Quick Answer: Fill power measures how much space one ounce of down occupies in cubic inches. Higher fill power means lighter weight and better insulation per ounce. For Canadian winters, 600 fill power is a reasonable minimum. 700+ is premium. The number does not tell you how warm the duvet is on its own; warmth depends on both fill power and the total amount of fill (fill weight).
In This Guide
Reading Time: 6 minutes
What Is Fill Power?
Fill power is a standardised measurement of down quality. It tells you how much space one ounce of that down fill occupies when allowed to loft freely. The higher the number, the larger and more springy the down clusters, and the more air they trap per ounce.
The practical result is simple: a duvet filled with 700 fill power down provides the same warmth as a duvet filled with 400 fill power down, but the 700 fill power version needs less fill to achieve it. That means it is lighter, less bulky, and more comfortable to sleep under.
How Fill Power Is Measured
The fill power test is standardised by the International Down and Feather Bureau (IDFB) and the Canadian government's Fill Power Certification process. One ounce of down is placed in a cylinder, allowed to loft for a specified period, and then the volume in cubic inches is measured. A result of 700 means that one ounce of that down lofts to fill 700 cubic inches of space.
Why Large Down Clusters Insulate Better
A down cluster is a three-dimensional structure of interlocking fibres that resembles a dandelion puffball. Larger clusters trap more air within their structure. Air is an excellent insulator because it does not conduct heat well. The down cluster holds the air in place, preventing the convective heat loss that would occur with moving air. Higher fill power down from cold-climate geese (Hutterite, Icelandic, Hungarian) tends to have naturally larger cluster structures than down from warmer-climate birds, which is why cold-climate sourcing commands a premium. This is not just marketing; the structural biology of cold-climate waterfowl produces genuinely superior down clusters.
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Fill Power Ratings Explained
| Fill Power Range | Quality Tier | Characteristics | Who It Suits |
|---|---|---|---|
| 300-450 | Entry-level | Lower loft, heavier for same warmth, shorter lifespan, typically duck down or mixed fill | Budget buyers, guest rooms, occasional use |
| 450-600 | Mid-range | Good warmth-to-weight ratio, serviceable quality, usually duck or lower-grade goose down | Everyday use, moderate climates, value-focused buyers |
| 600-700 | Premium | Excellent loft, lighter weight, typically goose down or Canadian standard down. Highland Feather Cordoba is 600. | Most Canadian households, serious everyday use |
| 700-800 | Luxury | Exceptional loft and lightness, cold-climate sourcing (Hutterite, Hungarian, Icelandic). Highland Feather Iceland is 725. | Those who want the best quality and longest lifespan |
| 800+ | Ultra-luxury | Maximum loft, very expensive, rare in mainstream retail. Eider duck down can reach 1,000+. | Luxury buyers, specialty retailers, high-end gifting |
Brad, Owner, 40+ years of experience: "The fill power number is useful but it can also be misleading if you do not understand fill weight. A 700 fill power duvet with only 18 ounces of fill is a summer weight. A 600 fill power duvet with 50 ounces of fill might be much warmer. You need both numbers to understand what you are actually buying."
Fill Power vs Fill Weight: Why You Need Both Numbers
This is where many buyers get confused. Fill power tells you the quality of the down. Fill weight (measured in ounces) tells you how much down is in the duvet. Together they determine warmth.
Fill Power and Fill Weight Together
- High fill power + low fill weight = light, good quality, warm enough for moderate climates. Think summer or all-season weight.
- High fill power + high fill weight = warm, heavier, excellent insulation. Winter weight at premium quality.
- Low fill power + high fill weight = very heavy duvet, adequate warmth but bulky and less comfortable. Often used in budget hotel-style duvets.
- Low fill power + low fill weight = light, not very warm. Good only for warm rooms or decorative use.
For example: the Highland Feather Iceland uses 725 fill power (high quality) in an all-season queen fill of approximately 40 ounces (adequate warmth for most Ontario bedrooms). The Cordoba uses 600 fill power in similar fill weights. The Iceland is lighter and airier at the same warmth level because the fill power is higher, meaning less down achieves the same insulation.
Fill Power for Canadian Winters
What Fill Power Makes Sense in Ontario?
For most Ontario homes heated to 18-22°C overnight, 600 fill power in an all-season fill weight is a practical minimum. At 600 fill power and 40 ounces of fill (queen), you get a warm, quality duvet that handles Brantford winters in a heated bedroom. If your bedroom runs cooler or you prefer a lighter duvet at the same warmth, stepping up to 700-725 fill power lets you use less fill for the same result, which means a lighter, airier duvet. The premium is real but so is the practical difference.
| Scenario | Recommended Fill Power | Recommended Fill Weight (Queen) |
|---|---|---|
| Warm bedroom (22°C+), average sleeper | 600-700 (summer fill) | 25-30 oz |
| Typical Ontario heated bedroom (18-22°C) | 600-725 (all-season) | 35-45 oz |
| Cooler bedroom or cold sleeper | 700-750 (all-season or winter) | 45-55 oz |
| Very cold room, old home, minimal insulation | 700+ (winter fill) | 55+ oz |
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Is 700 fill power good for a duvet?
Yes. 700 fill power is in the premium tier and above what most everyday duvets offer. At 700+, you are getting down with large cluster structures that provide excellent insulation with a noticeably lighter weight than 400-550 fill power alternatives. Highland Feather's Iceland at 725 fill power is a practical example of what 700+ delivers in a real product.
What is the difference between 600 and 700 fill power?
At 600 fill power, one ounce of down occupies 600 cubic inches. At 700, it occupies 700 cubic inches. That 100 cubic inch difference per ounce adds up across a full duvet. To achieve the same warmth, the 700 fill power duvet needs less down fill, making it lighter and airier. Both are premium quality; the 700 is noticeably lighter at the same warmth level.
Can I tell fill power by feel?
Somewhat. A higher fill power duvet will loft more easily and feel airier when you shake it or compress and release it. However, fill weight also affects the feel significantly. The best way to assess a duvet is to compare similar fill weights at different fill powers side by side.
Does fill power affect how long a duvet lasts?
Yes. Higher fill power down, particularly cold-climate goose down, tends to have more resilient cluster structures that hold their loft over time. Entry-level down at 300-450 fill power typically breaks down faster with washing and use. A 700+ fill power duvet from a quality manufacturer, properly cared for, can last 15-20 years.
Is 850 fill power duvet worth the premium?
At 850+, you are in diminishing returns territory for everyday bedding. The difference between 725 and 850 is real but subtle in a home bedroom context. The extreme premium fill powers are more relevant for technical outdoor gear (sleeping bags, parkas) where weight and pack size matter more than in bedding. For a duvet, 700-750 fill power is the practical sweet spot of quality and value.
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