How Duvet Tog Ratings Are Measured: The BSI Standard

calendar Jun 25, 2026 / · 6 min read · togs bedding tog-rating  ·
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A tog is a unit of thermal resistance — and it is measured in a laboratory

The number printed on a duvet label is not a marketing grade or a rough guide. A tog is a defined physical unit of thermal resistance, and the value assigned to a duvet is the result of a standardised laboratory test rather than an estimate by the manufacturer. One tog equals 0.1 square metre kelvin per watt (0.1 m²·K/W) — a measure of how strongly a material resists the flow of heat through it. The higher the figure, the more the duvet slows heat escaping from your body to the cold air above the bed, and the warmer it sleeps. Understanding how that number is produced is the difference between buying a duvet on faith and buying one on evidence.

This explainer walks through what a tog is, the British Standard that governs the measurement, the apparatus that performs it, and what the rating does — and does not — tell you as a buyer.

What the tog scale actually represents

The tog was devised as a deliberately user-friendly unit of insulation for the textile trade. Rather than expressing thermal resistance in the small, awkward decimals of pure SI units, the tog scales it up so that everyday bedding lands on convenient whole and half numbers. A summer duvet sits around 4.5 tog; an all-rounder around 9 to 10.5 tog; a winter duvet at 13.5 tog; and the warmest specialist duvets reach 15 tog. Because the unit is standardised, those numbers are directly comparable across brands: a 13.5 tog duvet from one maker resists heat loss to the same degree as a 13.5 tog from another, regardless of fill or price.

That comparability is the whole point. Tog is to warmth what a clothing size ought to be to fit — a common yardstick. The reason it works for duvets, where clothing sizing famously does not, is that the figure comes from a measured test, not a designer's judgement.

The British Standard behind the number

The measurement is defined by the British Standards Institution. The relevant standard, BS 4745, sets out how the thermal resistance of textiles is determined — the test method, the apparatus and the conditions — so that any accredited laboratory measuring the same duvet should arrive at the same tog value. The standard specifies plate-based methods for measuring how much heat passes through a sample under controlled conditions, which is then converted into a tog figure. The full standard is published by BSI and catalogued in standards repositories such as the ANSI Webstore listing for BS 4745; the document itself is the authority manufacturers test against.

Because the method is standardised, the tog rating carries a small, defined tolerance rather than being treated as exact. In practice a modest margin of error is accepted on the warm side of the figure, which is why two duvets labelled identically can feel marginally different while both being honestly rated. The standard is what stops the number drifting into pure marketing.

How the test physically works

The instrument at the heart of tog measurement is the togmeter — a guarded hot-plate apparatus whose lineage traces back to the Shirley Togmeter, developed by the Shirley Institute in the 1940s specifically to rate the thermal resistance of textiles. The principle is straightforward. A sample of the bedding is placed between a heated plate and a cooler face. The apparatus holds the warm plate at a steady temperature and measures the heat flow required to maintain that temperature difference across the sample. A material that lets heat through quickly needs more power to hold the gradient; a well-insulating duvet needs less. From the measured heat flux and temperature difference, the thermal resistance — and therefore the tog value — is calculated.

The detail that matters for buyers is that the test measures the assembled duvet's resistance, not just the raw fill weight. Loft (how much air the fill traps), construction and casing all affect the result, which is why two duvets of the same fill weight can carry different togs. The number reflects insulating performance as built, not a proxy like grams of down.

What the rating tells you — and what it doesn't

A tog rating answers exactly one question: how warm is this duvet? It is silent on everything else that makes a duvet good or bad. It says nothing about whether the fill is natural down or synthetic hollowfibre; nothing about breathability or moisture handling; nothing about weight, drape or whether you will overheat. Two 10.5 tog duvets can feel completely different in the hand and through the night while being identically warm by the standard, because warmth and feel are separate properties.

That is the single most useful thing to carry into a purchase. Choose the tog for the season and your bedroom temperature first — our tog rating chart by season maps the numbers to the calendar — and then choose the fill type for how you sleep. If you run hot, a lower tog or a breathable fill matters more than chasing a high number; the case for a light option is laid out in our guide to the 4.5 tog summer duvet. The tog gets you to the right warmth; the fill gets you to the right comfort.

Tog, R-value and clo: the same idea in different units

Tog is not the only way to express thermal resistance, and it helps to know where it sits among its relatives. The clo, used widely in clothing and physiology research, measures roughly the insulation of a typical indoor outfit; one clo is approximately 1.55 tog. The R-value, common in building insulation and increasingly on outdoor sleeping mats, measures the same underlying property — resistance to heat flow — on a different scale. All three answer the identical question of how strongly a material slows heat loss; they simply package the result for different industries. The reason bedding settled on the tog is convenience: the scale was deliberately chosen so that real duvets land on tidy numbers between about 1 and 15, which is far friendlier on a shop label than the small SI decimals or the building trade's R-values would be.

This is also why you should be wary of any duvet sold without a tog figure at all. A reputable manufacturer tests to the standard and prints the result, because the number is cheap to obtain and reassuring to buyers. A vague claim like "extra warm" with no tog attached is a claim that has not been measured — exactly the marketing-grade language the standard exists to replace.

Putting it to use

The takeaway is reassuring: the tog on the label is one of the few genuinely reliable specifications in bedding, because it is a measured lab result governed by a published standard rather than a sales claim. Trust the number for warmth, compare it freely across brands, and spend your judgement on the things the standard does not cover — fill, casing and how the duvet actually feels. For a worked example of choosing at the warm end of the scale, see our comparison of the 13.5 tog winter duvet.

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