Weidian sizing
On Weidian the size row is usually written at the selling end rather than inherited from a brand. A listing is often one seller, one style, one batch, and the chart underneath it was typed by whoever put the photographs up. That makes the label a name chosen by a person rather than a value drawn from a standard, and two listings carrying the same letter can describe garments that differ in the ways that matter when the parcel arrives. A batch change is the quiet version of the same problem, and it leaves no trace in the chart.
When a chart is present, the first question is what was measured. A garment measurement is taken with the item lying flat and is a statement about the cloth. A body measurement is taken around a person and is a statement about a fit the seller has in mind. Charts rarely say which one they are, and a chart done correctly can still answer a question you did not ask. Reading the column heading is not enough; the units, the tape position and the category all change the meaning. Two charts can be honest and still describe the same garment differently.
The apparel size chart on this site takes the opposite route. Its figures come from a community compilation of sizes measured on public product pages, and the page labels that source together with a check date. This site has not independently verified those numbers, and it says so where they appear. Where no measured sample exists for a category and a market, no row is shown at all, because a guessed row would look identical to a measured one on the page. An empty row is easier to act on than a filled row nobody can trace back to a source.
The practical move is to start from an item you already own. Measure that garment in the way the listing says it measures, put your own number into the size converter, and take the suggested label with the deviation note attached to it. Then read the seller chart as a second opinion rather than as the primary source. One number you took yourself is worth more than three columns somebody else typed. It is also the only measurement that survives a listing being taken down.
Taobao sizing
Taobao listings tend to be brand-led, which changes what a chart is worth. A store selling one brand often reuses a single chart across many products, so a chart read once can be reused inside that store for as long as the brand holds its cut. The reuse stops at the store boundary. A chart copied into a different shop, or a different brand, is a new claim with no history behind it and no obligation to match the first one. A chart that travels without its measurements is a rumour with a table drawn around it.
Brand sizing drifts by design. Two brands can print the same letter on garments cut differently, and neither is wrong, because the letter is a name and not a measurement. That is why a chart which lists a body measurement is really a statement about how the brand expects the garment to sit, including the ease it builds in. Reading that as a garment measurement produces a number that is too large, and reading a garment measurement as a body measurement produces one that is too small.
Label conventions add a third layer. A listing may carry a letter label, a numeric label, or a Chinese numeric label, and a chart written for one market may print a column for another market that was never measured. A printed conversion column is a convenience for shoppers rather than a reading taken from the item. Unless a column carries its own source and date, it is best treated as decoration and left out of the decision. A decoration column is not evidence, and treating it as evidence moves the answer in a direction nobody chose.
The method that survives all of this is narrow on purpose. Read the chart for the exact garment and the exact category, not for the brand in general. Note where the chart says the tape was placed, and keep your own measurement as the anchor rather than the chart. Convert that number through the size converter, which returns a suggested label with an explicit warning that manufacturers differ, rather than a promise about a specific listing.
1688 sizing
The 1688 side of the same trade serves buyers who are reselling, and the size row reflects that audience. A listing often covers a range of sizes in a compressed line, or offers a single free size that is meant to fit a spread of people rather than a measurement. Wholesale pages are written to move volume, so the part a retail shopper needs most, the measurement of one garment, is frequently the part left out.
Suppliers also vary more than brands do. Two suppliers working from the same style can use different cloth, a different cut and a different finishing allowance, which means the size row is only ever a statement about the supplier in front of you. A chart copied from one supplier to another is a coincidence rather than a specification. The further a listing sits from a consumer-facing shop, the less of its size row was written for somebody who intends to wear the item.
Where a chart does exist, it is usually a garment measurement taken flat, with a tolerance stated as a range. That range is a claim about how much the finished items vary, and it is not a measurement of the item that will be put in your box. It is still useful, because it tells you how much room the seller expects between two items cut from the same pattern. It tells you nothing about which end of the range your parcel will land on.
What helps here is a record rather than a conversion. If a measurement of the specific item exists, write down where it came from and when it was taken. If it does not, the label stays unverified, and the honest output is a blank rather than a substitution. The apparel size chart follows that rule already: no measured sample for a combination means no row for that combination, and the centimetre figures are left to whatever you type into the input. Whatever number you enter there is yours, and it stays comparable next season.
The traps between them
The first trap is that a label is a name. Three marketplaces can print the same letter on three garments that fit differently, and none of them has to be wrong, because the letter was never a length. What travels between marketplaces is not the label but the number you took yourself. Anything else is a translation of an intention held by somebody else, and intentions are not standardised across a border.
The second trap is the difference between flat and body measurements, which is the single most common way a correct chart produces a wrong decision. A flat measurement describes the cloth as it lies; a body measurement describes a circumference with room built in. Choosing one when the chart used the other moves the answer in a predictable direction, and the size of that movement is exactly what most charts leave unsaid.
Shoes add their own version. Three label systems sit on the same listing, an insole can be measured instead of a foot, and a chart built on one method is not interchangeable with a chart built on another. The measurement you actually own is the length of your own foot, and the size converter is built to take that number and return a suggested label with the deviation warning attached. Everything after that is a check against the listing rather than a conversion.
What stays constant across all three marketplaces is short: your own measurement, the date it was taken, and the source of any chart you used. Keep those three and a reorder becomes a comparison instead of a fresh guess. One limit belongs on the record too. Every one of the ninety-three records in the current window traced back to Weidian, so the structure described here is read from one marketplace, and nothing on this page carries measured evidence from the other two. The patterns hold across markets; the numbers stay with whoever takes the measurement.