How To Choose a Sleeping Bag

By Mark Roberts

Baffled by sleeping bag options? Temperature ratings getting you down? Lost in loft?

There are lots of decisions to make when you’re buying a sleeping bag. And there’s no one-size-fits-all option that’s perfect for every possible situation. Here’s how to choose the right sleeping bag for your next camping trip.

Choosing a Sleeping Bag

  1. Decide what you need your sleeping bag for
  2. How warm should your sleeping bag be?
  3. What do sleeping bag seasons mean?
  4. Sleeping bag temperature ratings explained
  5. Down vs synthetic sleeping bags
  6. Other sleeping bag features to think about

 Throwing out a green sleeping bag inside a large geodesic tent

Decide What You Need Your Sleeping Bag For

The conditions you plan to sleep in and the activities you do will make a big difference to the type of sleeping bag you need. Answering some of these questions will help you to decide on the finer details.

What’s the lowest temperature you’re likely to camp in?

If you never camp in winter, there’s no sense in buying the warmest sleeping bag money can buy. Equally, opting for an ultralight summer sleeping bag might be a risk if you want to camp in spring and autumn. The country you use it in will make a big difference too. Continental climates (e.g. central Europe, North America) can be baking in summer and incredibly cold in winter, even at lower latitudes.

What activities are you going to use your sleeping bag for?

Weight and pack-size might not be important if you’re pitching camp right next to your car boot. But a lightweight and highly packable sleeping bag makes a big difference on backpacking and bikepacking trips where you could be carrying it for miles. High altitude camps or expeditions in extreme cold require a sleeping bag that combines exceptional warmth with a low weight.

Is your sleeping bag likely to get wet?

This will make a big difference to the type of insulation you opt for. The longer your trip, the harder it will be to keep your sleeping bag dry – especially in bad weather.

How much do you want to spend?

The main factor that affects sleeping bag price is the type and quality of the insulation inside. The good news is: the size of your budget might make all of these decisions for you!

Pouring coffee in front of a lightweight backpacking tent on a bikepacking trip

How Warm Should Your Sleeping Bag Be?

Try to match your sleeping bag to the lowest night-time temperatures you expect to camp in, combined with how you personally sleep (whether you’re a warm or cold sleeper). If in doubt, opt for a sleeping bag that’s slightly warmer than the conditions you’re expecting. Hardcore alpinists and adventure racers might be willing to put up with a shivery night’s sleep, but most of us would rather be snug and warm after a long day.

Warm vs cold sleepers

Remember that your sleeping bag works by trapping the heat that your body generates – you (yeah, you!) make a big difference to how warm your sleeping bag feels. And even the warmest sleeper can have a cold night after a tiring day with not enough to eat. Here are a few general rules:

  • Women sleep colder than men
  • Older people sleep colder than younger people
  • Unfit people sleep colder than fit people
  • Light people sleep colder than heavy people
  • Restless sleepers sleep colder

It's not just about your sleeping bag!

The other thing to bear in mind is that there are lots of external factors that can affect how warm you are at night and your sleeping bag is only part of that. Here are a few of the most important ones:

  • How insulating your sleeping mat is – you can lose up to 3x more heat to the ground than to the surrounding air
  • How much protection your shelter provides – bothy/hut > mountain tent > backpacking tent > bivvy bag
  • How dry your sleeping bag is – wet fabrics/fill conduct heat away from you – even in synthetic bags
  • How humid it is – damp cold always feels colder than dry cold
  • How many people you share a tent with – more people radiate more heat
  • What clothes you sleep in – always change into dry clothes before bed
  • How much you've had to eat – fats and proteins release more energy, more slowly
  • How hydrated you are – better to need a wee in the night than sleep dehydrated!
  • Your exertion levels for the day – you'll always feel colder after a hard day
Snuggled up inside a green mountaineering sleeping bag

What Rating Sleeping Bag Should I Get?

What do sleeping bag seasons mean?

One of the best general guides to sleeping bag warmth is the ‘season rating’. The higher the season rating number, the warmer the sleeping bag will be. Our season ratings are based on typical seasonal temperatures in the UK. 2-season sleeping bags are only really suitable for mild summer camping, while 4-season sleeping bags are designed to keep you warm below freezing. However, these ratings are a little subjective (we all sleep differently) and different brands will probably have different ideas on what makes a 3-season sleeping bag. Luckily, there is a more scientific rating you can use…

Sleeping bag temperature ratings explained

The temperature figures you’ll usually see quoted on sleeping bags are the ‘EN13537’ ratings – a standardised European rating system. First introduced in 2005, the EN13537 tells you what a sleeping bag’s useable temperature range is. Because the test is standardised, these figures are really useful for comparing the warmth of sleeping bags, even between different brands.

