It is the first cold week in November and your old coat no longer keeps you warm. If you want to buy a winter coat quickly, you will run into terms such as fill power, loft, cuin, hydrostatic head and synthetic insulation. Most offers look much the same in the photos. But what keeps you warm in the end is not the look, it is the filling and how well the coat keeps out wind and wet.

Which coat suits you depends a lot on how you use it. If you walk ten minutes to the train or bus in the morning, you need something different from someone who spends hours walking in sleet at the weekend. This article helps you make sense of the details you see in the shop.

Down or synthetic: the basic difference

Down is the fine under-plumage of geese or ducks. It traps a great deal of air and therefore keeps you warm at a low weight. A good down jacket can be squashed down small and then puffs back up afterwards. Its weak point is moisture. Once down gets soaked, it collapses and loses a large part of its warmth.

A synthetic jacket is filled with polyester fibres. At the same weight it is slightly less warm and does not pack down as small. In return, it keeps a good part of its insulation even when damp and dries faster. It is also usually easier to wash and cheaper. For cold, wet weather and for people who move a lot and sweat while doing so, synthetic filling is often the more practical choice.

Down jacket fill power: what the number means

Fill power, also called loft, shows how much space a given amount of down takes up. It is usually stated in cuin, meaning cubic inches per ounce. The higher the value, the more air the down traps and the better it keeps you warm relative to its weight. Values around 600 cuin are considered decent, from about 700 cuin people talk of high-quality down, and very light specialist jackets go higher still.

Fill power alone does not tell you how warm a jacket is, though. The total amount of down inside counts too. A jacket with 800 cuin and a low fill weight can be cooler than one with 600 cuin and twice as much filling. Also look at the ratio of down to feathers, such as 90 to 10. The more feathers, the heavier and less lofty the filling. Reputable sellers state where the down comes from and whether live plucking is ruled out. If there is no information at all, ask.

Weather protection: outer fabric and hydrostatic head

The filling keeps you warm, the outer fabric keeps out wind and water. Many down jackets only have a water-repellent surface. That copes with light snow and a brief drizzle, but not with steady rain. If you are often out in the rain, you need a jacket with a waterproof membrane and taped seams, or you wear a rain jacket over your down jacket.

For waterproof jackets, the hydrostatic head is given in millimetres in the description. The higher the value, the more water pressure the fabric can withstand. Breathability matters just as much, so that sweat can escape. A completely sealed jacket without ventilation quickly turns into a sauna when you climb the stairs.

With quilted down jackets, it is worth looking at the seams. Stitched-through seams are cold spots, because there is hardly any filling there and wind gets through more easily. Higher-quality jackets for very cold days therefore have baffle chambers with fabric walls or offset seams. For everyday city use, simple quilting is usually enough, but for long spells outdoors in bitter cold it tends not to be.

Fit and details that matter day to day

A winter coat should leave some room for a thick jumper, but not be so loose that cold air gets in. The sleeves should still reach over your wrist when your arms are bent. With down jackets, sizes vary a lot depending on the cut. Read the size chart carefully instead of relying on your usual size. The article Buying clothes online: sizes and fit explains more.

• A hood that can be pulled tight and does not block your view when you turn your head.
• Cuffs on the sleeves and a chin guard at the zip.
• A length that suits how you use it: short jackets are more practical for cycling, long ones keep you warm while waiting at the bus stop.

Worked example: price per day worn

Say you wear your winter coat on 80 days a year. A cheap jacket for 90 euros is worn out after two winters, the filling clumps and the zip sticks. That makes 160 days of wear and about 56 cents per day. A higher-quality jacket for 240 euros lasts, for example, six winters. That makes 480 days of wear and exactly 50 cents per day.

If you buy the more expensive jacket in the end-of-season sale with 30 percent off, you pay 168 euros and come to 35 cents per day. Winter coats are often reduced from January onwards, although by then there is less choice of sizes. You can read more about this in January sales and end-of-season clearance. When you see a discount in the EU, look at the lowest price of the last 30 days, which retailers there have to state; outside the EU, for example in the UK, the US or Canada, other rules apply to price reductions. You can find current codes under discount codes.

Ordering and sending back

Try the coat on at home with the jumper you actually wear in winter. Leave the labels on until you are sure. For online purchases in the EU you have a 14-day right of withdrawal; outside the EU, for example in the UK, the US, Canada or Switzerland, your own country's rules apply. If you order several sizes intending to keep one, find out the return costs beforehand. The article Shopping more sustainably: avoiding returns has tips on this.

Frequently asked questions

How much cuin should a good down jacket have?

Values around 600 cuin are decent for everyday use, and from about 700 cuin the down is considered high quality. How warm the jacket is also depends on the fill weight, though.

Is a synthetic jacket worse than down?

Not worse, just different. It copes better with moisture and is easier to care for, but it is heavier and does not pack down as small.

Can you wash a down jacket?

Usually yes, with a special down detergent and a gentle programme. The important part is drying it thoroughly, often in a tumble dryer with a few dryer balls. The instructions on the care label take priority.

When is the best time to buy a winter coat?

In terms of price, often in January and February, when the season is coming to an end. If you need a particular size, though, you will find more choice in autumn.