Down Fill Power Guide: Everything to Know About Down Apparel Insulation

Insulation 101: What is down and how does it work?

Most all that we know about insulation we have learned from animals surviving and thriving in cold weather climates. Many birds, though primarily geese and ducks, are able to withstand frigid temperatures and icy water because down—the fuzzy undercoat of their feathers—is a super insulator, trapping warm air in tiny pockets of fluffy feather clusters close to the body, creating a lightweight and highly effective barrier against cold.

Down products work by filling material baffles with down feathers, keeping the air pockets that form between the feathers to trap heat against the body—and keeping cold air out. Thanks to these air pockets down jackets are also breathable, allowing for thermal regulation. Down is an ideal insulator, since it is the most lightweight method for lining outerwear in a way that allows for comfort throughout a wide range of temperatures and activities.

Goose Down vs Duck Down

Generally goose down is considered more premium than duck down, for a number of reasons. First, geese are larger than ducks, so in turn goose down tends to have larger, fluffier clusters of down feathers, allowing it to achieve higher fill power and better warmth-to-weight efficiency. In short, goose down is lighter, warmer, and more compressible than duck down. On the other hand, good quality duck down is more readily available and more cost-effective, as down is usually harvested from birds that are used for food and ducks are more plentiful.

Down Fill Power vs Fill Weight

Down fill power is a rating that measures how many cubic inches of space one ounce of down can fill. The higher the down fill power rating, the more efficient a jacket will be at trapping heat and providing insulation.

The test of down fill power is conducted in a lab. One ounce of down is dropped into a cylinder and a weight is placed on top. The down compresses, and the volume of the down in cubic inches after one minute becomes the rating (i.e. 800 cubic inches equates to 800 down fill power). The higher the quality of the down, the more air is trapped, and the less the fabric compresses. This is known as loft.

Down fill power ratings can vary from around 400 down fill to 900, but are typically in the 600-800 range. Higher down ratings will offer a better warmth-to-weight ratio, though they aren’t the only measure in determining how warm a jacket is.

What is Fill Weight?

A thin jacket with a few ounces of the highest quality down will never be as warm as a thick jacket bursting at the baffles with lower quality down. A jacket’s warmth is ultimately determined by both the down fill power as well as the total weight of the down used. Apparel companies therefore consider what the potential use cases are of a jacket (everyday wear vs. high output activities) and try to hit the sweet spot in balancing a consumer’s desire for relative warmth with their need for speed.

Down vs. Synthetic Insulation

The most notable downside to natural down insulation is that it holds water and lumps together when wet, causing it to lose much of its insulating properties. Cold spots can form in a jacket or blanket when down clumps together, due to exposure to moisture or improper storage (reminder: don't store your jackets or sleeping bags compressed!). Synthetic down, on the other hand, is created in even sheets that still insulate when wet and won't clump or move around over time. Synthetic insulation also benefits from not being an animal product—and in fact can be made from recycled plastic, as explored below—which is a significant selling point for many users. 

Many modern jackets filled with natural down utilize a durable water repellent (DWR) treatment on the shell, and occasionally a water repellant treatment on the feather fibers themselves to prevent clumping if the shell becomes saturated. These measures help the jacket shed precipitation, but unless the shell is completely waterproof, in consistently wet conditions the insulation will be impacted. This is the main argument for the use of synthetic fill insulation in outerwear and sleeping bags for wet winter scenarios.

What is Sustainable Down?

Often the best insulated jackets and down gear comes with a Responsible Down Standard (RDS) certification, which means that the down has been vetted by an organization that ensures ethical manufacturing practices. In other words, the animals are not force fed or live-plucked. Their down is harvested only after they have been killed for food.

