Down Versus Synthetic Insulation: What Fill Power Measures and What It Doesn't

Fill power is the number printed on every down jacket, and it is the number most shoppers read as a temperature rating. It is neither.

Fill power measures volume, not warmth

Fill power is cubic inches per ounce: one ounce of down is placed in a graduated cylinder under a weighted piston, and the volume it occupies is the rating. That is the IDFB method used by labs such as IDFL. It is why 800-fill and 650-fill describe quality per unit of mass, nothing more.

Warmth is fill power multiplied by fill weight. A jacket with 800-fill down and a thin layer of it will be colder than a 650-fill jacket stuffed full. The reason to pay for higher fill power is that the same warmth arrives in a lighter, more compressible package — the Uniqlo Ultra Light Down Jacket, which Uniqlo rates at 750+ fill power by the IDFB method, is warm for its weight rather than warm in absolute terms. The Patagonia Down Sweater uses 800-fill-power down at a claimed 369 g for the whole jacket.

What changes when either fill gets wet

This is the real dividing line, and it is not close.

Down insulates by lofting into a structure that traps still air. Wet clusters collapse and stick together, the trapped air disappears, and the jacket stops insulating until dry again — slow work with a garment that holds water. Hydrophobic treatments on down slow the wetting, they do not prevent it, and they wash out over time.

Synthetic insulation is continuous polyester filament. It absorbs far less water, keeps most of its structure when damp, and dries quickly. Patagonia states that the 60-g PrimaLoft Gold Insulation Eco in the Nano Puff maintains 98% of warmth when wet. The same jacket claims 337 g total, so that advantage costs about the same packed weight as a light down piece.

The practical rule is climate rather than temperature: consistently damp cold favours synthetic, dry cold favours down.

Compression, rebound, and how long each lasts

Down compresses smaller and rebounds better than any synthetic fill, and with reasonable care it holds that loft for many years. Synthetic fills pack nearly as small in the shop and then slowly lose rebound — repeated compression and washing flatten the filaments, so a synthetic jacket that was equivalent to a down one when new is measurably less warm after several seasons.

Care follows from that. Down wants a down-specific wash, a long low tumble dry to break up clumps, and loose dry storage; synthetic tolerates a normal machine wash and dries faster. Both fail if put away damp.

Where the money goes

Down costs more per jacket and buys warmth-to-weight and compressibility. Synthetic buys wet-weather reliability, easier care and a lower price, and pays for it in bulk and in a shorter loft life. For a layer that lives in a bag for emergencies, synthetic is the lower-maintenance choice. For a layer worn every day in dry cold, down is the one that stays warm for its weight.

The fill is one part of a system — what goes over it and under it decides how much insulation you keep. How to layer under a rain shell covers the shell side, merino versus cotton the layer against skin, and a three-jacket system for 40–70°F the temperature bands.

The one check before buying

Look for fill weight alongside fill power. Brands publish the first number everywhere and the second one rarely, and a jacket that states only fill power is withholding half of what determines how warm it is.