How Warm Is That Jacket, Really? The Truth About Fill Power and Ratings

You're standing in a store looking at jackets with numbers like "800 fill power" or "rated to -10°F" printed everywhere.

Maybe you've seen a north face jacket with confusing specs and wondered what they actually mean.

Here's the truth: those temperature ratings aren't as straightforward as you might think.

What Temperature Ratings Tell You?

Temperature ratings serve as guidelines, but many factors affect whether a garment will be too warm or not warm enough for you. The biggest factor is your activity level.

Here's a reality check: if you're in a 0°F environment you may need something rated to -20°F if you're not actively moving around.

That 20-degree difference matters when you're sitting still versus hiking uphill.

Your physical condition also plays a huge role. Age, metabolism, weight, and circulation issues all change how cold you feel.

Some people run hot, others run cold. A jacket's rating assumes an "average" person, but you might not be average.

CLO Values: The Science Behind Warmth

CLO measures clothing insulation levels. A CLO value of 1 corresponds to the insulating value of clothing needed to maintain a person in comfort sitting at rest in a room at 21°C (70°F).

Think of CLO as your warmth currency. If someone was to be resting in a temperature of around -30°C they would need to wear garments around 8 CLO. You can add up CLO values from different clothing pieces to reach your target warmth level.

Real-world CLO examples:

• Flannel shirt: 0.30 CLO

• Average down jacket: 0.55 CLO

• Heavy winter coat: 2-3 CLO

• Full winter outfit for extreme cold: 4+ CLO

The equation is simple: Insulation + Activity + Time = Optimal Comfort. The longer you're in the cold, your temperature rating is going to dip.

Down Fill Power vs. Synthetic Insulation

Fill power measures down quality, not warmth. Fill power is a number that indicates the relative quality of down.

The number comes from a lab test that measures how many cubic inches of loft one ounce of that down fill produces.

Higher fill power means:

• Lighter weight for same warmth

• Better compression

• Higher price

But here's what matters more: the amount of insulation. A 900-fill-power down jacket that has only 30g of down fill won't be warmer than a 450-fill-power jacket with 100g of down fill.

Fill Power Range	Quality Level	Best For
500-600	Budget-friendly	Casual use, minimal packing
650-750	Good performance	Most outdoor activities
800+	Premium	Weight-critical activities

Synthetic insulation doesn't use fill power. Synthetic insulation is only as insulating as 550-600 FP insulation irrespective of the garment or quilt it's put into. Instead, look for weight per square meter (like 60g/m²).

How Temperature Ratings Get Messy?

Jackets don't have standardized temperature ratings like sleeping bags do. 

Unlike sleeping bags, in which ISO ratings are established for providing apples-to-apples comparisons to warmth, down jackets do not have temperature ratings of their own due to the many dependent factors.

This means manufacturers basically guess at temperature ratings. One brand's "-10°F jacket" might feel completely different from another's. The numbers are educated estimates, not scientific measurements.

Activity Levels Change Everything

Your body generates different amounts of heat based on what you're doing. This is measured in MET (Metabolic Equivalent) levels:

• Light activity (under 3 METs): Sitting, driving, office work

• Moderate activity (3-6 METs): Walking, light hiking

• Vigorous activity (6+ METs): Running, heavy lifting, intense hiking

The moderate MET levels are measured between three and six METs. On the moderate side you would have your pickers and packers and maybe delivery folks. Each activity level needs different insulation amounts.

Environmental Factors That Matter

Wind and moisture change everything. The comfort rating of your insulated clothing may need to be warmer than just the air temperature when you factor in wind chill and precipitation.

Water is insulation's enemy. Water is a better conductor of heat than air, thus if clothes are damp — because of sweat, rain, or immersion — water replaces some or all of the air between the fibres of the clothing, causing thermal loss.

Layering: Your Temperature Rating Insurance

Don't rely on one piece of clothing. Thermal insulation is thus optimal with three layers of clothing: a layer near the body for hygiene, an outer close-knit or closely woven layer as a wind breaker, and between the two, a "thick" layer that traps the air.

The layering system gives you flexibility. Too warm? Remove the middle layer. Getting cold? Add a base layer underneath.

Making Smart Choices

When choosing insulation levels, consider these factors:

Your typical activity level during cold weather. Are you mostly stationary or constantly moving?

Duration in cold affects your needs. The duration you will be in the cold temperatures, how acclimated you are to working in that environment and what other products you are wearing all matter.

Your personal temperature tolerance. If you tend to stay colder than others in other situations, size up your insulation needs.

Environmental conditions beyond just air temperature. Factor in wind, humidity, and precipitation.

Temperature ratings give you a starting point, but your experience, activity level, and environment determine what actually keeps you comfortable. 

Don't trust the numbers blindly—trust your knowledge of how you handle cold and choose your insulation level accordingly.

Frequently Asked Questions

What does the temperature rating on a jacket actually mean?

It’s a general guideline, not a scientific standard. Jacket temperature ratings are manufacturer estimates based on an average person at moderate activity. Real warmth depends on your movement, layering, environment, and body type.

Why do I still feel cold even when wearing a jacket rated for the temperature?

Because activity level, wind, moisture, and personal factors (like age or circulation) affect comfort. If you're inactive, you'll need more insulation than the rating suggests.

What is CLO, and how does it relate to jacket warmth?

CLO is a scientific measure of insulation. A value of 1 CLO keeps an average person warm at 70°F while resting. Higher CLO = more insulation. Layering helps you build up CLO to stay warm in extreme cold.

What is fill power, and does higher fill power mean a warmer jacket?

Not always. Fill power measures quality of down, not quantity. A 900-fill jacket with little down can be colder than a 650-fill jacket with more fill. Warmth depends on both fill power and fill weight.

How does synthetic insulation compare to down?

Synthetic insulation is roughly equal to 550–600 fill power down in warmth. It retains heat better when wet and is more budget-friendly, but it's typically bulkier than down.

Can I trust temperature ratings across different jacket brands?

No. There’s no standard for temperature ratings in jackets (unlike sleeping bags). Brands guess based on testing, but one brand’s -10°F rating may not match another’s.

How do wind and moisture affect insulation?

Wind strips heat away, and moisture kills insulation—especially down. Wet garments lose their ability to trap warm air, making them much less effective.