Down Fill Power Explained
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On this page
- How fill power is measured
- What fill power changes (and what it doesn't)
- Worked example: how many ounces do you save?
- Common fill power grades in quilts
- Fill power and baffle height
- Fill power in the real world: loft over a season
- Does treated (hydrophobic) down change fill power?
- How to choose a fill power
- Fill power red flags when shopping
- Bottom line
- FAQ
Quick answer: Down fill power is the volume, in cubic inches, that one ounce of down fills under a standard test. Higher fill power means more loft per ounce, so you need less down for the same warmth. For quilts, 800 FP is the value sweet spot, 850 FP is the most common premium choice, and 950 FP saves another ounce or two at a steep price.
Down fill power explained in one line: it's a measure of how fluffy the down is, not how much is in the quilt. A 20°F quilt filled with 950 FP down and one filled with 800 FP down can be equally warm; the 950 FP version simply needs fewer ounces to get there. Understanding that distinction is the key to comparing quilts on price and weight.
This page covers how fill power is measured, how it translates into ounces saved, how it interacts with fill weight and baffle height, and how to decide which level to buy. For a side-by-side of specific grades, also see our fill power comparison.
How fill power is measured
In the US method, a 1-ounce (30 g) sample of down is conditioned (steamed or tumbled and allowed to recover in controlled humidity), placed in a clear cylinder of standard diameter, and compressed under a light, standardized weight. The height it settles at is converted to cubic inches. If the sample occupies 850 cubic inches, it's 850 fill power.
Testing labs such as IDFL run these tests to the IDFB standard, and reputable quilt makers buy down with test certificates. A few points to know:
- It's a lab number. It doesn't change with the quilt design; it's a property of the down batch.
- There's tolerance. A lot rated 850 FP may test a bit above or below on a given day, and labeling rules allow some margin.
- Cluster-to-feather ratio matters. Most quality quilt down is 90/10 or better (90% clusters, 10% feathers and fiber by weight). Feathers add weight without much loft.
US vs European numbers
European brands often test with a different method (measuring in cm³ per 30 g, with different conditioning). The European method generally yields lower numbers for the same down, so a European "750" may be similar to a US figure in the low-to-mid 800s. The exact conversion varies by lab and sample, so treat it as a rough guide and compare like with like. Most US cottage quilt makers quote US figures.
What fill power changes (and what it doesn't)
| Property | Higher fill power effect |
|---|---|
| Weight for a given warmth | Lower (fewer ounces needed) |
| Packed size | Smaller, because there's less material |
| Warmth for a given weight of down | Higher |
| Price | Higher, sometimes much higher |
| Durability of loft | Roughly similar; good care matters more |
| Performance when wet | No inherent advantage; see hydrophobic down |
Fill power does not tell you how warm the quilt is. Warmth comes from total loft, which depends on how many ounces are in it and whether the baffles let that down expand. A 950 FP quilt that's underfilled can sleep colder than a generously filled 800 FP quilt.
Worked example: how many ounces do you save?
Let's say a maker targets a total lofting volume of about 10,200 cubic inches to hit a 20°F rating in a regular/standard quilt. Divide that volume by the fill power to get the ounces of down needed:
| Fill power | Down needed (oz) | Saved vs 800 FP |
|---|---|---|
| 650 FP | ~15.7 oz | -3.0 oz (heavier) |
| 700 FP | ~14.6 oz | -1.9 oz (heavier) |
| 800 FP | ~12.8 oz | baseline |
| 850 FP | ~12.0 oz | ~0.8 oz |
| 900 FP | ~11.3 oz | ~1.5 oz |
| 950 FP | ~10.7 oz | ~2.1 oz |
That's simple arithmetic (10,200 ÷ FP) and ignores shell fabric, baffles and overfill, which stay roughly the same. In real quilts, the gap between 850 and 950 FP is typically 1–2 oz on a 20°F quilt and 2–4 oz on a 0°F quilt, because colder quilts carry more down. For the whole-quilt picture, read our quilt weight guide.
Cost per ounce saved
Cottage makers commonly charge a meaningful upgrade fee for 950 FP over 850 FP, often enough to move a quilt up a full price tier. If the upgrade saves 1.5 oz, you're paying a premium of dozens of dollars per ounce. That's worth it for thru-hikers chasing a sub-10 lb base weight or anyone who needs the smallest possible packed size; it's poor value for weekend trips.
