Performance & Ratings -- Reading the Numbers
Jacket labels are full of numbers -- fill power, millimetres, grams, denier -- and brands rarely explain them. This guide decodes the ratings that actually matter: how warm a jacket is, how much weather it takes, and how much it weighs and packs down -- so you can compare two coats on paper and trust the result.
Warmth Ratings
Temperature Rating
A guide to the lowest comfortable temperature a jacket is built for -- useful as a rough band, not an exact promise.
How to read it
Some brands quote a comfort temperature range (for example 'comfort to -10C'). It is a starting point set in still, dry conditions for an average person -- wind, damp and your own metabolism shift it a lot.
Use it as a band
Treat it as light / mid / heavy guidance rather than a guarantee. Layering underneath changes the real-world figure more than the rating itself. See the insulation hub.
TOG Rating
A measure of thermal resistance -- how well a layer slows heat escaping. Higher TOG means warmer.
Where you see it
TOG is more common on bedding but appears on some insulated clothing. It rates the insulating power of the material, independent of fit or wind.
The limit
Like all warmth numbers, TOG measures the fabric, not the finished jacket -- gaps at the cuffs, hem and neck leak heat a TOG figure cannot capture.
CLO Value
A technical unit of insulation used in performance gear -- one CLO is roughly the warmth of a light business suit at rest.
The science figure
CLO is the lab unit behind warmth claims. Higher CLO means more insulation. It lets engineers compare fills precisely, though brands rarely print it on the label.
Why it matters to you
It underpins the warmth ratings you do see. The practical takeaway is the same: more loft and more fill weight raise the CLO. See thermal insulation.
Weather Ratings
Hydrostatic Head (Spec)
The headline waterproof number in millimetres -- how tall a column of water the fabric resists before it leaks.
Quick scale
1,500mm is the legal minimum for 'waterproof'; 10,000mm handles most British days; 20,000mm+ is full mountain spec. Pressure from straps and sitting eats into the margin.
Full detail
The complete explanation, with how pressure changes the real figure, lives in the waterproofing hub.
Breathability (MVTR / RET)
How fast a fabric lets sweat vapour escape -- quoted as MVTR (higher is better) or RET (lower is better).
Two scales, one idea
MVTR in g/m2/24h: 10,000-20,000 is good. RET resistance: under 6 is highly breathable, over 20 is poor. Both describe the same thing -- how clammy you get on the climb.
Read it with waterproofing
A high waterproof number with poor breathability leaves you wet from sweat. Always check the pair together -- see the waterproofing hub.
Wind Resistance
How well a jacket blocks moving air -- often the difference between feeling warm and feeling frozen.
Why it counts
Wind strips away the warm layer your body builds up, so a windproof shell can feel far warmer than its thickness suggests. Most waterproof membranes are windproof too.
How it is shown
Rarely a single number -- look for 'windproof' or a CFM air-permeability figure (lower CFM means less wind gets through). See the waterproofing hub.
Waterproof vs Water-Resistant
The single most misread distinction on a spec sheet -- one keeps rain out for hours, the other for minutes.
The honest gap
'Waterproof' means a membrane or coating plus taped seams holding water out under pressure. 'Water-resistant' sheds a passing shower then wets through. Softshells are resistant; hardshells are waterproof.
Don't be caught out
If the spec says water-resistant, do not expect all-day rain cover. See softshell vs hardshell.
Weight, Pack & Durability
Jacket Weight
The total grams of the garment -- the headline figure for travel, walking and packability.
Why grams matter
Every gram you carry uphill or pack for a trip counts. A lightweight shell might be 250g; a heavy insulated parka ten times that. Lighter usually means thinner fabric and less insulation.
The trade-off
Cutting weight costs warmth or durability. Match the weight to the job -- light for town and travel, heavier for deep winter. See the insulation hub on warmth-to-weight.
Packed Size
How small a jacket squashes down for carrying -- the practical partner to weight.
The convenience number
A jacket that folds into its own pocket or a small stuff sack lives in a bag until needed. Down compresses smallest; synthetic and waterproof shells less so.
What drives it
Thin face fabric plus compressible insulation. Heavy parkas do not pack; lightweight down and windbreakers do. See jacket types.
Denier (D)
A measure of how thick and tough the face fabric's threads are -- higher denier means a heavier, more durable jacket.
Reading the D
10-30D is ultralight and packable but delicate; 70D+ is rugged for hard use. It is a durability dial, not a warmth one -- a low-denier down jacket can still be very warm.
Match it to use
Light denier for travel and town, heavy for the hills and rough wear. Full detail in the construction hub.
Abrasion Resistance
How well the outer fabric survives rubbing, scuffs and pack straps -- the unglamorous spec that decides how long a jacket lasts.
Where wear happens
Shoulders under a rucksack, elbows, cuffs and hems take the punishment. Tougher face fabrics (higher denier, ripstop weaves) shrug it off; ultralight fabrics wear through faster.
The balance
More abrasion resistance usually means more weight. A ripstop weave is the clever compromise -- light yet tear-stopping. See the materials hub.
Frequently asked questions
What is a good waterproof rating for a jacket?
For everyday British weather, around 10,000mm hydrostatic head is comfortable. Casual urban waterproofs may sit at 5,000mm; serious hill shells run 15,000-28,000mm. Always read it alongside breathability.
What do the breathability numbers mean?
MVTR measures how much vapour escapes (higher is better, 10,000-20,000 is good). RET measures resistance to vapour (lower is better, under 6 is highly breathable). Both tell you how clammy a jacket feels under effort.
Does a higher temperature rating mean a warmer jacket?
Roughly, but treat it as a band not a promise. Ratings are set in still, dry lab conditions; wind, damp and your own layering shift the real figure significantly.
What does denier (D) tell me?
Denier measures thread thickness, so it tells you durability, not warmth. Low denier (10-30D) is light and packable but delicate; high denier (70D+) is tough for rough use.
Why does jacket weight matter?
Lighter jackets are easier to travel with, carry and pack, but cutting weight usually costs warmth or durability. Match the weight to the use -- light for town, heavier for deep winter or hard wear.