Guide

What Do Sleeping Bag Season and Temperature Ratings Mean?

A sleeping bag could be one of your most important camping purchases — it has a major part to play in how much of a restful night you get and your energy for the next day. We're here to help you understand what the gobbledegook means and how to pick the right sleeping bag for you. It's worth pointing out that everyone sleeps differently — some are warm sleepers, some sleep a bit cooler — and your tent, sleeping mat, even metabolism and calorie intake will all be factors that affect your body heat.

There are lots of things to consider when buying a sleeping bag — synthetic vs down, shape, weight — but perhaps the most important consideration is the temperature rating. How warm will you actually be?

Temperature rating

There is a European Standard for sleeping bag temperature ratings, EN13537. Key to this rating is that it is legal (in the EU) for sleeping bags to be labelled in a specific way. The standard requires manufacturers to run a thermal manikin test on their sleeping bags to measure four temperature ratings, in degrees Celsius:

For the purposes of the manikin test, a standard man is 25 years old, 1.73m tall and 73kg; a standard woman is 25 years old, 1.60m tall and 60kg.

So with these temperatures you can pick the right bag for your expedition. However, for the more casual camper you may want a less technical guide to find something that's a good all-rounder — that's where seasons come in.

Seasons

Seasons are a less technical rating system for bags. All sleeping bags must carry the above temperature ratings by law (in the EU), but for marketing or guidance the retailer may also use a season rating. As this isn't regulated, there may be different interpretations of seasons. The below is a generally accepted guide:

Our advice? Use both these sets of information together. First think about when and where you are likely to camp and use your sleeping bag — this gives you a season guide. Then identify the temperature specifications you need from that. Just remember, if you go for a winter bag or one with a low limit, check the comfort limit too — if it's too low you may get very hot if you also do a lot of summer camping. If that's the case, consider a lighter sleeping bag and a fleece liner for the colder months.

Down vs synthetic fill

Once you know roughly what temperature rating you need, the next decision is what the bag is actually filled with. This affects the temperature rating you get for a given weight, how the bag copes if it gets wet, and how much you'll pay — so it's worth understanding before you buy.

Down fill (the fine plumage from ducks or geese) has the best warmth-to-weight ratio of any insulation — for a given temperature rating, a down bag will almost always pack smaller and weigh less than a synthetic equivalent. It's also more durable over years of use and compresses down further, which matters if pack space is tight. The downsides: down is significantly more expensive, and it loses much of its insulating power when wet, since the individual feathers clump together and stop trapping air. Many modern down bags now use a hydrophobic (water-resistant) treated down to reduce this risk, but it's still the weak point of down compared to synthetic.

Synthetic fill is made from polyester fibres engineered to mimic the loft of down. It's cheaper to buy, dries faster, and — crucially — continues to provide some insulation even when damp, since the fibres themselves don't absorb water the way down does. It's a sensible choice for damp UK conditions, for anyone on a budget, or for kit that's likely to get wet regularly, like a bag used for canoe camping. The trade-off is that synthetic bags are bulkier and heavier than a down bag with the same temperature rating, and the fill breaks down and loses loft faster over repeated use and washing, so synthetic bags typically need replacing sooner.

As a rule of thumb: choose down if weight and pack size matter most and you can keep it reasonably dry (or budget for a hydrophobic-down bag); choose synthetic if you're camping somewhere wet, need something budget-friendly, or want a bag that's more forgiving if it gets soaked.

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