Take the maximum user height the maker publishes, not the bag’s own outside length — those are two different measurements and listings mix them freely. A bag’s overall length is routinely well above the height the maker says it suits, because the hood and the foot box are not sleeping space.
The two numbers, and why they differ
An internal length describes the cavity from the top of the shoulders to the closed end of the foot box. An overall length includes the hood, the draught collar and the shaping at the foot, and it is always the larger figure.
A maximum user height is the maker’s own translation of the internal dimension into the question you are actually asking. Where it is published, use it. Where only an outside dimension appears, treat it as an upper bound rather than a fit.
Why too long is a real problem
An oversized bag is not simply spare room. Bags tested to EN 13537 or ISO 23537 are measured on a heated mannequin, and the figures assume a body filling the bag well enough to warm the air inside it. A large volume of empty space at the foot is more air to bring up to temperature, and it moves the fit further from what the test assumed.
Too short is worse in a different way: it compresses the insulation at the shoulders or the feet, and compressed insulation has less loft, which is the whole mechanism.
Read shoulder girth in the same breath
Length is only half the fit. Shoulder girth and hip girth decide whether the bag closes around you or is pushed open all night. Broad shoulders in a narrow mummy bag press the insulation flat at exactly the point where the draught comes in.
Makers publish girths less often than lengths, and where they are absent that is a specification the listing simply does not give you. It is worth saying so rather than inferring it from a photograph.
The foot box and the hood
A shaped foot box takes the shape of feet lying naturally rather than pressing them flat, and it is why two bags of the same stated length can feel different at the bottom. A drawcord hood takes up length when it is cinched; a bag that fits with the hood open may be short with it closed.
Long and short versions are not just longer
Many makers offer a long version with a greater fill weight as well as more length, and publish the extra grams. That is a genuine difference in the amount of insulation you are carrying, and it is worth noting when comparing weights between sizes — the figures on the page usually describe the regular length.
If you are adding a liner
A liner has to match the bag’s shape as well as its length. A rectangular liner inside a mummy bag concertinas around the legs; take the length and shoulder width from the bag specification and check whether the liner has a shaped foot box before ordering.
What people ask before buying
Is a longer bag warmer because there is more of it?
No, the opposite tends to be true. A bag rated on a heated mannequin assumes a certain volume of air to warm, and empty space at the foot end is more air to heat. Length is a fit question rather than a warmth upgrade.
What if I fall between two sizes?
Compare the shoulder girth as well as the length, because the wider size often comes with the longer one and changes the fit far more than a few centimetres of length. Where a maker publishes a maximum user height, that figure is the one to use rather than the bag's outside dimension.
Last reviewed 6 September 2026