What Tog Duvet Do You Need? Summer, Winter, All Year Round

Tog is a unit of thermal resistance - it measures how well a material stops heat escaping. The higher the tog, the more warmth the duvet holds in. One tog equals 0.1 square-metre kelvin per watt, a figure defined for the textile industry (the tog is a derived unit of thermal insulance). In practice you never need the maths: British duvets are sold on a simple 1 to 13.5 tog scale, and all you read is the number on the label.

The key point is that tog rates warmth, not weight or loft. A thick, airy duvet can have a low tog, and a thin, dense one can have a high tog. Two 4.5 tog duvets keep you equally warm even if one looks twice as puffy as the other. When you shop for summer, ignore how thick a duvet feels in the packaging and read the tog.

For most people in a UK summer, a duvet between 1 and 4.5 tog is the comfortable range. A 4.5 tog duvet suits mild British summer nights and cooler bedrooms; a 1 to 2.5 tog duvet - often sold as a "summer" or "cool" duvet - is the pick for anyone who sleeps hot or lives somewhere that turns muggy in a heatwave.

Room temperature matters as much as the label. A well-ventilated north-facing bedroom might stay comfortable under 4.5 tog even in July, while a warm loft room can feel stifling under anything above 2.5 tog. If your bedroom runs hot, start low: it is easy to add a sheet or a light blanket on a cool night, but a too-warm duvet leaves you kicking it off at 3am. For the rest of the room, our guide to making a bedroom cooler for sleep covers airflow, blackout and bedding together.

British duvets follow a broadly standard seasonal scale, and most UK ranges are built around four ratings: 4.5 tog for summer, 7 to 7.5 tog as a light mid-season option, 10.5 tog for spring and autumn or year-round use, and 13.5 tog for winter. Use the scale as a starting point, then adjust for how warm your bedroom runs and whether you sleep hot or cold.

If you want one duvet for the whole year, the usual UK answer is 10.5 tog. It is warm enough for spring, autumn and most centrally heated bedrooms in winter, and it is the rating most ranges stock as their year-round option. It is a compromise rather than an optimum: 10.5 tog is warmer than most people want in a July heatwave, and cooler than a 13.5 tog duvet in an unheated bedroom in February.

Whether that compromise works depends mostly on your bedroom rather than the duvet. A well-insulated home that holds 18 to 20C overnight suits a single 10.5 tog duvet all year, especially if you switch to a cotton cover in summer and add a blanket on the coldest nights. A draughty older house, or a bedroom you deliberately keep cool, is where one rating starts to fail at both ends of the year.

If you sleep hot, invert the compromise. Pick a lighter duvet as your year-round base - 4.5 tog, or 7 to 7.5 tog if your winters are cold - and add a wool blanket or throw when the temperature drops. Layering over a light duvet gives you far more control than trying to sleep under one that is slightly too warm for nine months of the year, and it is cheaper than buying two duvets.

An all-season duvet (two duvets that popper or clip together - typically a 4.5 tog and a 9 tog) is the most flexible option if you would rather buy once than store several duvets. Use the 4.5 tog layer alone in summer, the 9 tog alone in spring and autumn, and both together - 13.5 tog - in deep winter.

The trade-off is that even the lightest layer of an all-season set is 4.5 tog, so committed hot sleepers may still want a dedicated 1 to 2.5 tog summer duvet for the warmest weeks. If storage is not an issue, owning a separate summer and winter duvet gives the widest comfort range; if it is, an all-season set covers three of the four seasons from one purchase.

Tog decides how warm a duvet is; the fill and cover decide how it handles heat and moisture once you are under it. Two duvets at the same tog can feel very different if one breathes and the other traps humidity.

Natural fills tend to regulate temperature best. Wool is naturally breathable and wicks moisture, so it rarely feels clammy. Down and feather are light and breathable, though warmer at higher fills. Silk is light and cool against the skin. Among covers, cotton and cellulose fibres such as Tencel lyocell (a soft fibre made from wood pulp) breathe better than tightly woven polyester.

Synthetic fills - microfibre and hollowfibre - are cheaper, easy to wash and a common choice for allergy sufferers, but denser synthetics can trap heat and hold moisture, which is why a budget synthetic duvet can feel warmer than its tog suggests on a humid night. If you sleep hot, pick a low tog first, then favour a breathable natural fill or a cotton or Tencel cover.

Three bedding numbers get confused constantly. Tog is the warmth of the duvet. Thread count describes the cover fabric or your sheets - the number of threads per square inch - and affects feel and durability, not warmth. Fill power measures the loft of down, or how much space a given weight of down fills; higher fill power gives the same warmth from less weight, but it is a quality measure, not a warmth rating.

For staying cool in summer, tog is the number that counts. A high thread-count sheet over a low-tog duvet is still a cool setup; a low thread-count sheet over a 13.5 tog duvet will still leave you too warm.

A few habits leave people too warm in summer. The most common is buying by thickness - a plump duvet looks cosy in the shop, but loft says nothing about tog, and a fluffy 10.5 tog duvet will still overheat you in July.

The next is using one duvet all year. A 10.5 or 13.5 tog duvet is built to hold winter warmth in, so keeping it on through summer is a leading cause of night-time overheating. The fix is simply a lower-tog duvet for the warm months.

Two more are worth avoiding: ignoring the room - the same tog feels very different in a cool north-facing bedroom and a hot loft, so match the duvet to your actual room rather than a generic chart - and, if you share a bed, forgetting that partners differ. When one of you sleeps hot and the other cold, a dual-tog duvet (a different rating on each half) or two separate duvets keeps both of you comfortable.

If you would rather not buy a second duvet, a few changes to the rest of the bed help. Switch to breathable cotton or linen sheets, which move heat and moisture better than polyester blends. A cooling mattress topper or a cooling pillow tackles the heat that builds up underneath and around your head, where a lighter duvet cannot reach. One layer worth reconsidering in summer is a weighted blanket: the weight comes from an extra filled layer, so it insulates as well as it soothes, and the blanket that helps you settle in January is quietly adding warmth in July. If you sleep under one year-round, how heavy a weighted blanket should be covers picking the weight, and why defaulting to the lighter option is usually right.

Airflow does the rest: a fan, an open window with the door ajar for a cross-breeze, and blackout that keeps the day's sun out all help the room shed heat before bedtime. Bedding and room work together - a low-tog duvet in a hot, stuffy room still leaves you too warm.

A summer duvet lasts longer and performs better with a little care. Check the care label before washing - many synthetic summer duvets are machine-washable at 40C, while down, feather, silk and wool duvets often need specialist or gentle cleaning. Wash the cover regularly, and air the duvet itself on a dry day to keep the fill lofty and fresh.

When the seasons turn, store the summer duvet clean and fully dry in a breathable cotton bag rather than a sealed plastic one, which can trap moisture. Keeping natural-fill duvets out of a tightly compressed vacuum bag for long spells helps them hold their loft, so the duvet performs the same next summer.