| Shape | Tapered, body-following, hooded |
| Benefit | Warmest for its weight |
| Best for | Backpacking, cold conditions |
| Trade-off | Less roomy than rectangular |
A mummy bag is a sleeping bag shaped to follow the body — wider at the shoulders, tapering to the feet, with a hood — to minimize dead air space and maximize warmth for weight. It’s the standard shape for backpacking and cold conditions, at the cost of the roominess of a rectangular bag.
Why it’s warm
Less empty space and a sealing hood mean less heat to generate and lose — the warmest-for-weight sleeping bag shape.
Mummy vs rectangular
Roomier but colder-for-weight is the rectangular bag — see mummy vs rectangular.
Frequently asked questions
What is a mummy bag?
A mummy bag is a sleeping bag cut to follow the body's shape — broad at the shoulders and tapering to a narrow, often hooded foot box — so there's little empty space for your body to heat. This makes it warm and light, the standard for backpacking.
Mummy vs rectangular sleeping bag?
Mummy bags are warmer for their weight and more packable thanks to the snug, hooded shape; rectangular bags are roomier and more comfortable for those who like to move, but heavier and colder for the weight. Backpackers favor mummy bags; car campers often prefer rectangular.
Why are mummy bags warmer?
Because the close, tapered fit and hood minimize the dead air space your body must heat, and seal warmth in around your head and shoulders where a lot of heat escapes. Less empty space means less heat lost, so a mummy is warmer than a roomy rectangular bag of the same fill.
Sources
- Sleep systems — The Mountaineers