Winter coats are sold on a single number. It sits on the swing tag in large type, usually between 550 and 900, and shoppers treat it the way they treat a thread count: bigger is warmer, so pay for the bigger one. Two jackets on one rail, marked 650 and 850, and the choice looks settled before you have picked either of them up.
The number is real and it is measured properly. It just answers a narrower question than most buyers think. It describes how much space a fixed weight of down takes up when it is left alone to puff, and nothing at all about how much down went into the coat you are holding. Those are two separate facts, and only one of them gets printed.
A cylinder, thirty grams, and a light disc
The test is mechanical and almost charmingly simple. A weighed sample of down, conditioned with steam and then dried, goes into a transparent cylinder of known diameter. A light disc is lowered onto it and left to settle. Somebody reads the height reached, converts it to volume, and reports cubic inches.
The sample is always the same mass, thirty grams under the protocol the industry now shares, so the result is a volume per unit weight and nothing else. A reading of 800 means that thirty grams of that particular down, undisturbed and freshly fluffed, filled around 800 cubic inches of cylinder. It is a loft measurement. Warmth follows from loft only because trapped still air is what insulates, and a material that occupies more space per gram is holding more of it.
The protocols themselves come from the International Down and Feather Bureau, the trade body that writes the test methods for composition, cleanliness, and loft. Its current Testing Regulations date from June 2023, it certifies laboratories through an annual round of blind samples that a lab must pass twice running before it is listed, and more than thirty labs worldwide hold that certification. The Bureau is explicit that it does not write labeling rules. It supplies the ruler; each country decides what may be said with it.
Loft per gram is not warmth per garment
Here is the arithmetic the swing tag skips. Insulation depends on total loft, which is loft per gram multiplied by the number of grams present. A shell holding 60 grams of 850 delivers less trapped air than one holding 150 grams of 650, and the second coat will be warmer on a cold platform even though it carries the lower figure and probably the lower price.
The industry has a rough conversion for this. Matching the warmth of an 800 loft duvet with 550 loft down takes roughly 40 to 50 percent more weight of fill. That extra weight is the whole point of paying for a high figure: you are buying the same warmth in a lighter, more packable garment, which matters enormously on a mountain and very little on a school run.
Fill weight is the number that would settle the question, and it is the one manufacturers most often leave off. Serious outdoor brands publish it. High-street coats rarely do. When it is missing, the next best evidence is the garment's own weight on a kitchen scale, read against how thick the panels actually look, because a light coat with a big printed figure and thin quilting is a coat with very little down in it.
The rough bands, and where they stop mattering
Readings below about 500 come from down mixed with a good deal of small feather and fiber. The 600 to 650 band covers most decent bedding and mid-price jackets. From 700 to 750 the clusters are large enough to be genuinely light for their warmth, and above 800 the price climbs faster than the performance for anybody not counting grams in a pack.
Two coats at 650 and 700 feel much the same in ordinary use. A thinly filled 850 and a generously filled 600 are not comparable at all, which is why one figure is a poor way to shop and a good way to sell.
The word down means different amounts in different markets
Loft says nothing about how much of the filling is actually down rather than feather, and that proportion is governed by national rules that do not agree with each other. The Bureau's own summary of world labeling standards lays the differences out plainly.
| Market | Governing document | What a plain "down" claim requires |
|---|---|---|
| United States | USA-2000 labeling standard, ASTM D-4522 | At least 75 percent down cluster in anything advertised as down |
| Europe | EN 12934 | A minimum of 60 percent defined down, with a class system layered on top |
| China | GB/T 14272-2021 for apparel | All filled apparel needs at least 50 percent down, and each grade carries its own loft requirement |
| Canada | Federal labeling guide | Percentages must be listed, with the bare "Down" label exempted |
So a jacket sold as down in Shanghai, Munich, and Denver can legally contain three different proportions of the stuff. Chinese and Korean rules go further than the others by tying a loft figure to each declared percentage, which is closer to what a shopper actually needs than a bare volume reading on its own.
The percentage rules behind an unqualified claim
American advertising sits under a separate document, and it is more useful than its dry title suggests. The Federal Trade Commission's business guidance on advertising and labeling feather down, issued in October 2014, sets out what the words on the tag have to be backed by.
- Pure, all, or 100 percent. None of those may appear unless the filling really contains only down, and the same holds for feather claims.
- An unqualified "down" label. Efficient mass production should yield more than 70 percent down content, so that is the floor a plain claim is measured against.
- A stated percentage. A label reading 50 percent down and 50 percent waterfowl feather allows a deviation of no more than 5 points, and only from unavoidable process variation. A producer may not, in the Commission's phrase, shoot for the tolerance.
- Species. Goose down and duck down are separate claims, and a modest manufacturing deviation is the only slack allowed.
- Cleanliness. Modern processing reaches an oxygen number below 10, so filling dirtier than that fails what a buyer is entitled to expect.
Note that the American figures do not line up neatly. The advertising guidance talks about 70 percent down content while the labeling standard asks for 75 percent down cluster, because cluster and content are counted differently. That gap is a fair warning against reading any of these numbers as interchangeable.
Buy the coat you will wear, not the figure
Pick up both jackets. Squeeze a panel between finger and thumb and feel how much resistance there is before you reach the shell on the other side. Hold each one by the collar and feel the difference in hanging weight. Look for a fill weight in grams on the tag or the product page, and if one brand gives it and the other does not, that silence is information.
Then decide what the garment is for. If you carry it in a bag, ski in it, or walk somewhere genuinely cold with a pack on, high loft down bought by fill weight is money well spent and will outlast the synthetic alternatives by years. If it hangs by the front door and goes to the bus stop, a mid-range figure with generous fill and a shell you can wash will keep you warmer for less, and it will not matter in the slightest that the tag said 650.