Bone weighing (称骨) turns a birth date and hour into a "weight" in the old units of 两 and 钱 (one 两 is ten 钱), then matches the weight to a four-line verse. It is a folk practice that still circulates widely, in almanacs and online. The verses predict nothing, and this guide does not read them. What can be measured is the arithmetic behind them: how the weight is added up, how often each weight occurs, and where the tables come from.
The most common result is 3两9钱, and the middle result is 4两. We added up the weight for every hour of every day from 1950 to 2009 — 262,980 birth times — and 3两9钱 came up for 5.4% of them, with nearly half falling below 4两 and about one in seven reaching 5两 or more. Our bone weight calculator works out the weight for any birth date and hour.
How the weight is added up
The final weight is the sum of four values drawn from separate tables for the birth year, lunar month, lunar day, and two-hour birth period (时辰).
| Table | Entries | Lightest | Heaviest |
|---|---|---|---|
| Year, by its place in the 60-year cycle | 60 | 5钱 | 1两9钱 |
| Lunar month | 12 | 5钱 | 1两8钱 |
| Lunar day | 30 | 5钱 | 1两8钱 |
| Two-hour period | 12 | 6钱 | 1两6钱 |
Take someone born on 15 August 1990 between 7 and 9 in the morning. In the lunar calendar that is the 25th day of the sixth month of the 庚午 year, in the 辰 period. The four weights are 9钱 for the year, 1两6钱 for the month, 1两5钱 for the day and 9钱 for the hour, which add up to 4两9钱.
The smallest possible total is 2两1钱 and the largest is 7两1钱. The traditional verses run one step further, to 7两2钱, but no combination of the four tables reaches it, so that verse can never be anyone's.
How often each weight comes up
We calculated the distribution by taking every date from 1 January 1950 to 31 December 2009, converting it to the lunar calendar, and finding the weight for all twelve two-hour periods. This produced 262,980 results from 21,915 unique days. Using real dates matters, because not every lunar month has a 30th day, and a count that assumed it did would give the 30th twice its real weight.
| Result | Share of birth times |
|---|---|
| Most common weight: 3两9钱 | 5.4% |
| Next: 3两8钱, then 4两 | 5.2% and 5.1% |
| Below 3两 | 8.1% |
| Below 4两 | 46.6% |
| 5两 or more | 14.3% |
| Above 6两 | 1.0% |
The middle weight is 4两, and the average is just over 4两. The shape follows from the tables: each of the four parts can land on a light or a heavy value, and most birth times combine some of each, so the totals bunch in the middle and thin out towards the two ends.
The heaviest verses are correspondingly rare. A weight above 6两 requires high values from all four tables simultaneously, an alignment that occurs in only about one birth time per hundred.
The tables do not all agree
We compared the weight tables published on three widely used websites, cell by cell: all 114 entries across the four tables. They agree on 113. The one difference is the 20th day of the lunar month, which two of the tables weigh at 1两 and the third at 1两5钱.
Our calculator uses 1两5钱. The difference is minor, shifting the share of birth times reaching 5两 or more from 14.3% down to 13.9%. In either case, the most common weight remains 3两9钱. The calculator's page notes that results can differ between versions of the table.
Who wrote the tables
The reading is traditionally attributed to 袁天罡 (written 袁天綱 in the Tang histories), a face-reader of the early Tang dynasty. The Tang official history, the Old Book of Tang, has a biography of him. It describes him reading people's faces and bone structure (骨法) and making predictions from what he saw, but it says nothing about weighing birth dates, and no Tang-era text containing these tables has been found. The attribution is a tradition, not a record.
How we counted
Lunar dates were computed with an open-source implementation of the Chinese calendar, using the lunar year's cyclical name for the year table. Our count weighted each of the 21,915 days (1950–2009) and each two-hour period equally. This assumes births are spread evenly across the calendar and the clock, which is a simplification, as real birth rates fluctuate. The three weight tables were fetched from their websites and compared entry by entry. The Old Book of Tang passage was read in the full text of chapter 191.