How are sleeping bag temperature ratings tested?

The test is conducted on a heated mannequin, dressed head to toe in thin base layers and lying on a sleeping mat with a R Value of 4.8 (that’s a pretty warm mat - warm enough for winter). This mannequin is then placed in the test sleeping bag, in a temperature-controlled chamber, with sensors attached. The data that comes back tells us how well the sleeping bag prevents heat loss at various temperatures. This range is then separated into 3 different ratings...

What sleeping bag temperature ratings mean:

Comfort — the temperature at which a standard woman* can expect to sleep comfortably in a relaxed position.

Lower Limit — the temperature at which a standard man** can sleep for eight hours in a curled position without waking.

Extreme — the minimum temperature at which a standard woman can remain for six hours without risk of death from hypothermia (though frostbite is still possible).

*Based on a ‘standard woman’ being 25 years old, 160cm high and weighing 60kg

**Based on a ‘standard man’ being 25 years old, 173cm tall and weighing 73kg

Bivvying inside two sleeping bags on a bikepacking trip

The limitations of temperature ratings

It’s best to take these temperature ratings with a heavy pinch of salt. Well, a fist of salt. Not only are there so many different variables which affect how each of us sleep, the heavily controlled lab conditions are very different to the conditions most of us will use our sleeping bags in. We rarely sleep in a humidity-controlled room, with a fully lofted sleeping bag and a military-grade face mask! Traditional advice is to add 5°C to the Comfort rating when matching the ratings to outside night-time temperatures.

What is the AK Sleep Limit?

We created the AK Sleep Limit to use alongside the EN13537 measurement and make it easier to match your sleeping bag to real-world conditions. The Alpkit AK Sleep Limit is what the majority of our customers will find to be the tipping point between a comfortable and restless night’s sleep.

Two people sitting inside a tent on top of their sleeping bags after mountaineering

Down vs Synthetic Sleeping Bags

Sleeping bags use two types of insulation to keep you warm: down feathers and man-made synthetic insulation. Down sleeping bags have the best warmth-to-weight ratio and are the most compressible. This means they’re generally better for backpacking, bikepacking or any situation where you’ll have to carry your bag a long way. This also makes them ideal for adventures in extreme cold environments. Synthetic sleeping bags are better for damp conditions or for repeated heavy use. Their insulation is much easier to wash and care for than down.


Down Sleeping Bags
Pros Cons
The best warmth-to-weight ratio Loses all its warmth when wet
More compressible than synthetics Harder to look after in the damp
Lasts for years with good care Takes a long time to dry out
Natural, renewable and biodegradable More expensive


Synthetic Sleeping Bags
Pros Cons
Keeps its insulation when wet Not as warm as down at the top end
Water repellent and fast-drying Heavier for the same warmth as down
Very easy to wash and care for Less compressible than down
Cheaper than down Can lose loft after repeated heavy use


Down Fill

Down comes from the underbelly of ducks and geese, a natural insulation layer that sits underneath their protective outer feathers. These light and fluffy ‘filaments’ trap lots of insulating air for very little weight. Down quality is measured by loft – the amount of space each down filament takes up. Think of it as a measure of ‘fluffiness’!

What is fill power?

Fill power (FP) is the scientific measurement of loft. The higher the number, the higher the loft, and the higher the quality of down. High fill power down has a better warmth-to-weight ratio and will be more compressible. The number represents how many cubic centimetres a gram of down lofts to. 750FP means that 1g of down lofts to 750cm³.  Anything above 600FP should be excellent quality down.

What is fill weight?

This is how many grams of down is inside your sleeping bag. Sleeping bags often have the fill weight in their product name, e.g. PipeDream 400. Sleeping bag warmth depends on both fill power and fill weight – a 400g down bag with 750FP down might be warmer than a 500g down bag with 650FP down. This is where the temperature ratings we talked about earlier come in handy.

What is hydrophobic down?

Hydrophobic down is any down that has been treated with a durable water repellent (DWR). When down gets wet, the down filaments absorb water, collapse and clump together, losing all their insulation. Hydrophobic down dries quicker and repels water and the moisture from your body for longer. You’ll still need to keep your sleeping bag dry, but this treatment helps it to resist damp and recover better. The DownTek™ PFC-Free Water Repellent Down we use stays dry for up to 13.5x longer than untreated down.

