Warmth-to-Weight, Explained
Degrees of temperature rating per kilogram is the ranking rule behind our sleeping bag comparisons. This guide explains how to read it, when it genuinely decides a purchase, and the several situations where the heavier bag is the right answer.
What the Ratio Measures
Warmth-to-weight divides a bag’s stated temperature rating by its weight. A bag rated to 20°F weighing 1.5 kg is delivering more claimed capability per kilogram than a 20°F bag weighing 2.5 kg.
The reason to compare this way rather than on either figure alone is that each in isolation rewards the wrong bag. Rank by temperature rating and heavy synthetic bags win. Rank by weight and summer bags win. Neither answers the question a buyer is actually asking, which is what they get for what they carry.
Read this first: the ratio inherits everything wrong with the temperature rating that feeds it. Since no listing we examined states its testing basis, the numerator is an unverified claim. See what a temperature rating actually promises. The ratio compares claims efficiently — it does not verify them.
How to Read a Score
The absolute value is not meaningful on its own; the useful reading is positional. The category median in our data sits at roughly 20°F of rating and 1.8 kg of bag, so a bag that is materially lighter at the same rating — or materially warmer at the same weight — is doing something the middle of the market is not.
Two products can reach the same score by opposite routes: a light bag with a modest rating, and a heavy bag with an aggressive one. They are not interchangeable. Always read the ratio alongside the two figures that produced it, which is why our comparison pages show all three columns.
What Drives a Good Score
- Down fill. Down provides more insulation per gram than synthetic. Nearly every bag at the top of a warmth-to-weight ranking is a down bag.
- Mummy shape. Less fabric and less internal air to heat. Rectangular bags are penalised heavily.
- Lightweight shell fabrics. Thin nylon shells save meaningful weight and are less durable.
- An optimistic rating. Worth naming plainly: since ratings are unverified, a brand willing to rate generously scores better than an identical bag rated conservatively.
That last point is why we treat the ratio as a shortlisting tool. It rewards efficient design and generous rating equally, and the specifications cannot tell them apart.
The Missing Specification
Fill power — the loft measurement separating good down from mediocre down — is essentially absent from retail listings. In the bags we examined it appeared on effectively none of them.
This matters because fill power is the main determinant of a down bag’s efficiency. An 800 fill-power bag and a 550 fill-power bag can carry the same rating at very different weights, and the listing usually will not say which you are buying. If a brand publishes fill power, treat it as a positive signal about the brand as much as the bag.
When the Ratio Should Decide
Warmth-to-weight matters in direct proportion to how far you carry the bag.
- Multi-day backpacking. The sleeping bag is one of the three heaviest items in a pack. Efficiency here compounds over every kilometre.
- Weight-limited trips. Air travel, bike touring, or any trip with a hard weight ceiling.
- Fast-and-light trips. Where the whole kit is chosen on efficiency and the bag should match.
When to Ignore It
These are not edge cases. They cover most people who buy a sleeping bag.
Car camping
If the bag travels in a trunk, its weight is irrelevant. Comfort, width, a zip that survives abuse and a bag you can machine wash all matter more. A rectangular synthetic bag that ranks poorly here may be exactly right.
Wet conditions
Down’s efficiency advantage disappears when it gets wet, and it dries slowly. For persistent damp, a heavier synthetic bag is more reliable than a lighter down one, and the ranking will not reflect that.
Cold sleepers
Ratings assume a standard sleeper. If you sleep cold, the correct response is to buy a warmer bag and carry the weight. Optimising a ratio while shivering is a poor trade.
Durability and budget
Efficiency is expensive. The gap between a mid-range synthetic bag and a high fill-power down bag of the same rating is substantial, and for occasional use the heavier bag is often the better purchase.
Which Should You Choose?
- Carrying it more than a few kilometres: use the ratio to shortlist, then check fill type and whether the brand publishes anything about its rating basis.
- Car camping only: ignore the ratio. Buy for comfort, width and washability.
- Wet climate: prefer synthetic even at a ratio penalty, or down with a genuinely water-resistant shell.
- Sleeping cold: add margin to the rating first, then optimize weight within what is left.
Do Not Optimize the Bag Alone
A sleeping bag’s rating assumes adequate insulation underneath it. Compressed insulation beneath your body contributes very little, so ground insulation comes almost entirely from the pad.
Buying a highly efficient bag and pairing it with a summer pad wastes most of what you paid for. If your budget covers one upgrade, the pad is frequently the better place to spend it — with the caveat, covered on the sleeping pads page, that R-value is published so rarely that comparing pads is harder than comparing bags.
FAQ
What counts as a good warmth-to-weight score?
Read it positionally rather than absolutely. Against a category median of roughly 20°F and 1.8 kg, a bag delivering the same rating meaningfully lighter is performing well. Compare within a rating band rather than across the whole category.
Is a down bag always better on this measure?
Almost always, yes — down insulates better per gram. Whether that makes it the better bag depends on moisture, budget and how you look after it.
Why does the ranking not account for fill power?
Because retail listings essentially never publish it. It is the single most useful missing specification in the category.
Can a brand game this ranking?
Yes, by rating its bags optimistically, and we would have no way to detect it from specifications alone. This is the main reason we present warmth-to-weight as a shortlisting tool rather than a verdict.
Should I buy the highest-scoring bag I can afford?
Only if you carry it. For car camping the top of this ranking is a list of expensive, narrow, delicate bags optimized for a problem you do not have.