Materials
Down fill power explained: what the number measures and what it leaves out
Down fill power explained as a loft measurement, why it does not describe warmth on its own, and what to read alongside it on a winter jacket label.
Down fill power explained properly starts with what is being measured: volume per unit weight, not heat. A high number tells you the down is lofty for its weight. It does not tell you how much down is in the garment, and without that second figure the first one cannot be turned into warmth.
Fill power is a loft measurement
Fill power is determined by conditioning a sample of down, placing a known weight of it in a cylinder under a light disc, and reading the volume it occupies. The result is expressed as volume per unit weight, commonly cubic inches per ounce in North American labelling.
The physical meaning is straightforward. Down traps air, and trapped air is the insulator. Down that lofts more for the same weight traps more air per gram, which is why a higher figure allows a lighter garment to reach the same insulation.
Because the figure is a ratio rather than a total, it behaves in a way that catches people out. Doubling fill power does not double warmth, and it does not halve the weight of a garment either. What it does is change how much down you need to reach a given loft. A manufacturer can take that saving as reduced weight, as reduced bulk, or as reduced cost by using less down, and the label rarely says which choice was made.
Fill weight is the figure that is usually missing
Two jackets can state the same fill power and contain very different amounts of down. The one with more down is warmer and heavier. Because fill weight is the less flattering number, it is the one more often left off the label.
This is why fill power alone cannot rank two garments. A lower fill power with a generous fill weight can be warmer than a higher fill power with very little down in it, at the cost of packing larger and weighing more.
This is also why comparing two garments across brands is harder than it looks. One maker may quote fill power prominently and fill weight in the specification table; another may quote fill power alone and describe the garment as lightweight, which is true and uninformative. When fill weight is absent, the garment cannot be ranked against one that discloses it, and treating the missing figure as if it were average is a guess rather than a comparison.
Seasonal positioning adds another layer. The same construction sold as a three-season piece and as a winter piece often differs only in fill weight, with identical shell, baffles and down. Reading the two specifications side by side usually shows this immediately, which is a good reason to compare within a maker's own range before comparing across makers.
| Figure | What it measures | What it cannot tell you |
|---|---|---|
| Fill power | Loft per unit weight of down | How much down is in the garment |
| Fill weight | Mass of down in the garment | How efficiently that mass lofts |
| Both together | Approximate trapped air volume | Construction losses at seams and baffles |
Warmth tracks trapped air volume. Neither column on its own produces that figure.
Construction decides whether the loft survives
Down has to be held in place. Sewn-through construction stitches the outer and inner fabric together directly, which is light and creates a cold line at every seam. Baffled construction uses internal walls so the down can loft fully across the seam, which costs weight and material.
A garment with excellent down and sewn-through seams can underperform a garment with ordinary down and baffles in cold conditions, because the cold lines are where heat leaves. The construction type is usually stated, and it belongs next to the two numbers above.
- Sewn-through: lighter, cheaper, cold line at every seam
- Baffled: heavier, more material, down lofts fully across the seam
- Collar, hem and cuff closures decide how much warm air stays inside
- Hood construction is where many otherwise good garments lose the most
Down loses loft when wet, and treatments change that only partly
Wet down collapses, and collapsed down traps no air. Treated down is engineered to resist absorbing moisture for longer, which delays the collapse rather than preventing it. In sustained wet cold, that difference matters less than keeping the outer shell functional.
Moisture also accumulates slowly from the body over consecutive days of use. This is why multi-day cold weather use is a different problem from a cold commute, and why the garment that is right for one is not automatically right for the other.
What the label is legally required to carry
Labelling of textile articles in Canada is governed by the Textile Labelling Act, which sets out requirements for how fibre content is disclosed. That tells you what the filling is, which is a separate question from how well it lofts.
Cold weather exposure guidance is worth reading alongside the garment specification. The Canadian Centre for Occupational Health and Safety publishes material on working in cold conditions that frames clothing as one control among several, which is the right way to treat a jacket: part of a system that includes layering, shelter and time limits.
Reading a jacket label in order
Take the two numbers together, then check how the down is held, then check the closures. Fill power on its own ranks nothing.
- Find both fill power and fill weight, and treat a missing fill weight as missing information
- Check whether construction is sewn-through or baffled
- Check hood, collar, hem and cuff closures, which decide retained warm air
- Decide whether your use is sustained wet cold or short dry cold, and weight the shell accordingly
- Read the fibre content disclosure to confirm what the filling actually is
Before you buy
- Look for fill power and fill weight together and treat either alone as incomplete
- Check whether the garment is sewn-through or baffled before comparing warmth claims
- Inspect hood, collar, hem and cuff closures, which decide how much warm air is retained
- Decide whether your exposure is sustained wet cold or short dry cold before weighting the shell
- Read the fibre content disclosure to confirm the filling matches the marketing
Questions readers ask
- Is a higher fill power always warmer?
- No. It means more loft per unit weight of down. A garment with higher fill power and very little down can be cooler than one with lower fill power and a generous fill weight.
- Why do some labels omit fill weight?
- It is the less flattering figure for a lightly filled garment. Its absence is informative in itself: without it, the stated fill power cannot be converted into expected warmth.
- Does treated down solve the wet problem?
- It delays moisture uptake rather than preventing collapse. In sustained wet cold, keeping the outer shell working matters more than the treatment.
Sources
- Justice Laws Canada — Textile Labelling Act — checked 2026-11-06
- Canadian Centre for Occupational Health and Safety — cold environments — checked 2026-11-06
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