Fill Power Explained
In brief: Fill power measures the quality, or loft, of down: the volume that a fixed mass of down expands to fill under standard laboratory conditions. A higher fill power means each gram of down traps more air, so the garment is warmer for its weight and packs down smaller. It measures quality, not quantity, so a high fill power does not by itself make a jacket warm. To judge total warmth you also need the fill weight, the actual mass of down inside.
What is fill power?
Fill power is a measure of how much a given mass of down lofts, or expands, when it is allowed to settle freely. In the test, a weighed sample of down is placed in a measuring cylinder, conditioned, and the volume it rises to is recorded in cubic inches. The bigger that volume, the higher the fill power. Crucially, fill power describes the quality of the down clusters, not how much down is in the garment. Larger, more mature clusters trap more air and spring back more strongly, so they post a higher number. That is why fill power is often the headline figure on a premium down jacket: it is a shorthand for the grade of down being used.
How fill power is measured
Fill power is determined in a laboratory using a standardised method. A fixed sample of down, commonly 30 grams under the widely used IDFL and IDFB procedures, is cleaned and conditioned to a set humidity, then placed in a transparent cylinder where a lightweight disc rests on top. The height the down lofts the disc to is read off and converted into cubic inches per the sample mass. Because the result is so sensitive to humidity, temperature and conditioning, reputable figures come from controlled testing rather than estimates. Different testing standards have historically produced slightly different numbers for the same down, which is one reason two jackets can quote different fill powers for similar material.
US and European testing standards
For years, American and European fill power figures were not directly comparable, because the conditioning steps differed and tended to give European numbers a different reading for the same down. The European standard EN 12130 was revised in 2018 to use a 30 gram sample with steam conditioning, bringing it much closer in practice to the US and IDFB methods. The result is that modern fill power figures are far more consistent across regions than they once were, though it is still worth knowing which standard a number was measured to when comparing older garments or very precise claims. As a shopper, the practical takeaway is simple: compare like with like, and treat a single fill power number as a guide to down grade rather than an exact warmth rating.
| Fill power range | How it is usually described | Typical use |
|---|---|---|
| Around 550-600 | Good everyday down | Casual jackets and value outerwear |
| Around 650-700 | Strong quality down | Premium lifestyle and high-street down |
| Around 750-800 | High-grade down | Technical outdoor and luxury down jackets |
| 850-900+ | Top-grade down | Mountaineering and extreme-cold kit |
These bands reflect common retail convention rather than a formal grading standard. The exact wording brands use varies.
Fill power vs fill weight
This is the distinction that catches most buyers out. Fill power is the quality of the down, expressed as loft. Fill weight is the quantity, the actual mass of down stuffed into the garment, usually given in grams. Total warmth comes from combining the two: a high fill power paired with a generous fill weight gives a genuinely warm jacket, while a high fill power with a low fill weight gives a light, packable layer that is warm for its weight but not necessarily warm in absolute terms. A 600 fill-power coat carrying 250 grams of down can easily out-warm an 800 fill-power coat holding only 90 grams. If a brand quotes only fill power and stays quiet on fill weight, read the warmth claim with caution.
Goose down vs duck down
Fill power also tends to track the type and maturity of the bird. Goose down generally reaches higher fill powers than duck down because geese are larger and their clusters can grow bigger, with the very best goose down reaching around 900. Good duck down typically sits a little lower, often in the region of 750 to 800, and is frequently more affordable while still performing strongly. Neither is inherently better for every use: a well-made jacket with quality duck down can be an excellent buy, and the headline difference in real-world warmth is often smaller than the fill power gap suggests once fill weight and construction are taken into account.
Why higher fill power costs more
Higher fill power down commands a premium because it is harder to source. The largest, loftiest clusters come from more mature birds and make up a smaller share of any harvest, so top-grade down is scarcer and more expensive per gram. The pay-off is efficiency: because each gram does more work, a high fill-power jacket can hit a target level of warmth using less down, which means less weight and a smaller packed size. For travel, hiking and anyone who values packability, that efficiency is worth paying for. For everyday town wear, a mid fill-power jacket with a sensible fill weight often delivers all the warmth you need at a friendlier price.
Fill power at OD's Designer Clothing
At OD's Designer Clothing we stock down outerwear across a range of fill powers, from everyday lifestyle down to high-grade technical jackets, from premium labels such as Parajumpers, Peak Performance, The North Face and Berghaus. When you are choosing, we are happy to talk through fill power alongside fill weight so you buy for genuine warmth rather than a single headline number. We offer next-day delivery and free click and collect, and customers in the North West are welcome to visit our St Helens store to feel the loft and weight in person.