Down vs Synthetic Sleeping Bags: Which Should You Buy?
Fill type is the single biggest factor in how a sleeping bag performs, packs, and costs — bigger, in practice, than brand or shape. Get it right for the way you actually camp and everything else about the bag tends to fall into place. This guide goes deep on down versus synthetic fill so you can buy with confidence rather than guesswork.
Warmth-to-weight ratio
Down remains unmatched for warmth relative to weight. Because down clusters trap air in a three-dimensional structure far more efficiently than man-made fibres, a down bag rated to a given temperature will almost always weigh less and pack smaller than a synthetic bag rated to the same temperature. This is why down dominates among backpackers, thru-hikers, and anyone carrying their own kit for multiple days — every gram saved in the sleeping bag is a gram available elsewhere. Synthetic fill has closed some of this gap over the years with better fibre engineering, but it hasn't eliminated it; gram for gram, down still lofts warmer.
What fill power actually means
Fill power is a measure of down quality, not quantity. It describes how many cubic inches one ounce of down loft occupies — a higher number means each cluster of down traps more air, so you need less down by weight to achieve the same warmth. Budget down bags typically use 550–650 fill power, mid-range bags sit around 700–800, and premium ultralight bags push toward 850–900 fill power or higher. A 900-fill bag will be noticeably lighter and more compressible than a 550-fill bag with an identical temperature rating, which is why fill power is usually the biggest driver of price within the down category. Fill power says nothing about how much down is inside the bag (that's fill weight) — the two numbers work together to determine actual warmth.
Behaviour when wet
This is where the two fills diverge most sharply. Down clusters rely on trapping air between fine filaments; when they get wet, the filaments clump together, the structure collapses, and the bag loses most of its loft — and with it, most of its insulating ability. A soaked down bag can feel like a cold, heavy rag, and it dries slowly in the field. Synthetic fill behaves very differently: the individual fibres don't absorb water the way down does, so even when damp the fill largely retains its structure and continues to trap some air, meaning a wet synthetic bag still provides usable warmth. Synthetic also dries considerably faster. For anyone camping somewhere genuinely wet, this single difference can outweigh every other consideration.
Hydrophobic down — the middle ground
Modern hydrophobic down (sold under various brand names, often DWR-treated at the cluster level) has narrowed the gap considerably. Each down filament is coated with a durable water-repellent treatment, so the down resists absorbing moisture for longer and dries faster than untreated down when it does get wet. It isn't waterproof and it won't perform like synthetic in sustained heavy rain or full submersion, but for the everyday risk of condensation, a damp tent wall, or light drizzle finding its way in, hydrophobic down meaningfully reduces the traditional weak point of down fill. It typically adds a modest cost premium over untreated down of the same fill power, and is now common enough in mid-to-premium bags that it's worth actively looking for.
Compressibility and pack size
Down compresses far more than synthetic and, importantly, recovers its loft reliably after being packed away — even after years of repeated compression in a stuff sack. Synthetic fill can be compressed too, but the fibres are stiffer and bulkier, so a synthetic bag packs down to a noticeably larger size for the same warmth, and repeated hard compression gradually damages the fibre structure. If pack volume in your rucksack is tight — bikepacking, multi-day backpacking, anything where every litre of pack space counts — down's compressibility is a major practical advantage, separate from the weight saving.
Cost
Down is more expensive, and the gap widens with fill power: a high-fill-power hydrophobic down bag sits at the premium end of the market. Synthetic fill is cheaper to manufacture and the resulting bags cost noticeably less for a comparable temperature rating, which is a large part of why synthetic remains popular for family camping, festival use, and anyone buying multiple bags on a budget. If you're kitting out a household rather than a single serious backpacker, synthetic often makes the budget go much further.
Durability and lifespan
Looked after properly, down is genuinely long-lived — good down clusters can retain loft over many years and hundreds of compression cycles, which is part of why premium down bags command a high resale value. Synthetic fill has a shorter practical lifespan: the fibres gradually break down and lose loft with repeated use, compression, and washing, so a heavily used synthetic bag will typically need replacing sooner than an equivalent down bag. Over a long ownership period this can offset some of down's higher upfront cost.
Care and washing
Down bags need specialist care — a down-specific cleaner (never standard detergent, which strips the natural oils and DWR coating from the plumage), a large-capacity washing machine or laundrette machine to avoid clumping, and a long, gentle tumble-dry with clean tennis balls or dryer balls to break up clusters and restore loft. Over-washing shortens a down bag's life, which is exactly why a liner (see our separate guide) is worth using to cut down on how often the bag itself needs washing. Synthetic bags are considerably more forgiving — most can be machine washed on a gentle cycle with ordinary technical-fabric wash and dried on a low heat without the same risk of ruining the fill structure, making them a lower-maintenance choice for frequent use.
Ethical sourcing
Down is an animal by-product, and sourcing matters. The Responsible Down Standard (RDS) is the main independent certification in this space — it verifies that the down in a certified product comes from geese and ducks that haven't been subjected to live-plucking or force-feeding, and it tracks the down through the supply chain from farm to finished product. Most reputable outdoor brands now use RDS-certified (or an equivalent in-house verified) down as standard, but it's still worth checking for the certification specifically if ethical sourcing is a factor in your buying decision, rather than assuming it by default. Synthetic fill sidesteps this question entirely, which is part of its appeal for buyers who'd rather not think about animal welfare in their gear at all.
Decision framework: which should you buy?
Choose down if: you're backpacking multi-day routes where every gram and every litre of pack space counts; you can reasonably keep your kit dry (or you're willing to pay more for a hydrophobic-down bag as insurance); and you camp often enough that down's higher upfront cost is offset by its long lifespan.
Choose synthetic if: you're camping in typically damp UK conditions where soggy tents and condensation are routine; you're buying on a budget or kitting out several bags at once for family or group camping; or your trips involve a real risk of the bag getting properly wet — canoe camping, coastal trips, humid climates — where synthetic's ability to keep insulating when damp matters more than raw warmth-to-weight performance.
Many experienced campers end up owning one of each — a lightweight down bag for serious backpacking, and a cheaper, more forgiving synthetic bag for car camping, festivals, or anything where the bag is likely to take a beating. If you can only justify one, be honest with yourself about how you actually camp most often, and buy for that.