Packed size is a volume in a particular sack, and the sack is a choice the maker makes. Two bags with identical insulation can publish very different packed figures because one was measured in a compression sack and the other in the plain stuff sack it ships with.
What is actually being measured
Most listings give packed size as a diameter and a length, describing a cylinder, or occasionally as a volume in litres. Both describe the bag inside something, and the something matters:
A supplied stuff sack is the honest default and the one most makers use. It gives a figure you can reproduce at home.
A compression sack with straps pulls the same bag much smaller, and a figure quoted that way is not comparable with a stuff-sack figure. The straps and buckles also add weight that is normally outside the published bag weight.
A storage sack — the large mesh or cotton bag a good bag ships with for the cupboard — is not a packing figure at all.
Why the fill decides the number
Down is rated by fill power, the volume a fixed mass lofts to. A high fill power gives more loft per gram, which means less material for the same warmth and therefore a smaller pack. That is why a high fill power bag can be both lighter and smaller than a lower one at the same stated rating.
But fill power alone tells you nothing until you also read the fill weight — the grams actually in the bag. A high fill power with a small fill weight is a small, light bag with less insulation in it, and the packed size reflects that rather than contradicting it.
Synthetic insulation is heavier and bulkier for the same stated rating, keeps more loft when damp, and costs less. Its packed figures are simply larger, and that is the trade being made rather than a failing.
The figure that constrains a rucksack
Packed length is normally less of a problem than packed diameter, because a sleeping bag is soft and will take the shape of whatever it is pushed into. Many walkers pack the bag loose in a drybag at the base of the pack rather than in a sack at all, which fills corners a cylinder leaves empty.
If you do that, the published packed size stops describing your pack and becomes a rough guide to volume. That is fine — but it means the comparison between two bags should be made on volume rather than on a cylinder’s dimensions.
Comparing two bags properly
Note, for each: which sack the figure was measured in, the stated packed dimensions or litres, the fill weight, the fill power if it is down, and whether the insulation is down or synthetic. Then compare within those groups.
A bag that publishes only a packed size and no fill weight is describing the outcome without the input. It is worth saying that the specification is incomplete rather than reading the small number as a virtue.
And the shell around it
Shell denier changes packed size a little and durability a lot. A very light shell packs smaller and abrades sooner, which is a decision about how the bag is used rather than about the number on the page.
What people ask before buying
Should I store a bag in its stuff sack?
Makers state otherwise, and it is one of the few pieces of care advice that is nearly universal in this category. Long-term compression is what flattens loft, and a bag stored loose in a large cotton or mesh sack keeps the figure the specification describes.
Does a compression sack reduce weight?
No, it adds it. A compression sack with straps and buckles weighs more than a plain stuff sack, and any packed size quoted using one has that weight sitting outside the published bag weight unless the maker says otherwise.
Last reviewed 6 September 2026