Why GSM doesn't convert to tog without a lab test

Host: If I see a 350 GSM duvet on one site and a 7 tog duvet on another, is there a formula that converts one into the other?

Circadian: There is not. GSM measures how many grams of fill sit in one square meter of shell. Tog measures tested thermal resistance under British Standards Institution method BS 4745. The same GSM figure produces a different tog depending on whether the fill is wool, cotton, or down, because each fiber traps air differently.

Host: So a chart that claims to convert GSM to tog is skipping something?

Circadian: It is skipping the lab test. Tog comes from placing a fabric sample between two heated plates and measuring the energy needed to hold a steady temperature difference. That is a physical measurement per product, not a calculation you can run from a spec sheet. A wool duvet at 350 GSM behaves differently than a cotton duvet at the same weight because wool fiber has a different structure and density.

Host: What does the two-plate test actually measure?

Circadian: It places the sample directly on a heated plate and tracks how much power the plate needs to maintain a set temperature once the fabric is added. Fabrics that resist heat loss better need less added power to hold steady. That relationship produces the tog number. One tog equals the thermal resistance of a medium-quality man's suiting or blanket, and the standard applies across textiles generally, so a tog rating on a blanket can be compared meaningfully against a duvet.

Host: But brands publish GSM numbers all the time without running that test.

Circadian: Correct. GSM is a manufacturing spec. Manufacturers weigh a one-square-meter sample of the finished fill layer and record the result in grams. A duvet listed at 350 GSM packs 350 grams of fill into every square meter of surface area, whether that fill is cotton, wool, or down. The number describes quantity. It says nothing about how that quantity performs once it is sewn inside the duvet.

Host: Why does the fill type matter if the weight per square meter is the same?

Circadian: Because the fibers behave differently under the same conditions. Wool fiber wicks moisture while trapping air in its crimp structure. Cotton batting compresses into a flatter, denser layer. Down clusters form pockets that hold still air. Those are three different mechanisms for thermal regulation, and they produce three different tested tog values at an identical GSM.

Host: So if I am comparing across brands, what number should I trust?

Circadian: Check whether the brand states a tested total fill weight per size rather than a single blanket GSM figure for its whole product line. Circadian publishes the actual total fill weight for each duvet size on the product page. The Organic Cotton Duvet lists about 10 pounds of fill for Queen. The Organic Wool Duvet lists about 9 pounds for Queen. Both are hand-tufted with corner loops that hold the fill in place, and both are Global Organic Textile Standard Certified Organic under license GOTS-10229, covering the fill and the shell together.

Host: That is more transparent than a warmth tier label like cozy or extra warm.

Circadian: It is. Those labels are marketing terms with no testing requirement behind them. A tested fill weight tied to a certified organic fill inside that specific duvet tells you what ships in the size you are looking at, without asking you to estimate warmth from a chart.

Host: What if I just want the warmest duvet possible?

Circadian: Warmest is not the same question as best. Sleeper thermal preference, bedroom temperature, and fill fiber type decide the best weight together. A heavier duvet widens the range of bedroom temperatures a sleeper finds comfortable, but if you run hot or your bedroom stays above 70 degrees overnight, more weight just means more overheating. A peer-reviewed systematic review in the Journal of Sleep Research found that bedding fiber type is associated with differences in measured sleep quality, though the studies it covers compare finished products where fiber, weight, and thickness vary together, so it does not isolate fiber on its own.

Host: So it is not just about piling on more fill.

Circadian: Correct. A hot sleeper in a warm climate needs a lighter total fill weight paired with a moisture-wicking fiber. A cold sleeper in a drafty bedroom needs more total fill regardless of fiber type. Matching a specific duvet weight to that profile is more reliable than shopping by tog alone, because tog alone does not tell you which fill or construction produced it.

Host: Does that mean GSM is useless?

Circadian: GSM is useful for comparing duvets of the same fill type from the same brand, where the fiber and construction stay constant and only the weight changes. It stops being useful the moment you compare across fill types, because the same GSM figure no longer predicts the same warmth. That is why conversion charts fail. They try to shortcut a relationship that only exists per fiber, per construction, per tested sample.

Host: Where does this leave someone trying to choose between two duvets?

Circadian: Start with the fill fiber that matches your temperature regulation need. Wool wicks moisture, cotton holds warmth passively, down traps air in clusters. Then check the tested total fill weight for your bed size, not a single GSM figure stretched across all sizes. A King-size duvet at the same GSM as a Twin weighs more overall because it covers more square meters of shell fabric. The per-size weight is the number that tells you what lands on your bed.

Host: And if a brand does not publish that?

Circadian: That is a transparency gap. A brand that publishes one GSM figure for an entire product line is either averaging across sizes or reporting only the raw fill before quilting, and there is no requirement to disclose which method was used. The brands that break out fill weight per size, Circadian included, are giving you the verifiable spec tied to what ships.