How to read a sleep-gear spec sheet

Under ISO 23537 (and the older EN 13537 it replaced) a bag is tested on a heated mannequin and produces three temperatures. Comfort is where a standard cold sleeper — the standard uses an average woman — stays warm lying relaxed on their back. Limit is where a standard warm sleeper stays warm curled up. Extreme is a six-hour survival figure with a genuine risk of hypothermia, and it should never enter a purchasing decision.

Most makers name a bag after its limit, some after something in between, and a few after nothing in particular. That is the whole reason this site leads with the comfort figure wherever one exists: it is the only one of the three that describes a night you would want to have.

Where a maker publishes only the lower limit, the comfort figure normally lands ten to twelve degrees warmer. Where a maker publishes nothing — which is true of 16 of the 32 bags and quilts here, including most of the premium American brands — there is no lab number to fall back on, and the useful substitutes are measured loft in inches and fill weight in ounces, both of which those makers do publish.

How many degrees warmer the published comfort rating is than the number in the product’s name. It is arithmetic, not an accusation: most makers name a bag after the lab limit, and comfort runs ten to twelve degrees above limit, so a gap that size is normal and expected. What is worth knowing is how big it is on the specific bag in front of you.

honesty gap = ISO comfort rating − temperature in the name

The widest in this catalogue is Sea to Summit Spark 15, sold as a 15 with a comfort rating of 28 °F — 13 degrees of daylight between the name and the night. The tightest is Marmot Trestles Elite Eco 20, at 4 °F. Neither maker is doing anything improper — both publish the lab figure openly — but one of those two names describes the night much better than the other.

A 60 °F night needs no bag at all, so the useful question is how many degrees below that a bag buys for every ounce you carry. That is a comparison of insulation efficiency rather than of absolute warmth, and it can only be computed against a lab comfort figure, so it is blank on anything that publishes none.

warmth per ounce = (60 − ISO comfort °F) ÷ weight in ounces

The winner is Therm-a-Rest Vesper 20 Quilt. It is worth saying plainly that this metric flatters bags with a cold comfort rating and a light shell, and says nothing about durability, fit or whether the thing is pleasant to sleep in — it is one axis, not a verdict.

Fill power is cubic inches of loft that one ounce of down produces in a standard test cylinder. It measures efficiency, not warmth. Fill weight is how many ounces of down are actually in the item. Multiply them and you get the volume of loft the insulation can produce, which is the physical thing warmth comes from.

loft budget (thousands of in³) = fill weight in oz × fill power ÷ 1000

The practical band to shop in is 800 to 900 fill. Below about 650 the down is bulky enough that a synthetic bag becomes competitive on packed size without any of down’s moisture problems; above 900 the returns are an ounce or so for a price step that is often three figures. And because high fill-power clusters are larger and more fragile, a 950-fill bag punishes bad storage faster than a 650-fill one does.

Down wins on warmth per ounce and packed size by a wide margin and holds loft for decades if it is looked after. Synthetic keeps insulating when it is soaked, costs a third to half as much, needs no special storage, and loses loft steadily with compression cycles rather than suddenly with neglect.

The decision is really about how likely your insulation is to get wet and stay wet. Hydrophobic down treatments — a durable water repellent applied to the clusters — slow wetting out and speed drying, but they do not make down waterproof, and the marketing around them consistently oversells that. Of the down items here, the ones without any treatment are mostly the premium American brands, who consider the treatment a durability compromise.

RDS on a spec sheet means the down’s chain of custody was audited under the Responsible Down Standard against live-plucking and force-feeding. It is now close to universal on anything sold at these prices, which makes its absence the notable thing rather than its presence.

A quilt deletes the hood and the insulation under your back on the correct theory that both are doing very little: compressed down traps no air, and the hood is the part you can replace with a hat. What it hands you in exchange is the job of sealing the edges, and that is where quilts are won and lost.

The attachment systems divide into three kinds. A cord-and-clip arrangement lets the quilt float on the straps and move with you while the edges stay down — this works best. A strap that pulls the quilt taut against the pad seals the edge by crushing the loft you are lying against, which is self-defeating. And a sheet-based system clips the quilt to a fitted sheet already on the pad, which seals completely at the cost of buying into one maker’s whole system.

Two numbers predict a warm night in a quilt better than its temperature rating does: the quilt’s wrap width, and the pad’s width. A pad narrower than your shoulders lets the edges fall to the ground every time you turn over. Of the 12 quilts here, almost none publish a lab rating — so the physical numbers are all there is to compare.

R-value is thermal resistance — how well the pad stops heat moving from you into the ground. ASTM F3340, adopted across the industry from 2020, put a pad between a heated plate and a cold plate and measures it the same way for everyone. Before that standard existed, brands used their own methods and the numbers genuinely were not comparable, which is why an R-value on an older pad, or one whose maker does not name the standard, is worth discounting.

R per ounce = R-value ÷ weight in ounces

The other thing worth knowing is that stacked R-values add. Thermal resistances in series sum, so an R-2 foam pad under an R-4.5 air pad gives roughly R 6.5 — which makes the cheapest item in this catalogue also the cheapest route to a winter-capable system.

