Northern Warmth

Sizing & Fill Guide

Two sleeping bags laid out on a mountain ridge at sunrise

Fill weight is the number that decides how warm a bag is. This guide explains how to pick it, and what the other measurements on our listings mean.

Choosing a fill weight

Fill weight is the mass of down inside the bag, in grams. It is the single most useful figure for comparing bags — including against laboratory-tested ones.

Fill weight	What it is for
400 g	Mild summer nights, hostels and bothies, and as a warm-weather bag that packs small. Not intended for frost.
600 g	Late spring to early autumn camping in the UK, and van nights out of the wind.
800 g	Cool three-season nights and shoulder-season camping, when the ground is cold and the mornings are damp.
1000 g	Cold winter nights under canvas, and exposed pitches where the temperature falls away after dark.
1200 g	Hard winter use in a tent, when carrying a little more weight beats sleeping badly.

If you are between two, take the heavier one. A bag that is too warm can be opened; a bag that is too cold cannot be fixed at 3 a.m.

Down and feather

Down is the soft three-dimensional cluster from under a bird’s outer plumage. It traps air, and trapped air is what keeps you warm. Feathers have a quill, lie flat and add weight without much loft.

A ratio like 90/10 means 90% down clusters to 10% feather by weight. The higher the first number, the more of the fill is doing useful work.

Shell fabric

The shell has two jobs: keep the down in, and shed the condensation that runs down a tent wall. A tight downproof weave stops clusters working their way out through the fabric.

The rest of the system

Down compressed under your body loses almost all its loft, which means the underside of your bag insulates you barely at all. The ground takes the heat instead.

A mat is not an accessory. Roughly half the warmth of a sleep system comes from what is beneath you. If you are choosing between a heavier bag and adding a mat, add the mat.

Looking after down

Store the bag loose, never compressed — long compression permanently damages loft. Use the stuff sack for carrying, not for storage.

Wash rarely, with a down-specific detergent, on a cold gentle cycle. Tumble dry on low with two clean tennis balls until the clusters separate completely; expect two or three cycles. Do not dry clean.