What Is Fill Power?

Fill power is the number that tells you how good the down inside your comforter or pillow actually is. It measures loft — the fluffiness of the down — by capturing how many cubic inches a single ounce of it expands to fill when it is allowed to puff up completely. Let one ounce loft in a measuring cylinder, read off the volume, and that figure is the fill power. A higher number means bigger, better-formed clusters that hold more air, which is why fill power is treated as the headline quality grade for down. And it is the number most people quietly misread — as a warmth dial it is not.

The short version: fill power measures the quality and loft of the down, not how warm the finished piece is. A higher fill power gives you more warmth from less weight — a lighter, fluffier, longer-lasting comforter — but the total warmth still depends on how much down is inside. Everything below is why.

What fill power actually measures

Down is the soft, three-dimensional undercoat plumage of a duck or goose — clusters of fine filaments that splay out in every direction and trap pockets of still air. That trapped air is what keeps you warm, so the more air a given amount of down can hold, the better it insulates. Fill power is simply a way of putting a number on that ability: it is the volume, in cubic inches, that one ounce of down occupies once it has fully lofted. An ounce of 700-fill-power down puffs up to fill 700 cubic inches; an ounce of 550 fills 550. Bigger, more mature clusters loft higher, so a higher fill power is a direct sign of higher-quality down.

Because it is a quality measure rather than a warmth measure, fill power tells you how efficient the down is, not how much heat the product traps. High-fill-power down does more with less — it reaches a given loft, and a given warmth, using fewer ounces. That efficiency is why enthusiasts chase the number: it is what lets a premium comforter feel like almost nothing on the bed while still sleeping warm.

Fill power versus fill weight — the distinction everyone confuses

This is the single most important thing to understand about down, and it trips up almost everyone. Fill power is the quality and loft of the down — how fluffy each ounce is. Fill weight is a completely different number: it is simply how many ounces of down are stuffed inside the comforter or pillow. Fill power describes the ingredient; fill weight describes how much of it is used. Warmth comes from the two of them together, never from either one alone.

Here is why that matters in practice. A comforter with a high fill power can be both warm and remarkably light, because each lofty ounce pulls more than its weight in insulation. A low-fill-power comforter needs more ounces — more fill weight — to reach the same warmth, so it ends up heavier and denser to sleep under. Same warmth, very different feel. And crucially, warmth itself is set by fill weight for a given quality of down: a summer-weight and a deep-winter comforter can use the exact same 700-fill-power down, and the winter one is warmer purely because it holds more ounces of it. If you read fill power as "warmth," you will assume the highest number is the toastiest comforter — and you will be wrong. It is the most efficient, not the warmest.

The practical takeaway: read both numbers. Fill power tells you how good and how light the down is; fill weight tells you how warm the finished piece will sleep. Our guide to choosing a comforter walks through matching fill weight to your season and bedroom, which is the other half of this decision.

What the fill-power bands mean

There is no single official grade, but the market clusters into commonly-published bands, and they are a useful shorthand. Down in the 400 to 500 range is modest quality — perfectly usable, but you will need more of it by weight to stay warm, and it tends to flatten sooner. The 600 to 700 band is the sweet spot for most buyers: good, honest down that lofts well, sleeps light enough and lasts. Once you climb into 750 to 900 and beyond, you are into premium territory — the lightest, loftiest, most durable down, the kind that gives you a cloud-like comforter weighing next to nothing. Treat these as typical ranges rather than a rulebook: testing methods differ between regions, so a European fill-power figure and a US one are not always on the same scale, and two brands quoting "650" may not match. Use the band as a guide, then read the fill weight too.

Higher fill power costs more — and lasts longer

High fill power is not free. The largest, maturest down clusters are rarer and costlier to source, so fill power tracks closely with price. What you are buying with that premium is real, though: besides the lower weight, higher-fill-power down is more durable. Those big, resilient clusters spring back into loft after being compressed, night after night, for years, whereas lower-fill-power down packs down and stays flat sooner. A premium down comforter that is fluffed regularly can genuinely last a decade or more, which spreads that higher upfront cost across a very long life. To see how these quality tiers show up in finished products, our down comforter guide covers the range.

Fill power only applies to down

One honest limit worth stating plainly: fill power is a measurement of natural down clusters, so it only means anything on down and down-blend products. Down alternative — engineered polyester built to imitate down — does not loft the way real clusters do, so it is not rated by fill power at all. Makers describe it with fill weight, fiber denier or GSM instead. If you ever see a fill-power number on something labelled down alternative, be sceptical, because the measurement does not properly apply. If you are weighing natural down against a synthetic fill, our down vs down alternative guide explains how each one traps warmth and where fill power fits in.

Fill power in down pillows

Fill power is not just a comforter number — it matters just as much in a down pillow, at a smaller scale. A higher fill power means the pillow's down lofts better and springs back into shape more readily, so it holds its support longer between fluffs and resists going flat over time. That resilience is what makes a good down pillow worth its price for people who want lasting, mouldable neck support. As with comforters, the total feel still depends on how much down is packed in — the fill weight — which sets how tall and firm the pillow sits. Our best down pillows guide looks at how loft and fill combine for different sleep positions.

How to actually use the number

• Read fill power as quality, not warmth. A higher number means fluffier, lighter, longer-lasting down — not a hotter comforter.
• Always read fill weight too. Fill power sets how light and efficient the down is; fill weight sets how warm the piece sleeps. You need both.
• Use the bands as a guide: roughly 400-500 is modest, 600-700 is good everyday down, 750-900+ is premium — but expect brand-to-brand variation.
• Pay up for fill power if you value light weight and longevity. Higher fill power costs more but lasts longer, so it can be the better long-term buy.
• Ignore fill power on synthetics. It only applies to real down; down alternative uses different measures entirely.

Read that way, fill power is a genuinely useful number — as long as you ask it the one question it can answer. It tells you how good and how lofty the down is, and leaves how warm to the fill weight. To translate all of this into a warmth level for your season and bedroom, our TOG rating guide covers the warmth side, so between the two numbers you can pick a comforter that is both the right quality and the right warmth for the way you sleep.