Comforter Fill, Warmth and Sizes, Explained
Fill power measures how much space an ounce of down fills, so a higher number means more loft for the weight; comforter sizes run wider than their mattress.
By Elissa SanciContributing writer.
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The short answer
A comforter's warmth comes from its fill: how much there is and how well it traps air. For down, the number to read is fill power, which says how much space one ounce of down takes up. A higher fill power means fluffier down that traps more air for its weight. Synthetic and down alternative fills have no fill power rating, so compare those on weight and the maker's warmth label.
What fill power means
Fill power is the volume, in cubic inches, that one ounce of down fills in a test cylinder under a light weight, as the Wikipedia entry on down feathers describes. Down rated 600 fills about 600 cubic inches per ounce.
Two comforters with the same fill power can still differ in warmth, because one may hold more ounces of fill than the other. Fill power tells you the quality of the down; the amount of fill tells you how much of it there is. A light, high fill power comforter can be as warm as a heavier one with lower fill power.
The rating can also drop after packing. The testing lab IDFL notes that comforters, pillows and bulk down may lose 5 to 20 percent of their measured fill power after being compressed, shipped and stored. A new down comforter can look flatter than expected for that reason.
Label words matter too. Wikipedia notes that "100% down" means only down clusters, while a product labeled simply "down" can be mixed with feathers or fiber.
Comforter vs duvet
The Sleep Foundation draws the line this way:
| Comforter | Duvet (insert) | |
|---|---|---|
| Cover | Built in, used as is | Goes inside a removable cover |
| Washing | The whole comforter | Usually just the cover |
| Warmth | Often thinner | Often thicker and warmer |
A duvet insert is a simple filled shell meant to live in a duvet cover. A comforter is finished on the outside and goes straight on the bed.
Comforter sizes chart
A comforter is wider and longer than the mattress so it can hang over the sides. These ranges come from the Sleep Foundation's size chart; makers vary within them.
| Bed size | Mattress (W x L) | Comforter (W x L) |
|---|---|---|
| Twin | 38 x 75 in | 66 to 68 x 86 to 90 in |
| Twin XL | 38 x 80 in | 68 x 86 to 90 in |
| Full | 54 x 75 in | 78 to 88 x 86 to 92 in |
| Queen | 60 x 80 in | 86 to 90 x 90 to 100 in |
| King | 76 x 80 in | 90 to 108 x 86 to 108 in |
| California king | 72 x 84 in | 90 to 110 x 92 to 108 in |
Some comforters are sold in combined sizes such as full/queen or king/California king. Read the actual width and length on the listing and compare it with your mattress, adding its depth on both sides if you want the edges covered.
Choosing warmth
- Sleep hot, or live somewhere warm: look for a lighter fill or a label like "lightweight" or "summer".
- Sleep cold: more fill, or higher fill power down, traps more air.
- Allergies or a down free home: choose a down alternative fill and compare weights.
Our guides to bed sizes and mattress firmness cover the rest of the bed. Comforters sit in the home and kitchen department.