Fill Power, GSM & Tog: How to Spec Duvet and Summer-Quilt Warmth

Fill power, GSM and tog are three different measurements, and you cannot convert directly between them. Fill power measures the loft of down (cubic inches per ounce), GSM measures areal weight (grams per square meter of fill or fabric), and tog measures thermal resistance — the actual warmth of the finished duvet. Only tog measures warmth. The practical rule for a wholesale order: use fill power plus fill weight for down, GSM for microfiber and summer quilts, and tog for anything sold into the UK, EU or Australia — and settle the contract on fill weight in grams.

Fill power — loft quality, down only

Fill power is the number of cubic inches one ounce of down lofts to, measured by a steam-conditioned lab test. Commercial down runs roughly 550–650 (good), 700–750 (very good) and 800–900+ (premium). Higher fill power means the down traps more air per gram, so you reach a target warmth at a lower fill weight — lighter, more breathable, more compressible. It is a quality and efficiency number, not a warmth number by itself, and it is meaningless for microfiber, wool, silk or cotton fill.

GSM — areal weight, the default for synthetics and summer quilts

GSM is grams per square meter. It describes both shell fabric weight and fill areal weight, and it is the standard warmth proxy for microfiber, wool, silk and summer quilts, where fill power does not apply. One caution that causes constant confusion between buyers and factories: a GSM figure can mean fill-only weight or total quilt weight (fill plus shell). Total-quilt GSM runs heavier. Always state which you mean. GSM is comparable within one fill type but not across types — 300 GSM of hollow-fiber microfiber is not as warm as 300 GSM of down.

Tog — thermal resistance, the metric that measures warmth

One tog equals 0.1 square-meter-kelvin per watt of thermal resistance; the unit was devised at the UK's Shirley Institute as a friendlier alternative to SI units. Crucially, tog is measured on the finished article with a guarded hotplate or togmeter (per BS 4745 and the ISO textile thermal-resistance methods), not calculated from weight. The standard scale runs 1.0–4.5 (summer), 7.5–10.5 (spring/autumn and all-season), 12–13.5 (winter) and 15 (extra warm). All-seasons duvets clip two togs together — 4.5 plus 9 equals a 13.5 winter set. Tog dominates the UK, Ireland, EU and Australia; the US and Canada instead read fill power and fill weight.

Season, tog and GSM at a glance

Fill weight is the number you actually order

Fill power, GSM and tog describe the product; the line a factory fills against is grams of fill, with a tolerance — typically plus or minus five percent. Lock the fill weight per size, and with down use fill power to trim it: higher fill power hits the same warmth at fewer grams, cutting both weight and cost. A specification of '700 fill power, 300 g queen ± 5%' is enforceable; 'warm duvet' is not.

How to spec quilt warmth for a wholesale order

You can map fill power, GSM and tog to a season, but you cannot convert one into another. The only warmth number you can guarantee is a lab-measured tog on the finished duvet.

Speccing warmth factory-direct

BeddingTextilePro makes duvets and cooling summer quilts factory-direct from Nantong, China, and quotes in whichever metric your market uses — fill power and fill weight for down, GSM for microfiber and summer quilts, and tog ratings for UK, EU and Australian buyers — with 100-set MOQ, full OEM/ODM and third-party thermal and fill-weight testing on request. Send your target season and market and our team will translate it into a spec and quote within one business day.

Frequently asked questions

Can I just convert GSM to a tog rating?

No. GSM is areal weight and tog is measured thermal resistance — different physics, and the ratio changes with fill type and construction. A GSM-to-tog chart is valid only for one fixed fill and build. If your buyer needs a guaranteed tog, require a lab-measured tog (BS 4745 / ISO 5085) on the finished product rather than a conversion.

Should I spec fill power or fill weight in my purchase order?

Both, because they do different jobs. Fill power grades the down's quality; fill weight in grams, with a tolerance, is the enforceable quantity the factory fills against. Lock fill weight per size and set a minimum fill power — for example 700 fill power at 300 g ± 5% — so you control both warmth and cost.

Is a heavier duvet always warmer?

Not across fill types. More grams of the same fill is warmer, but a 700+ fill-power down duvet can beat a heavier low-fill-power or microfiber one because it lofts more and traps more air per gram. For hot-climate markets, high fill power lets you hit the target warmth at lower weight.

Which warmth metric should appear on the label for each market?

UK, Ireland, EU and Australia expect a tog rating; the US and Canada read fill power and fill weight; GSM is understood everywhere for synthetics and summer quilts. A source factory should be able to state all three so one product can ship to multiple regions.

What tests should I demand before approving bulk?

For down: third-party fill power, species and composition, plus cleanliness and downproofness. For any duvet: measured fill weight, shell GSM, and a finished-article tog or Rct (BS 4745 / ISO 5085 / ISO 11092), plus OEKO-TEX STANDARD 100 for harmful-substance safety and a sealed reference sample matched to the report.

Sources & references

1. 1.Tog (unit) — definition, scale and measurement (Shirley Institute origin)
2. 2.IDFL — TOG Ratings Explanation (BS 4745 method, ±0.5 tog tolerance)
3. 3.IDFL — Explanation of Down and Feather Tests (fill-power method)
4. 4.AWTA Product Testing — Thermal Resistance / TOG value testing (ISO 11092)
5. 5.Sizes.com — the tog unit
6. 6.OEKO-TEX — STANDARD 100 (tested for harmful substances)