Two silk pillowcases sit in the same shop. One is marked 19 momme, the other 22 momme at a third more, and a listing nearby says 6A grade with no momme figure at all. The shopper reasonably concludes that momme is a quality scale and that 22 beats 19 the way a five star review beats a four.
It is not a quality scale. It is a weight, in pounds, of a piece of cloth cut to one fixed size. The number tells you how much silk is in a given area and nothing whatsoever about how good that silk is.
The unit is a pound of cloth cut to a fixed size
One momme is the weight in pounds of a piece of fabric 45 inches wide and 100 yards long. That is the whole definition. The word comes from a Japanese unit of mass equal to 3.75 grams, which is why the scale looks unfamiliar to anyone raised on grams per square metre.
Convert it once and keep the result. A 45 inch by 100 yard piece covers about 104.5 square metres, and a pound is 453.6 grams, so one momme is roughly 4.34 grams per square metre.
| Momme | Grams per square metre | Where you meet it |
|---|---|---|
| 6 | About 26 | Chiffon and georgette, near transparent |
| 12 | About 52 | Light blouse and lining weight |
| 16 | About 69 | Shirts, scarves, everyday charmeuse |
| 19 | About 82 | Standard bedding and heavier blouses |
| 22 | About 95 | Bedding sold as premium, dresses with body |
| 30 | About 130 | Upholstery, heavy jackets, structured pieces |
Read the middle column and the marketing loses some of its grip. Moving from 19 to 22 momme adds about 13 grams per square metre, noticeable in a pillowcase you sleep on for years and close to irrelevant in a scarf.
Degumming removes a quarter of the raw weight
The reason weight became the number silk is sold on has to do with what happens to the fibre before it is woven. A silkworm spins two fibroin filaments held together in a coating of sericin, a natural gum. Sericin is stiff and dull, and almost every silk you would want to wear has had it boiled off in a step called degumming.
That step costs about a quarter of the raw weight. A 2024 study from the University of Ljubljana, published in the journal Polymers and available in full through PubMed Central, puts the loss at roughly one quarter and describes what mills once did about it. Silk was bought and sold by weight, so a quarter of the value evaporated in a vat between the cocoon and the loom.
Mills sold the lost weight back with tin
From the second half of the nineteenth century into the early twentieth, finishers put the weight back chemically. Silk was loaded with metal salts, tin chlorides most often, in a process called weighting. It restored the missing mass, it thickened the hand, and it gave the cloth a fuller fall that buyers liked.
The Ljubljana team ran the historical recipe and weighed the results. Successive tin weighting cycles raised fabric weight by 7.8, 18.9, 27.7 and 42.2 percent after one, two, three and four rounds, or roughly 8 percent per cycle. Four rounds of chemistry could hand a mill nearly half again as much sellable weight without a single extra metre of silk.
The bill came due decades later. Tin phosphate deposits inside the fibre restrict the movement of the polymer chains, and heavily weighted silks tend to split, then shred, then go powdery. A 2018 master's thesis at the University of Rhode Island tested whether humidification could recover any breaking strength in artificially aged tin weighted habutae, and noted that earlier attempts to reverse metallic salt damage had largely failed.
Restrictions on how much weighting was acceptable arrived in the 1930s, once the deterioration was too obvious to argue with. That history is the reason to keep weight and quality in separate columns in your head. For fifty years or so, the trade sold weight that actively shortened a garment's life.
Grade letters describe the yarn, not the cloth
The other number on silk listings is the grade, usually written 6A, 5A, 4A and downward. That scale is real and it comes from a Chinese national standard for raw silk, GB/T 1797, which classifies reeled yarn on technical properties such as size deviation, evenness and cleanness. It grades the thread before weaving.
So the two numbers answer different questions. The grade speaks to how uniform the filament was when it came off the reel. The momme figure speaks to how much of that filament ended up per square metre of finished cloth. High grade yarn can be woven into a thin 12 momme lining, and mediocre yarn can be woven heavy. Neither number is a substitute for the other, and a listing that gives one and hides the other is telling you which one flatters it.
Note also what a fibre content label is required to say. The Federal Trade Commission's textile labeling guidance asks for generic fibre names and percentages by weight in descending order, and any fibre at 5 percent or more must be named. Fabric weight is not part of that requirement anywhere. When a seller prints a momme figure, it is a voluntary marketing choice, which is precisely why it is inconsistent and occasionally optimistic.
Match the number to the use, not to the price
Weight is a specification. Ask what the garment has to do, then pick the band that does it.
- Bedding you will wash weekly. The 19 to 22 momme band is where sellers converge for a reason: below it, seams and hems take a beating from repeated laundering.
- A blouse meant to hang rather than float. Somewhere around 16 momme gives enough mass to fall from the shoulder instead of clinging.
- Scarves and overlays. Light is the point. Paying up for 22 momme buys weight you did not want.
- Anything tailored or structured. Higher weight holds a shape, which is why heavy silk turns up in jackets and furnishing fabric.
- Listings with no momme figure at all. Ask, and read the answer as a claim rather than a measurement, because nobody is auditing it.
One caution about the top of the range. Heavier silk is not automatically more durable in use. A 30 momme cloth carries more fibre per square metre, but weight says nothing about weave tightness, dye process, or finishing, and a heavy piece finished badly will still snag and mark.
What to do with a momme figure at the till
Convert it before you compare anything. Multiply by 4.34 and you have grams per square metre, the unit every other fabric on your shelf is described in, which lets you set a silk listing beside a cotton shirting whose feel you already know.
Then price the difference honestly. Three momme between two pillowcases buys about 13 extra grams per square metre, and deciding that is worth it for something your face rests on nightly is fair enough. If a dress at 22 momme costs half again as much as the same dress at 19, ask whether you wanted the extra body or simply read the bigger number as the better one. The unit was never designed to rank silk. It was designed to tell a merchant what a bale weighed.