Down Fill Power and Insulation Types, Explained
Fill power is how many cubic inches an ounce of down fills, so higher is loftier; warmth also depends on how much down a jacket holds, and synthetics work wet.
By Adolph GreenContributing writer.
On this page
The short answer
Fill power tells you the quality of the down in a jacket: how much space one ounce of it takes up. A higher number means fluffier down that traps more air for its weight. It does not tell you how much down the jacket holds, so a thin jacket with high fill power can be less warm than a puffier one with lower fill power. Synthetic insulation has no fill power rating; it is the better choice when the jacket will get wet.
What fill power means
Fill power is measured in cubic inches per ounce. A lab puts one ounce of down in a cylinder, sets a light weight on top and reads the volume. Field Mag describes the reading being taken after one minute, and gives the example that down filling 800 cubic inches is 800 fill power.
Typical ranges, as Wikipedia's down feather article gives them:
| Fill power | Where it is used |
|---|---|
| About 400 to 900 | The range for most outdoor gear |
| 500 to 650 | Suits most conditions |
| 800 to 900 | Very light or very cold weather gear |
Field Mag notes that ratings are typically in the 600 to 800 range.
Fill power vs fill weight
Warmth comes from two numbers: fill power (the quality) and fill weight (the amount of down). Field Mag makes the point that a thin jacket with high quality down will not be as warm as a thicker jacket with lower quality down. When a maker lists both, compare both. At the same weight, higher fill power down insulates better, as Wikipedia notes.
Why a new jacket can look flat
Fill power drops when down is compressed, shipped and stored. The testing lab IDFL says jackets and sleeping bags may lose 10 to 40 percent of their measured fill power this way. Wearing the jacket, shaking it out and tumble drying it help bring the loft back, according to IDFL.
Down vs synthetic insulation
| Down | Synthetic | |
|---|---|---|
| Rating | Fill power | No fill power rating |
| When wet | Clumps, loses most of its warmth, can mildew if left damp | Keeps insulating and does not clump |
| Build | Loose clusters | Even sheets |
Many down jackets have a water repellent (DWR) coating on the shell, and some treat the down itself to resist clumping. Field Mag notes that unless the shell is fully waterproof, steady rain will still reach the down. If a jacket will often get wet, synthetic insulation keeps working where down does not.
Reading the label
Wikipedia summarizes the US labeling rules this way:
- 100% down: only down.
- Down: can be a mix of down, feathers and fiber.
- Goose down: at least 90% goose down; the rest goose feathers.
A Responsible Down Standard (RDS) mark means an independent body has checked how the down was sourced. Field Mag explains that it rules out force feeding and live plucking, and that the down comes from birds raised for food.
Shopping on Wiki Shopping
Jackets on Wiki Shopping sit in the clothing, shoes and jewelry department. Fill power and insulation type are not yet filters, so check the product listing for the fill power, the fill weight and the insulation name before you buy.