Fill Power Explained: 600 vs 700 vs 800 vs 850

The fill power test was developed to give buyers a consistent, comparable quality metric. It works well for what it measures - the lofting efficiency of the down at the point of manufacture. What it doesn't capture is how that down performs over time, how it responds to moisture, or how it's been distributed within the jacket. Those variables matter too. Fill power is the starting point, not the complete picture.

The fill power scale in practice, from budget to expedition:

A few things worth noting about this scale. Most high street brands operate in the 550–700 range and very few reach 800+. The jump from 700 to 800 is significant: roughly 14% more volume per ounce, which translates directly to warmth-to-weight performance. The jump from 800 to 850 is meaningful but smaller; at this level, other variables such as fill weight, construction and baffle design begin to matter more than the fill power number itself. Beyond 900 FP, diminishing returns set in rapidly. The clusters are very large, require more careful handling during manufacture, and are significantly more expensive to source. Most expedition jackets treat 850–900 FP as the practical ceiling.

The scale is useful as a starting framework, but fill power should never be read in isolation. A well-constructed jacket at 750 FP with the right fill weight will perform better in the mountains than a poorly specified jacket at 850 FP. The number tells you the quality of the raw material. Everything else - construction, fill weight, baffle design, shell fabric, synthetic insulation additions - determines what the jacket actually does with it.

750 fill power is excellent quality and the standard for serious outdoor use. A well-constructed 750 FP jacket with adequate fill weight will keep you warm in genuinely cold conditions. The Jöttnar Thorne uses 750 fill power; a deliberate choice for a jacket designed for sustained outdoor and daily mountain use, where durability and fill weight matter as much as fill efficiency.

800 fill power is premium quality. It’s noticeably lighter and more packable than 750 FP at equivalent warmth. The step up to 800+ is most valuable when pack weight is critical: alpine approaches, multi-day routes, travel. The difference is real and measurable, but won't be obvious to most people in everyday use.

850 fill power is the top of the practical range, used in jackets where warmth-to-weight is the dominant consideration, such as expedition use, altitude, or other weight-critical applications. One ounce of 850 FP down occupies 42% more volume than 600 FP down. This is the fill power used in the Jöttnar Fenrir and Asta; chosen specifically for mountain use where every gram of weight saved has a compound effect over a full day.

If you're choosing between a 750 and 850 fill power jacket from different brands, the fill power number is only one variable. A 750 FP jacket with significantly more fill weight may be warmer than an 850 FP jacket with less. And a well-constructed 750 FP jacket will outlast a poorly made 850 FP jacket by years. Fill power is a useful quality signal but it's not a complete specification.

Fill power measures quality - how efficiently the down insulates per ounce. Fill weight measures quantity - how many grams of down are actually in the jacket. Warmth is a function of both. Neither number alone tells you how warm a jacket will be.

A practical example: two jackets, both claiming 800 fill power. Jacket A has 800 FP and 120g fill weight - lightweight, packable, warm. Jacket B has 800 FP and 200g fill weight - heavier, but warmer in serious cold. Jacket B will be significantly warmer despite identical fill power.

Why don't companies (including us) publish fill weight prominently? Partly because fill power makes for a cleaner headline. But there's also a legitimate reason: fill weight comparisons across brands are less useful than they appear, because jackets are constructed differently - different baffle configurations, hybrid fills, different volumes in hoods and collars. A jacket must be tuned to achieve its intended performance, and loading up on fill weight alone doesn't guarantee the outcome. We're happy to discuss fill weight in context for any Jöttnar jacket - please just get in touch.

Fill power and down-to-feather ratio are separate but related quality indicators. The down-to-feather ratio, for example 90/10 or 93/7, describes what percentage of the fill is down clusters versus feathers. Higher down content means better insulation, lighter weight, and less chance of feather poke-through. Higher quality down at higher fill power tends to come from older, larger birds that also produce a cleaner yield, so premium fill power and high down ratios often go together. Jöttnar uses 850 fill power in a 93/7 ratio in the Fenrir and Asta and the two numbers together describe down at the top end of what is practically achievable.

Fill power tells you about warmth in dry conditions. Hydrophobic treatment tells you about performance in the conditions you'll often actually encounter – wet, damp, raining. An 850 FP jacket with DownTek™treatment maintains most of its loft and insulating value when wet; an 850 FP jacket without it can lose up to 90% of its insulating value in sustained moisture. For anyone spending time in UK or North Atlantic mountain environments, DownTek™ treatment is not optional - it's as important as the fill power rating itself.

The two numbers that matter when buying a down jacket: fill power - the quality of the down, ranging from 600 to 900+; and down-to-feather ratio - the proportion of down to feather, with 93/7 being among the highest achievable. You won't find fill weight declared on Jöttnar product pages, and as we explain in this guide, cross-brand fill weight comparisons are less useful than they appear. What you will find is the fill power and down-to-feather ratio for every jacket - the numbers that tell you
what the down itself is, before construction variables come into play. If you want to go deeper on specification for a specific jacket, get in touch.