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Start with the nights you will actually spend out

Three-season wild camping in the UK is a broad church. It runs from a balmy July night on the South Downs to a frosty April bivvy in the Cairngorms, and your sleeping bag has to cope with all of it without wrecking your pack weight. The trick is not to buy the warmest bag you can afford, but the lightest bag that will still keep you comfortable on the coldest night you realistically expect.

As a rough guide, aim for a comfort rating of around 0°C to -2°C if you camp at height or push into spring and autumn, and around +2°C to +5°C for lowland summer trips. Remember that your sleeping mat does half the work: a bag rated to -2°C on a mat with an R-value of 1 will feel far colder than the same bag on a mat with an R-value of 4. Budget for a decent mat before you spend extra on the bag.

Down, synthetic and hybrid fills

Down is the gold standard for wild camping because it offers the best warmth for the weight and compresses to almost nothing. Fill power — usually quoted between 600 and 850 — describes how much space one ounce of down lofts into. Higher fill power means more warmth per gram, so a 750 fill bag is lighter and packs smaller than a 600 fill bag of the same rating. The catch is moisture: down loses loft when it gets damp, and damp down is miserable. Hydrophobic treatments help significantly, and a water-resistant shell fabric keeps condensation at bay.

Synthetic fills are heavier and bulkier for the same warmth, but they keep working when wet and cost far less. They also tolerate being stuffed away damp without permanent damage. For wet, humid trips on the west coast, or for a first bag you will use hard, a synthetic fill is a sensible, forgiving choice. Continuous filament synthetics hold their loft longer than chopped-staple versions, which can migrate and clump over time.

Hybrid bags blend the two: down through the body for warmth and packability, synthetic around the feet and sometimes the hood, where condensation and damp groundsheets do their worst. They are a genuinely practical compromise for UK conditions.

• A 0°C down bag typically weighs 800–1,000g and packs to about 3–5 litres.
• An equivalent synthetic bag weighs 1.3–1.7kg and packs to 8–12 litres.
• Hybrids usually land somewhere in between, around 1.1–1.4kg.

Reading temperature ratings properly

Most bags carry ratings based on the EN/ISO testing standard, which gives three numbers: comfort, limit and extreme. The comfort rating is the temperature at which a standard woman can sleep relaxed. The limit rating assumes a standard man curled up and slightly chilled. The extreme rating is a survival figure and should be ignored entirely for planning purposes.

Buy to the comfort rating, not the limit. A bag with a limit of -5°C often has a comfort rating near 0°C, and that is the number that matters when you are lying still at 2am. If you sleep cold, are tall, or use a thin mat, add a few degrees of margin. Ratings assume you are wearing a dry base layer and a hat, and that your mat is adequate.

Weight and packed size in the real world

On a long walk-in, every litre of pack volume and every hundred grams counts. A down bag around 900g leaves room in a 45-litre pack for food, shelter and a stove; a bulky synthetic bag can dominate the whole load. If you are choosing where to save weight, the sleeping bag is one of the best places to spend money, because the saving is immediate and you feel it with every step.

Quilts are worth considering if you sleep warm. By removing the hood and back panel, a quilt saves weight and packs smaller, but it demands a warmer mat and a good hooded hat, and it suits still sleepers better than restless ones.

Fit, features and the small details

A bag that is too big has cold air to heat; one that is too tight compresses the loft and creates cold spots. Check the shoulder and hip girth, and the internal length — if you are near the top of a size range, size up rather than squeeze.

• Hood — a well-shaped, drawcord hood traps a surprising amount of warmth.
• Draft collar — a snug collar around the neck stops warm air escaping.
• Zip baffle — a full-length baffle behind the zip is essential in UK conditions.
• Shell fabric — a light water-resistant outer handles condensation and spindrift.

Sleep dry: change out of damp walking clothes, wear a clean base layer and dry socks, and keep a hat on. A silk or fleece liner adds a degree or two and keeps the bag cleaner.

Looking after it so it lasts

Store your bag loose in a large cotton or mesh sack, never compressed, so the fill keeps its loft between trips. Wash it rarely, using a specialist down or synthetic cleaner, and dry it thoroughly — down needs a low tumble dry with a couple of dryer balls to break up clumps. A well-cared-for down bag can last a decade or more, which makes buying quality secondhand a genuinely smart move.

Match the bag to your mat, your shelter and the season you actually walk in, and you will sleep warm without carrying a kilo you do not need.