Buying Guides
Jacket Temperature Ratings Are (Mostly) Made Up — Here's How to Judge Warmth
Sooner or later every winter-jacket listing shows a number: rated to −20 °C, a "thermal index," a five-snowflake scale. Here's the uncomfortable truth: there is no industry standard behind any of it. Unlike sleeping bags — which have a real lab test (EN/ISO 23537) with defined comfort and limit temperatures — jacket temperature ratings are marketing artifacts each brand invents for itself.
Why a single number can't work
Whether you are warm at −20 °C depends on variables no jacket label can know:
- Activity. Walking briskly, you produce several hundred watts; standing at a bus stop, a fraction of that. The same parka is sweltering for one and inadequate for the other.
- Wind. −5 °C with a 40 km/h wind strips heat faster than −15 °C in still air.
- What's underneath. A parka over a t-shirt and the same parka over a merino base plus fleece are two different systems.
- You. Metabolism, body size, and cold tolerance vary enormously between people.
A brand printing "−20 °C" on a hang tag is guessing at all four on your behalf. Some brands' guesses are honest; the point is you can't tell from the number.
The specs that actually predict warmth
Ignore the temperature claim, read these instead:
| Spec | What it tells you | What to look for |
|---|---|---|
| Fill weight (grams of down) | The single best warmth predictor for down | Everyday winter jacket: 100–200 g. Serious parka: 200–300 g+. If it's not listed, ask — or assume it's low |
| Fill power (550–900) | Quality/efficiency of that down | Warmth ≈ fill power × fill weight. Both numbers or no conclusion |
| Synthetic insulation weight (g/m²) | Warmth of synthetic layers | 60 g/m² = cool-weather layer; 100–133 = winter; 200+ = deep cold, low activity |
| Length | Coverage matters as much as loft | A knee-length parka at 150 g of down outperforms a waist-length jacket at 200 g for standing around |
| Hood, storm cuffs, hem drawcord | Air-sealing features | Warm air escaping at the openings is the biggest real-world loss |
| Shell fabric | Wind and wet protection | Windproof shell can add the equivalent of a whole midlayer in exposed conditions |
The baffle check for down jackets
Look at the construction between the puffy chambers:
- Sewn-through baffles (stitching goes straight through both fabric layers) create cold lines at every seam. Fine for town, cheaper to make.
- Box-wall baffles (a fabric wall separates inner and outer, keeping full loft over the seam) are what real expedition parkas use — warmer at the same fill weight, and a reliable tell that a brand is building for warmth rather than looks.
How to use brand ratings anyway
Within one brand, ratings rank the range usefully: the brand's "−30 °C" parka is warmer than its "−10 °C" jacket. Across brands, the numbers are incomparable — one brand's −20 assumes a wool base layer and walking pace; another assumes standing still in a t-shirt (almost none publish their assumptions). Use ratings to sort a single brand's catalogue, then verify the shortlist with fill weight, construction and length.
A realistic buying checklist
For a "stand around in real winter" parka, the listing should show:
- Fill power and fill weight (or g/m² for synthetic) — a brand hiding one of them has a reason
- Box-wall or hybrid construction if you're paying real money
- Hip coverage or longer, insulated hood, cinchable hem and cuffs
- Windproof (ideally water-resistant) shell — see our down vs. synthetic guide for the wet-climate caveats
- Room inside for a midlayer without compressing the fill
A jacket that ticks those five will handle deep cold regardless of what temperature is printed on the tag. A jacket that ticks none of them isn't rescued by the boldest claim.
The one-line answer
Temperature ratings on jackets are unregulated marketing. Judge warmth by fill weight × fill power (or g/m² synthetic), construction, length and seals — and by whether you'll be moving or standing still while wearing it.