Goose down has never been hotter. This natural insulation material is popping up inside a growing number of jackets and parkas these days, even in pants and bed comforters. It’s the material of choice for most high-end sleeping bags. If you’ve ever shopped for these items, you’ve likely come across the term “down fill power,” but what does it mean? Don’t fret. Here’s what you need to know.

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What Is Fill Power?

Fill power is a way to measure the loft, aka fluffiness, of down. What is down? It’s a layer of fluff underneath duck and goose feathers that keeps the birds (and you) warm.

Most companies use goose down, but the jury’s still out on whether it’s warmer than duck down. We’ve heard the argument that since goose down clusters are typically larger—and therefore loftier—they should be warmer, given the same quantity and fill power. But there’s no study that conclusively proves it. Since goose down has historically been the down of choice and duck down has only recently become more prominent, due to cost, this issue isn’t settled yet.

Obtaining a fill-power measurement involves filling a receptacle with one ounce of down insulation and then placing a weight on top. The volume of down under weight is measured, and that’s the fill power. A measurement of 500 cubic inches means the fill power of that down is 500. In simpler terms, the higher the down fill-power number, the warmer it is.

Five hundred is fairly low on the scale. If you don’t mind a bulkier jacket or sleeping bag, or if you’re sleeping in cool-but-not-cold conditions, 500 might be all you need (you’ll save some money). Most entry-level garments and sleeping bags will have at least 600 or 650 fill power. Eight hundred to 900 fill power is considered the top end. These days, 1,000 fill power is about the most you’ll see. These garments and sleeping bags are for extreme cold. Most people won’t ever be in conditions to use them unless they’re pursuing high-altitude mountaineering or Arctic expeditions.

Down Fill Power Isn’t Enough

But down fill power is only one variable. If you were comparing an identical quantity or weight of the insulation alone, the higher down fill power would be warmer. But compare two jackets—say, the 650-fill REI Co-op 650 Down Jacket and the 800-fill Montbell Superior Down Round Neck Jacket—the latter isn’t necessarily warmer. You should also look at fill weight. This is how many ounces of insulation are in the jacket, and the REI’s 4.2 ounces of 650-fill make it warmer overall than the Montbell’s 1.3 ounces of 800-fill.

The construction method also affects how much warmth a garment can retain. If something says it’s sewn through, it’s exactly what that sounds like. There are two layers of fabric sandwiched around the insulation, and it’s stitched into a square grid or elongated column pattern. In particularly cold weather, the parts of the material where the stitching runs can develop into cold spots.

Baffle box construction looks the same from the outside at first glance, but it has vertical walls between each box of insulation that run perpendicular to the face fabric. Because it’s warmer, baffled construction is typically reserved for more expensive jackets.

If all else is the same, an ounce of 800-fill down will keep you warmer than an ounce of 650-fill down, and it’ll compress to a smaller size, meaning you’ll have an easier time stowing it in a pocket or bag. Almost universally, higher-fill down will cost more, and once you get up to 800- to 1,000-fill power, you’ll be paying several hundred dollars for whatever you’re buying, whether it’s a technical outdoor parka, winter sleeping bag, or cozy bed comforter.

What About Synthetic Insulation?

Synthetic down substitutes, such as PrimaLoft, don’t use fill power as a measurement. That makes it harder to compare warmth between two jackets.

One approach would be to find out the weight of insulation in the garment, though manufacturers don’t always disclose this information. Even if you reach out to customer service to find the fill weight, the comparison is thrown off if the type of synthetic insulation differs from one garment to the next. PrimaLoft Gold is different from PrimaLoft Silver, which differs even more from 3M Featherless. There just isn’t an easy metric for comparing synthetic insulation, the way you can use down fill power for goose down.

Synthetic insulation doesn’t compress down as small as goose down, and it doesn’t yet match its warmth. To make a synthetic jacket as warm as a down jacket, the manufacturer has to stuff more insulation into it, making it bulkier and puffier. I buy goose down exclusively when working in temperatures below 30 degrees Fahrenheit.