Common fill power grades in quilts
| Grade | Where you'll see it | Good for |
|---|---|---|
| 600–650 FP | Budget and mass-market quilts, camping blankets | Car camping, budget first quilt |
| 700–750 FP | Some budget backpacking quilts | Value-focused backpacking |
| 800 FP | Value lines from cottage makers (often duck down) | Most backpackers; the value sweet spot |
| 850 FP | The standard premium spec for many cottage quilts | Thru-hikers, frequent backpackers |
| 900–950 FP | Top-tier goose down upgrades | Ultralight, alpine, winter quilts |
| 1000 FP | Rare, very expensive specialty down | Niche: maximum weight savings |
Duck down typically tops out around 800–850 FP, and very high fill powers (900+) are almost always goose because mature geese grow larger clusters. That's the main practical difference covered in duck vs goose down. An 850 FP duck down and an 850 FP goose down quilt will loft similarly; the label number is what matters.
Fill power and baffle height
Higher fill power down needs room to expand. If a maker puts 950 FP down in baffles sized for 800 FP, the down can't fully loft and part of the premium is wasted. Good makers size baffles to the fill power and target loft. This is one reason specs like baffle height and overfill matter as much as the fill power headline. See quilt baffles explained for how chambers are sized.
Ultra-high-fill-power down is also more sensitive to compression and humidity in day-to-day loft. The clusters are large and delicate, so careful storage (a large cotton or mesh sack, never stuffed long term) matters. See how to store a quilt.
Fill power in the real world: loft over a season
The lab number describes clean, freshly conditioned down. In the field, several things reduce effective loft regardless of grade: body oils and sweat migrating into the fill, nightly condensation in cold weather, and long hours compressed in a pack. A quilt that measured 3 inches of loft new may sit at 2.5 inches after a month on trail until it's washed.
That's why many experienced hikers care as much about overfill and care habits as about the fill power grade. Airing the quilt out at lunch, sleeping in clean base layers, and washing it properly once or twice a year keep a mid-grade quilt performing like its spec sheet. See how to wash a down quilt.
Does treated (hydrophobic) down change fill power?
DWR-treated "hydrophobic" down is tested for fill power after treatment, and the treatment usually has little effect on the number. Some hikers report treated down lofting slightly faster after compression. It doesn't make down waterproof; it slows how fast it absorbs moisture. Our hydrophobic down guide covers the tradeoffs.
How to choose a fill power
- Set your temperature rating first. Use top quilt temperature ratings to choose the degree rating.
- Decide how much weight matters. If your base weight is already low and you hike long miles, 850–950 FP makes sense. If you're building your first kit, 800 FP is excellent.
- Weigh the cold-quilt multiplier. The colder the rating, the more ounces of down, and the bigger the absolute savings from high fill power. A 0°F quilt gains more from 950 FP than a 40°F quilt.
- Check the total fill weight. Compare quilts by fill power × ounces of fill, not by fill power alone.
- Consider synthetic for wet trips. If you'll face weeks of rain without drying chances, compare against synthetic quilts.
A practical rule: for a 30–40°F summer quilt, buy the cheapest quality down you trust (800 FP is plenty). For a 10°F or colder quilt, the ounces saved by 900+ FP start to add up and the upgrade becomes easier to justify.
Fill power red flags when shopping
- No fill power listed, or a vague "premium down", usually means lower-grade fill.
- Fill power without fill weight. You can't judge warmth without both.
- Unrealistic claims such as 1000 FP at a budget price.
- Unspecified test standard on imported gear; ask whether it's US or European.
- No traceability information; see our Responsible Down Standard guide.
If you want to verify a quilt's actual weight against the spec sheet, a simple kitchen-style digital scale is enough. Check price on Amazon.
Bottom line
Fill power is a quality measure: more cubic inches per ounce. It lets makers hit a rating with less down, which means a lighter, smaller quilt at a higher price. It's only half the warmth equation; the other half is fill weight. Pair the two and you can compare any down quilt honestly. Next, read fill weight and overfill to finish the picture, then browse the best down top quilts.
Frequently asked questions
Is higher fill power warmer?
Not by itself. Higher fill power means each ounce lofts more, so a quilt can reach the same loft with less down. Warmth depends on total loft, which is fill power multiplied by fill weight and limited by baffle height.
Is 800 fill power down good enough for a backpacking quilt?
Yes. 800 FP is a very good quality level and is used in many value-oriented quilts. Moving to 850 or 950 saves roughly 1 to 3 oz on a 20°F quilt and shrinks packed size, at a noticeably higher price.
Why do European fill power numbers look lower?
The European test method conditions and measures down differently and generally produces lower numbers for the same down. Always check whether a figure is US (IDFL/IDFB) or European before comparing.
Does fill power drop over time?
Down slowly loses some loft from body oils, dirt and compression. Proper washing and storage restore most of it. See how to restore quilt loft.