Fill Power
Fill power is the volume in cubic inches that one ounce of down occupies under a standard load. Higher fill power means loftier down, so less weight is needed for the same warmth.
Fill power describes down quality, not how warm a jacket is. Warmth needs fill power and fill weight together, and a jacket that publishes only the first is telling you half the story.
The bands
| Fill power | Quality |
|---|---|
| 550 to 650 | Entry level, heavier for the warmth delivered |
| 650 to 750 | The practical travel range |
| 800 and above | Premium, lightest and most compressible, expensive |
The number people forget
Fill weight is the actual mass of down in the garment. A jacket with 800 fill power and 60 g of down is much less warm than one with 650 fill power and 150 g. If a product page gives fill power and no fill weight, ask before assuming it is warm.
Synthetic equivalents
Synthetic insulation is described by weight in g/m2 instead, which is more straightforward. Roughly 60 g/m2 is a light insulated jacket, 100 g/m2 is warm, and above 130 g/m2 is genuinely cold-weather kit.
Related terms
- Down vs synthetic insulation: the full comparison
- Packability: where high fill power pays off
- The practical travel wardrobe: when the insulated layer earns its space
- Glossary: all terms