What is RDS-certified Down?

The Responsible Down Standard is a voluntary standard that independently certifies ethically sourced down. The RDS logo guarantees that the down in your sleeping bag meets highest of animal welfare standards. At Alpkit, we only use RDS-approved or recycled down.

Taking out a synthetic sleeping bag on a campsite

Synthetic Insulation

These man-made polyester fibres are designed to replicate the insulation and compressibility of down. Polyester fibres don’t absorb water so synthetic insulation keeps its insulating structure even when damp. There are lots of different brands and types of synthetic insulation. We use PrimaLoft® Gold, the highest quality of synthetic insulation available, and Thermolite® Ecomade, a really durable, long-lasting fill.

How is synthetic insulation measured?

Most brands measure synthetic insulation by grams per square metre – often written as gsm or g/m². However, because there are so many different types of synthetic insulation, gsm only really helps to compare between sleeping bags using the same brand and fill type. This is where the temperature ratings are useful, again.

Reading before bed in Cross Fell bothy by head torch

Other Sleeping Bag Features To Think About

Baffle Construction

Baffles are the separate compartments that hold the insulation inside your sleeping. Baffles stop insulation from ‘migrating’ (sinking) to the bottom/sides of your bag and creating cold spots. There are two main types of baffle construction (this applies more to down sleeping bags than synthetic bags)

Stitch-through - means that each baffle has been stitched all the way through the sleeping bag fabric. Stitch-through is the lightest construction and the most compressible, but it can create cold spots along the lines of the stitching.

Box wall - construction uses a divider of fabric between each baffle so that each baffle is shaped like a… erm, box. Box wall construction allows the down to loft better. But the extra fabric does add weight and make it less compressible.

Sleeping Bag Shape and Size

Make sure you can actually fit inside your sleeping bag – get inside one before you buy if you can! We offer Regular and Long length bags for people over 6ft. There are two main types of sleeping bag shape to decide between, depending on whether you prioritise lightweight warmth or sleeping comfort:

Mummy - These sleeping bags have a tapered shape that’s wider at the shoulders and narrows towards the footbox. They look just like an Egyptian pharaoh's sarcophagus! This is the most ‘thermally efficient’ shape (the easiest to warm up and keep warm) and the lightest.

All but one of our sleeping bags are this shape. The ultralight PipeDream sleeping bags have a highly tapered shape to save weight while the SkyeHigh bags have a more relaxed mummy shape. The AlpineDream and ArcticDream leave extra space so you can wear more layers inside your sleeping bag.

Rectangular - These sleeping bags have much more space to spread out – not quite a ‘starfish’, but not far off either! Rectangular sleeping bags are better if you’re sleeping on a campsite for multiple nights in milder weather or in a bunkhouse/mountain hut/cold hostel. You can often zip these sleeping bags together to form a double bag, like with our Cloud Nine.

Zip side

We offer left and right zips on all our sleeping bags. It’s easier to reach across your body with your lead hand to unzip your sleeping bag. Left zips are better for right-handers and right zips are better for lefties.

Hoods, neck baffles and zip baffles

 Make sure the hood fits snugly and is easily adjustable. Most sleeping bags also include adjustable neck baffles to stop cold air being sucked in when you move. Zip baffles stop draughts from squeezing between your zip teeth.

Setting up camp in a sunny forest campsite

How To Choose The Right Sleeping Bag

To summarise, ask yourself these questions:

  • What am I using it for? How light and how small? 
    Bikepacking and backpacking? Summer car camping? Polar expeditions?!
  • How warm does it need to be? 
    What’s the lowest temperature you might camp in? Are you a warm or cold sleeper? Use seasons and temperature ratings to guide you.
  • Which insulation is better? 
    Down is lighter, warmer and more compressible. (Higher fill power down is a higher quality. Hydrophobic down is more damp-resistant.) Synthetic fill is easier to look after, you can get it damp mid-trip, and it’s cheaper – that can be important too!
  • Make sure it’s comfy 
    Try it for size. Tighter fits (tapered mummy) are warmer; wider fits are more comfortable. Does the hood adjust properly? Is the zip on the right side for your lead hand?

Also, remember that the ‘perfect sleeping bag’ doesn’t really exist! Most of us start off with an affordable 3-season bag and only buy another one if our main sleeping bag isn’t suitable for future trips. With the right mat, a sheltered tent and warm clothes, you can make a 3-season sleeping bag work for most of the year in the UK, as long as you’re not camping too high up.

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