Air pads give the most loft per ounce and pack smallest. They insulate with layers of aluminised film, synthetic batting, or occasionally down, welded inside the chambers. Film is lightest and is also why most ultralight pads crinkle: the film flexes against the face fabric every time you move. Makers have solved it two ways — suspending the film inside the baffles rather than bonding it, or skipping film for batting — and both cost a little weight or money.

Self-inflating pads have an open-cell foam core that expands when the valve opens. They are heavier and thinner for the same R-value, and much more forgiving: a puncture degrades the night rather than ending it, because the foam still holds you up. Closed-cell foam cannot fail at all, gives about R 2 for fourteen ounces, and doubles as the bottom layer of a winter stack. Camping mattresses are self-inflating pads with the weight limit removed.

Thickness runs from 0.75 inches to 4.6 here. Three inches is the practical floor for a side sleeper; two will leave a hip on the ground. Width matters as much and gets mentioned less — twenty inches is standard and narrow, twenty-two to twenty-five is what stops knees and elbows hanging onto cold ground.

Shoulder girth is the inside circumference at the widest point, and it predicts whether you will sleep in the bag or wrestle it. Under about 60 inches most people rotate the whole bag when they turn over, which drags the hood off their face and the zip under their hip. Over about 64 inches there is room to wear an insulated jacket inside without crushing its loft, which is what makes a roomy bag more versatile than a warm one.

A mummy is the most thermally efficient shape and the least comfortable; semi-rectangular trades some warmth for room; a rectangular bag is a duvet with a zip and belongs in a car. Some makers cut extra width at the elbows and knees and take it back at the shins, which gives a side sleeper room where it is needed without the penalty of a bigger bag overall.

Girth figures here run from 49 to 64 inches — a five-inch spread that matters more than most temperature differences. Several makers publish no girth figure at all, which is noted on each of those pages.

The single most useful table on this site. Your body compresses the insulation underneath it to nothing, so the pad carries the entire underside of the system — and ground conducts heat away far faster than the air around your shoulders does. A bag is only as warm as the pad it is on.

These are floors from accumulated practice, not measurements — there is no standard that maps R-value to bag rating, and there cannot be one, because it depends on you. Two adjustments are worth making. If you sleep cold, add about ten degrees to the bag’s comfort figure before reading the table, which usually moves you a full R-value step. And if the ground will be frozen rather than merely cold, move up a row regardless of air temperature: frozen ground is a much better heat sink than 30 °F air.

Denier is the mass in grams of 9,000 metres of the yarn, so it is a rough proxy for how much fabric is there. A 7-denier shell is a windbreaker you can see through; 20-denier is the normal backpacking compromise; 50 to 75 denier is campground fabric that shrugs off grit and dog claws. The number does not capture weave, coating or the tear strength of the specific yarn, so treat it as a scale rather than a spec.

On pads the top and bottom deniers often differ, and the bottom is the one that matters — a 30-denier top over a 70-denier bottom is a pad designed to live on rough ground. On bags, an ultralight shell is the single largest durability compromise in the product, and it is why the lightest bags here are also the ones that most reward a groundsheet and a careful owner.

Published two ways and rarely comparably: as a cylinder in inches, or as compressed volume in litres. Litres are more useful because a pack is measured in litres, and because a stuff sack’s stated dimensions assume a compression nobody achieves in a hurry. Where a maker publishes litres this site uses them.

Fill power is what drives packed size on a down item, which is the real argument for paying for it: between a 650-fill and a 900-fill bag of the same warmth, the difference in the pack is several litres, not several ounces. For pads, batting and down insulation both pack larger than film at the same R-value.

Every price on this site is an approximate figure from the maker’s own store at the last catalogue update, shown with a ~$ prefix. It is not a live price, this site sells nothing, and nothing here links to a retailer. Where a product has a size run, the headline price is the cheapest size and the full run is on its page.

Prices here run from $50 (Klymit Static V Base) to $795 (Western Mountaineering VersaLite 10). The thing worth knowing about that spread is how weakly it correlates with the published numbers: several of the most expensive bags here publish no lab rating at all, and the best warmth-per-dollar entries are mid-market.

Compression damage is cumulative with time, not instant. A week in a stuff sack on a trip is irrelevant; an off-season in one measurably flattens the down, and several seasons of that is what turns a warm bag into a cold one. Store down loose in the large breathable sack that came with it.

Body oils migrate through the lining and coat the down, and oiled down does not loft — that is the real reason a five-year-old bag feels thin. Wash it with a down-specific soap in a front-loader or a bathtub, never a top-loader with an agitator, and dry it on low for a long time with something in the drum to break the wet clumps apart.

To tell whether a bag is finished, measure its loft with a ruler against the figure its maker published — which is one more reason a loft spec is worth more than a temperature number. A bag that should loft five inches and now lofts three has lost around forty per cent of its insulating thickness. Wash it first; several makers here will re-loft and repair a bag decades old for a fraction of a new one.