How Duvet Tog Ratings Are Measured: The BSI Standard

Jun 25, 2026 / Oct 5, 2026 · 7 min read · togs bedding tog-rating ·

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The number on the label comes from a lab test

A tog is a unit of thermal resistance: how strongly a material slows the flow of heat through it. The figure on a UK duvet label is the result of a laboratory test carried out to a British Standard, BS 4745. That makes it one of the few numbers in bedding you can compare across brands. It also has limits that most labels do not mention.

This guide explains what one tog is, how the test is done, and what the rating can and cannot tell you when you buy a duvet.

What one tog is

The unit has a precise definition. According to Wikipedia's article on the tog, one tog equals 0.1 square metre kelvin per watt (0.1 m²·K/W). Put another way, the thermal resistance in togs is ten times the temperature difference, in °C, between the two faces of a material when one watt of heat flows through each square metre of it.

The tog was created in 1946 by F. T. Peirce and W. H. Rees of the Shirley Institute in Manchester. In their paper on heat transfer through textiles, they set the size of the unit "so that practical clothing may be described conveniently by a range of small integers." The same reasoning is why duvets land on tidy numbers rather than long decimals.

Other units measure the same property:

R value (RSI): the scientific base unit, 1 m²·K/W. One tog is 0.1 of it.
Clo: a US clothing unit, equal to 1.55 tog. Wikipedia notes it is described in the American standard ASTM D-1518.

All three answer one question: how well does this layer slow the loss of heat? Bedding in the UK uses the tog.

The British Standard behind the test

The test method is set out in BS 4745, published by the British Standards Institution. The current edition is BS 4745:2005, which replaced the 1990 version. Its full title, as listed on the ANSI Webstore, names three methods: a two-plate method at fixed pressure, a two-plate method at a fixed opening, and a single-plate method. The listing also says the method suits materials up to 20 mm thick; above that, heat lost from the edges of the sample starts to distort the result.

The full standard is sold by BSI and is not free to read, so most buyers only ever see its result: the tog on the packaging.

How the test works

The apparatus traces back to the Shirley Togmeter, which the Shirley Institute launched in the 1940s. Wikipedia describes it as "the standard apparatus for rating thermal resistance of textiles, commonly known as the Tog Test", and says it measures a sample of textile in one of two ways:

Between two metal plates, the set-up used for underclothing.
Between one metal plate and free air, the set-up used for outer layers.

The principle is simple. One side of the sample is kept warm, and the apparatus measures how much heat passes through the fabric to the cooler side. A fabric that lets heat through easily has a low resistance and a low tog. A fabric that holds heat back has a high one.

What the rating tells you

The tog tells you how warm a duvet is, and lets you compare that warmth across brands. Wikipedia notes that British duvets are sold in steps of 1.5 tog, from 3.0 tog for summer up to 16.5 tog for extra-warm. The testing laboratory IDFL gives the main British Standard range as 4.5 tog for a summer duvet up to 15.0 tog for very cold winters. It adds that 13.5 tog is the most common rating in the UK and Ireland, and that 12.0 and 10.5 tog duvets are becoming more popular as more homes have central heating.

Wikipedia also gives adult comfort bands: 1.0 to 2.5 tog is minimal, 3.0 to 4.5 tog is summer, 7.5 to 10.5 tog is spring and autumn, and 12.0 to 13.5 tog is winter.

Because tog values add up, some makers sell two duvets as one set. Wikipedia describes the usual pairing: one duvet of about 4.5 tog and one of about 9.0 tog. Used alone, they cover summer and spring or autumn. Joined with press studs or Velcro at the corners, they make a 13.5 tog winter duvet.

For choosing by season, our tog rating chart by season matches each number to the time of year.

What the rating does not tell you

This is where the label says less than it seems to.

The number is a minimum. Wikipedia states that the values printed on British duvets "are a minimum; actual values may be up to 3 tog higher." Two duvets with the same label can differ by that much.

The test assumes no cover. Wikipedia also notes the stated values assume there is no duvet cover trapping extra air. A cover adds some warmth the label does not count.

The test uses a new duvet lying flat. IDFL points out that tog ratings "are calculated in a laboratory using a new quilt on a flat surface." You are not flat, and how a duvet drapes around you matters. IDFL adds that after a few nights a duvet is no longer new, and that "poor quality quilts lose their tog ratings fast, particularly if they're washed." Its conclusion: a tog rating "is at best only a rough guide to how warm you'll be."

The number says nothing about quality. IDFL is direct about this: "of itself the tog rating tells you nothing about a quilt's quality." It does not tell you whether the filling is down or synthetic, how the duvet is made, or how long it will last.

Tog beyond duvets

The same unit appears on other things that keep you warm. Wikipedia lists duvets, sleeping bags and carpet underlay as common examples. Baby sleeping bags are a familiar one in the UK. IDFL's page notes that Grobag baby sleeping bags are tested to BS 4745 with a margin of 0.5 tog on the upper side, so a bag sold as 2.0 tog will not exceed 2.5 tog. It also says a first-size baby sleeping bag in winter should not be more than 4 tog, and gives 18 °C as the ideal room temperature for a baby. For young children, a lower number matters more than for adults, because the aim is to avoid overheating.

How to use the number

Treat the tog as a starting point, not the whole answer:

Pick the warmth band first. Choose a tog range for the season and how warm your bedroom is. A light duvet suits summer; our 4.5 tog summer duvet guide covers that end of the range.
Allow for the margin. If you run hot and sit between two ratings, the lower one is the safer pick, because the real figure may be higher than the label.
Then judge quality separately. Look at the filling, the casing and the washing instructions, because the tog cannot tell you how the duvet will hold up.

For a worked example at the warm end of the scale, see our guide to the 13.5 tog winter duvet, and our comparison of 13.5 tog and 15 tog duvets.

Where to buy

Every UK bedding retailer labels its duvets by tog, so you can compare like with like. Dunelm lets you filter its range by tog rating, and Argos and Marks & Spencer publish tog guides alongside their ranges. Compare ratings within one brand where you can, since the label is a minimum.