Down vs synthetic insulation, and what fill power really means

Published September 15, 2026

Fill power, fill weight, treated down, the main synthetic fills and how to compare two marked-down jackets that are not the same jacket.

Insulated jackets are the most-discounted category we browse, and the one where a listing's numbers are easiest to get wrong. "800 fill" on a jacket that weighs nine ounces and "650 fill" on one that weighs twenty are not comparable on the fill number alone, and a synthetic jacket does not carry a fill number at all. Here is what the numbers mean and how we compare jackets across a markdown page.

Down: fill power is quality, not quantity

Fill power measures loft: how many cubic inches one ounce of down occupies in a standard test. 600 fill is decent, 700 good, 800 excellent, 850 to 900 the top of the range. Higher fill power means more warmth per ounce and a jacket that packs smaller.

What it does not tell you is how much down is in the jacket. That is fill weight, quoted in grams or ounces, and most brands print it in the specifications if you look. A 650-fill jacket with 150 g of down is much warmer than an 800-fill jacket with 60 g. The 800-fill jacket is lighter and packs smaller for the warmth it does provide; that is the trade.

The way to compare two down jackets is to multiply: fill power × fill weight is a rough index of total loft, and total loft is warmth. If the listing does not give fill weight, use the jacket's total weight and the fill power together: within a brand's line, a heavier jacket at the same fill power is a warmer jacket.

Two more down terms:

Goose vs duck. Goose down reaches higher fill powers; at 650 to 750 the two are functionally the same, and duck is cheaper.
Treated / hydrophobic down (DownTek, Nikwax Hydrophobic Down and brand names like Patagonia's and Mountain Hardwear's) has a water-repellent coating that slows the collapse of down when it gets damp. It helps; it does not make down a wet-weather insulation.
RDS (Responsible Down Standard) certifies the sourcing. Most major brands now carry it.

Synthetic: warm when wet, heavier when dry

Synthetic insulation is polyester fibre engineered to trap air the way down does. Its advantages are that it keeps most of its warmth when wet, dries fast, costs less and needs no special care. Its disadvantages are weight and packed size for the same warmth, and a shorter life: synthetic fills flatten with repeated compression, and a jacket that has been stuffed into a pack for three seasons is noticeably thinner.

Synthetic jackets are rated by grams per square metre (gsm) of insulation, usually 40 to 133 g. A 60 g jacket is a light mid-layer; 100 g is a proper insulated jacket; 133 g and up is winter wear. Some brands quote the fill by name instead:

PrimaLoft Gold is the reference short-staple synthetic, with Silver and Black as cheaper grades. Patagonia's Nano Puff uses PrimaLoft Gold Eco.
PlumaFill (Patagonia Micro Puff), ThermoBall (The North Face) and similar fills are designed to mimic down clusters. They pack smaller than sheet synthetics and are closer to down in feel.
Coreloft (Arc'teryx), Thermal.Q and Stretchdown (Mountain Hardwear, the latter actually a down construction), Omni-Heat (Columbia's reflective lining, not an insulation) are the other names you will see. When in doubt, look for the gsm figure.
Active insulation (Polartec Alpha, Patagonia Nano-Air) is a breathable synthetic for high-output use. It is deliberately less warm at rest.

When to choose which

Cold and dry, or as a stationary layer: down. Nothing else matches the warmth-to-weight.
Wet climates, or a jacket you will sweat in: synthetic. A damp down jacket is a bag of clumps.
A jacket for around town and travel: either, on price. Down is lighter to carry; synthetic is more forgiving of a washing machine.
Kids and abuse: synthetic. It survives being sat on.
Sleeping bags: the same logic, with a stronger tilt toward down because the weight difference is larger at bag sizes.

Comparing two marked-down jackets

Here is the sequence we use when two different jackets land on the same page at similar prices:

Same fill type? Compare down to down and synthetic to synthetic. A down jacket and a synthetic at the same price are different purchases, not competitors.
Down: fill power and fill weight. Find both. If the brand hides fill weight, a jacket's total weight is the proxy.
Synthetic: gsm. A 60 g jacket is not a winter jacket at any discount.
Shell fabric. A 10-denier shell is featherweight and tears on a branch; 20 to 30 denier is the sensible middle; a heavier ripstop or a softshell face is what makes a jacket last. Brands' lightest jackets close out cheapest because they are the most fragile.
Hood, or no hood. The same jacket usually exists in both, with the hooded version around $20 to $30 more at list. On closeout the difference often collapses, and the hood is worth having.
Fit and length. Alpine cuts (Mountain Hardwear, Patagonia's lighter jackets) are short and trim. Casual cuts (Columbia, The North Face's lifestyle line) are longer and roomier. The size run tells you which is left.

The models we see most often

Patagonia Down Sweater (800 fill, recycled down, a 20-denier shell) and Nano Puff (60 g PrimaLoft Gold Eco). The archetypes. They rarely go deep but they do close out when colours change.
Mountain Hardwear Ghost Whisperer (800 fill, very light shell) and Stretchdown. Ghost Whisperer closeouts are frequent and worth acting on quickly.
The North Face ThermoBall and the Aconcagua (down). The lifestyle cuts. Deep closeouts every spring.
Columbia insulated jackets at every price. Columbia's fill and shell specs are often unremarkable; the price is the point, and the closeouts are the deepest of any brand here.

Care, briefly

Down can be washed at home with a down-specific soap and dried on low with dryer balls until every clump is gone; do not store it compressed. Synthetic goes through a normal wash and dries in an hour on a hanger. A closeout down jacket that has been on a shelf for a year is fine; loft recovers after a tumble.

The number on the listing is the start of the comparison, not the end. Two jackets, two fill weights, two shells: do the arithmetic and the better deal is usually obvious.