If you prefer your insulation to be animal free, synthetic down made from polyester is the most common down alternative. Though since polyester is made from oil, synthetic options can be more damaging to the environment. However the synthetic down production field is becoming increasingly sustainable, thanks to more innovations in the use of recycled polyester or plastic to produce insulation. Thermore Ecodown is made of 100% post-consumer recycled plastic bottles, and can be found in products from American labels like Stio, Orvis, and Scandinavian outdoor brands like 66 North.

Some brands are pioneering other sources of insulation. For example, Paka launched PAKAFILL in 2023 and received an Outdoor Retailer Innovation Award because the insulation is harvested sustainably from alpacas (without harm to the animal).

Since other types of insulation would react differently to the down fill power test, they do not have the same rating system. Despite the growth of sustainable synthetic insulation, goose and duck down still dominate the market.

Heat retention as measured with thermal testing by Montbell

Performance Considerations aka How to Choose the Right Down Fill Power for Your Needs

When determining what the best option is for a down jacket, understanding the performance properties will allow you to narrow down your options to the one that suits your needs. For example, if you're into backcountry skiing or snowboarding, hiking or climbing, an ultralight down jacket that prioritizes lightness and packability, alongside warmth, is ideal. The 6 oz Mountain Hardwear Ghost Whisperer, made with 1000-fill goose down and 100% recycled shell fabric is a famously popular choice. The Plasma 1000 Alpine Down Jacket from Japanese outdoor brand Montbell is another great option with high fill power and unmatched warmth-to-weight ratio at just under 5 oz.

If you're frequently recreating in below freezing temps, like snow camping, or expect to spend an afternoon belaying a climbing partner, you'll want to sacrifice some weight for increased warmth and durability. The Feathered Friends Helios Down Jacket is a personal favorite that delivers tremendous warmth from ethically-sourced 900+ fill power goose down, creating a great midweight jacket landing around 17 oz. Legit mountaineers trust Seattle's Feathered Friends, so you can too.

Understanding Fill Power Ratings

The temperature rating and range of a down jacket, quilt, or sleeping bag depends on factors like fill weight, insulation value, baffle design, and layering. Always consider activity level, layering, and wind/water resistance for accurate temperature suitability. But here’s a general guide:

• 1000 Fill Power: Ultra-warm and lightweight, suitable for extreme cold conditions all the way down to -20°F with proper design.
• 800 Fill Power: Ideal for very cold temperatures, ranging from -10°F to 20°F, depending on layering and activity level.
• 650 Fill Power: Lighter weight with lower fill power means more versatile for moderate cold, suitable for 20°F to 40°F, making it a good three-season or urban option.

“There is no bad weather, only bad clothing,” goes the oft-quoted proverb. Those who spend considerable amounts of time outdoors know how abjectly false this is. Winter weather can be brutal, especially at elevation, yet human ingenuity has thought up some pretty advanced ways to deal with our paltry insulation. There is bad weather. But rest assured: there are better jackets.

Frequently Asked Questions

What is down fill power?

Down fill power measures the loft and insulation efficiency of down. It indicates how many cubic inches one ounce of down can fill. Higher fill power, like 800 or 900, traps more air and provides better warmth with less weight, making it a top choice for cold-weather outdoor gear.

What are the best insulation materials?

The best insulation materials include goose or duck down, synthetic fibers like PrimaLoft, and wool. Down is lightweight and compressible, ideal for extreme cold and dry climates. Synthetics perform well in wet conditions, retaining warmth even when damp, while wool offers natural breathability and temperature regulation.

What temperature is 650 down fill good for?

650-fill down provides a balance of warmth and affordability, suitable for moderate cold, typically in the range of 20–40°F depending on garment or sleeping bag design. Its versatility makes it great for three-season camping and general outdoor activities in cool climates.

Is 800 fill down warmer than 600?

Yes, 800-fill down is warmer, lighter, and more compressible than 600-fill down for the same weight. This makes it perfect for ultralight gear and extreme-cold conditions, where efficiency and weight savings are critical. However, 600-fill down is still a good, cost-effective option for less demanding adventures.