Nearly every sleeping bag is filled with one of two things: down, the soft under-plumage of ducks and geese, or synthetic fibre, usually polyester. Both work. They trade off weight, bulk, cost and how they cope with water, and the right one depends on your trips.
A note before we start: we only sell down bags and quilts. We'll still tell you plainly when synthetic is the better buy.
What fill power means
Fill power measures the loft of down: how much space a fixed weight of it fills in a standard test cylinder. It's quoted in cubic inches per ounce, so 800 fill power (800FP) means one ounce of that down, about 28 g, lofts to about 800 cubic inches, or roughly 13 litres.
More loft means more trapped air per gram, and still, trapped air is what keeps you warm. Higher fill power gives the same warmth for less weight and a smaller packed size. As a rough guide, 550FP down needs something like 40 to 50% more down by weight to match the loft of 800FP.
Three things to know when you compare labels:
- US and European test methods differ, so fill power figures aren't always directly comparable between brands.
- A figure like 90% or 90/10 is the share of down clusters in the fill; the rest is mostly small feathers.
- Fill power is quality, not quantity. Most of a bag's warmth comes from its fill weight: how many grams of down are in it.
Duck or goose? Fill power is loft per gram, so 800FP duck and 800FP goose loft the same. We sell both: the mummy and envelope bags are goose down by the maker's label, and the Cozy 300 and UL250 are duck.
Fill weight does the warming
Here's how one bag, our Goose Down Mummy, lines up across its five fill weights. Same design, more down, colder nights (our planning guide):
- 400 g of down, 870 g all up: about +8°C.
- 600 g, 1,070 g: about +3°C.
- 800 g, 1,290 g: about −1°C.
- 1,000 g, 1,500 g: about −5°C.
- 1,200 g, 1,700 g: about −8°C.
Cut, shell fabric and baffle design change warmth too, so compare fill weight between bags of a similar design. Our temperature ratings guide explains the comfort and limit figures.
Weight and packed size
Down's great strength is warmth for its weight and bulk. Our UL250 summer bag weighs about 500 g and packs to about 20 × 13 cm. A synthetic bag of similar warmth will usually weigh more and pack larger. Compare any two bags with the same comfort rating and you'll generally see the difference.
That's why so many multi-day walkers carry down: on your back, every gram and every litre of pack space counts.
When down gets wet
Down's weakness is water. Wet clusters clump and collapse, and collapsed down barely insulates. Synthetic fibres keep more of their loft when damp, and they dry faster.
A soaking is rare. The slow build-up is more common: breath inside the hood, condensation dripping off the tent wall, humid huts, and several days without sun to air the bag.
Water-repellent down helps. The UL250 uses treated duck down. Treated down takes up less moisture and dries faster than untreated down, but it isn't waterproof, and a soaked bag is still a cold bag.
What keeps down dry in practice:
- Pack it in a waterproof bag or pack liner inside your pack. A compression sack squashes it small and shrugs off showers, but it isn't waterproof, so line your pack too.
- Keep your face out of the bag on cold nights, and vent the tent.
- Air the bag in the sun whenever you can, and dry it fully when you get home.
If your trip is likely to be wet for days on end, such as south-west Tasmania in a bad week, read our guide to Australian conditions and weigh up synthetic seriously.
Care and lifespan
Down needs a little more care. Store it loose in its large storage sack, never in the compression sack. Wash it rarely, with a down-specific wash on a gentle cycle, then tumble dry on low with two clean tennis balls until there are no clumps.
A liner keeps sweat and skin oils out of the down, so you wash the bag less often.
Looked after, down keeps its loft for many years. Synthetic fill is easier to wash, but it slowly loses loft each time it's compressed, so a synthetic bag that's stuffed into a pack every day gets flatter over time.
Cost
Down costs more up front. Our down bags start under A$300 for a summer bag and run to around A$1,000 for an expedition bag, and synthetic bags with similar ratings are usually cheaper. Because down weighs less and lasts longer, many hikers decide it's worth the price. For a few nights of car camping a year, it often isn't.
Down and synthetic side by side
| Down | Synthetic | |
|---|---|---|
| Warmth for weight | Higher | Lower |
| Packed size | Smaller | Larger |
| When damp | Loses loft; water-repellent down copes better | Keeps more warmth, dries faster |
| Care | Wash rarely, dry thoroughly, store loose | Easy to wash and dry |
| Lifespan | Many years with care | Loses loft with repeated compression |
| Up-front cost | Higher | Lower |
| Best for | Hiking, cold nights, years of use | Car camping, long wet trips, tight budgets |
When synthetic makes sense
- Car camping, festivals and school camps, where weight doesn't matter and the bag gets knocked around.
- Trips where the bag will be damp for days with no chance to dry it, such as canoe and packraft trips or long wet spells.
- A tight budget. A decent synthetic bag beats a down bag you can't afford.
- If you'd rather not use animal products.
When down makes sense
- Hiking, bikepacking and anything carried on your back.
- Cold nights, where down's warmth for its weight matters most.
- When you'll look after it and want a bag that lasts.
Our sleeping bag guide helps you choose the warmth. Our down sleeping bags and down quilts list the fill weight, fill power and the maker's ratings on every one.
Sources
- Fill power: definition, units and the loft comparison between fill powers.
- Allied Feather + Down: fill power: how fill power is tested, and US and European methods.