between two bags with the same temperature rating, the synthetic one will be heavier and bulkier. - **Water resistance:** synthetic wins. It is designed to keep insulating even when wet, and it dries faster. Moisture is down's biggest weakness. - **Cost:** down is the more expensive material, and lower fill power down can save money while keeping most of the benefits. Therm-a-Rest's own synthetic insulation, eraLoft, uses water resistant polyester that retains warmth when wet, with hollow core fibers that trap heat. The brand's bottom line is simple: for gram counting backpackers who want a light, compact load, down is the top choice; for very wet conditions, synthetic is more reliable. ## What about hydrophobic down? Hydrophobic down is down treated to resist water. Therm-a-Rest says the treatment it uses means the down absorbs 70 percent less moisture than untreated down and dries three times faster, without adding weight. That gives down more room for use in damp climates. Even so, the brand notes that a synthetic bag still outperforms treated down in very wet conditions. The treatment also washes out over time; Therm-a-Rest recommends a dedicated down re-treatment product to restore it. ## How to care for down Good care keeps down lofty for years. Therm-a-Rest's care advice for its down bags includes: - **Wash rarely.** Washing is hard on all down, so spot clean with a damp sponge and only machine wash when necessary. - **Use a front loader.** Wash in a large capacity front loading machine with a down specific wash, cold water and a gentle cycle with two rinses. Do not dry clean, bleach or iron. - **Dry slowly.** Hang drying is best, though it can take 24 to 48 hours or more. If you use a dryer, choose low heat, check often and add a few tennis balls to break up clumps. - **Store it loose.** Keep a bag uncompressed in a cool, dry place, in a large storage sack. Use the compression sack only for trips. ## Build a warm sleep system A sleeping bag is only half the story. What is underneath you matters just as much. Read our guide to [sleeping pad R-value](https://www.unknowndivide.com/sleeping-pad-r-value-explained/), and see our look at the [Therm-a-Rest NeoAir XLite NXT](https://www.unknowndivide.com/therm-a-rest-neoair-xlite-nxt-sleeping-pad/) for a light, warm pad. A merino base layer helps too; our [merino wool guide](https://www.unknowndivide.com/merino-wool-explained/) explains why. For around camp, a down jacket like the [North Face 1996 Retro Nuptse](https://www.unknowndivide.com/north-face-1996-retro-nuptse-jacket/) uses the same principles. ## The bottom line Fill power measures how much space an ounce of down fills. Higher numbers mean less down for the same warmth, so the gear is lighter and packs smaller. Choose warmth by temperature rating, choose weight and packability by fill power, and choose synthetic if you expect to be wet for days. For more gear advice, browse our [buying guides](https://www.unknowndivide.com/buying-guides/).