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Luban rule
The three Lo Ban rules, 52.2cm, 42.9cm and 38.8cm, measure different kinds of dimension and divide their cycles differently. A size that lands on a favourable segment of one rule can land on an unfavourable one of another, and two Vietnamese sources disagree at the sixth and eighth segments. The task is to find a range favourable on all three, not to look up one number.
Check door, bed and altar sizes on all three Luban rules with the arithmetic shown and a visual scale. When a size falls on an unfavourable division it names the nearest auspicious sizes, and it can list every good size in a range.
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Three rules, and why picking the wrong one invalidates everything
The Luban rule is not one rule but three, each with its own cycle length and set of divisions. The thông thuỷ rule, 52.2cm across 8 divisions, is for OPENINGS, doors, windows, vents, light wells, and measures the clear internal span, the gap you can actually pass through, excluding the frame. The dương trạch rule, 42.9cm across 8 divisions, is for SOLID objects such as beds, cabinets, tables and steps, measured over the outside. The âm phần rule, 38.8cm across 10 divisions, is for altars, ritual objects and graves.
Choosing the wrong rule is the commonest mistake and it invalidates the whole answer, because one number lands on three different divisions across the three rules. That is why this page does not ask you to pick a rule but asks what you are measuring and picks for you, while stating in one line why that rule was chosen. Anyone who knows the trade can still override it; anyone who does not is not stopped at the very first field.
The arithmetic is published rather than hidden. The size is converted to millimetres, divided by the cycle length to take the remainder, and the remainder divided by one division's width. A 820mm door on the thông thuỷ rule: 820 divided by 522 leaves 298; the rule has 8 divisions so each is 65.25mm wide; 298 falls in Phúc Lộc. Every step can be rechecked on a pocket calculator, which a printed lookup table does not allow.
The most useful part is that the page does not stop at telling you your size is unlucky. When a measurement lands on an unfavourable division, it lists the NEAREST auspicious sizes with the offset in millimetres, so you know how much to widen or trim. The "find good sizes" mode works the other way: give a from–to range and it returns every size in that range that lands well. That is usually the right mode while you are still designing rather than after the piece is built.
One small technical point decides results at the boundaries: divisions are closed at the lower end and open at the upper. A number landing exactly on a boundary belongs to the division above, not below. Tables printed on paper rules rarely state this convention, so two sources can disagree precisely on the numbers sitting on a line.
Finally, this is a traditional measuring convention from carpentry and construction, with no scientific basis. Dimensions must still meet engineering standards, building codes and real use: a door on a fortunate division that is too narrow to carry a wardrobe through is still a badly built door.
| Rule | Cycle | Divisions | Used for | How to measure |
|---|---|---|---|---|
| Thông thuỷ | 52.2cm | 8 divisions | Doors, windows, vents, light wells | Clear internal span, frame excluded |
| Dương trạch | 42.9cm | 8 divisions | Beds, cabinets, tables, steps | Overall outside dimension |
| Âm phần | 38.8cm | 10 divisions | Altars, ritual objects, graves | Overall outside dimension |
Frequently asked questions
How many Luban rules are there, and which one applies?
Three. The 52.2cm thông thuỷ rule for openings, doors, windows, vents, light wells. The 42.9cm dương trạch rule for solid items such as beds, cabinets, tables and steps. The 38.8cm âm phần rule for altars, ritual objects and graves. The page picks the right one from what you say you are measuring and explains the choice, so you need not memorise it.
For a door, do I measure the clear opening or include the frame?
Measure the clear opening, the gap you can actually pass through, excluding the frame. That is what "thông thuỷ" means. This is a common error: including the frame typically adds a few centimetres, enough to shift the result to a different division. For solid items such as beds or cabinets the opposite applies: measure the outside.
What if my size lands on an unfavourable division?
The page lists the nearest auspicious sizes with the offset in millimetres, both larger and smaller, so you know how far to adjust. If you are still designing, the "find good sizes" mode is more convenient: give a from–to range and get every size in it that lands on a good division.
Why do two lookup tables disagree on the same number?
Usually one of two reasons. Either the tables use different rules for the same object, or they treat boundaries differently: this page closes divisions at the lower end and opens them at the upper, so a number exactly on a line belongs to the division above. Printed paper rules rarely state their convention, so disagreements appear precisely at boundary numbers.
Which common door sizes land on a good division?
Rather than memorise a list, use the "find good sizes" mode over the range you are considering, say 80 to 90cm for a room door, and pick from what comes back. That is always correct for the appropriate rule and your chosen unit, whereas a memorised list easily mixes the rules up.
Can an altar be measured with the dương trạch rule?
By trade convention, no: altars, ritual objects and graves use the 38.8cm âm phần rule, which has 10 divisions rather than 8. This is exactly why the page asks what you are measuring before computing, the same number gives a quite different division under the wrong rule.
Does the Luban rule have a scientific basis?
No. It is a traditional measuring convention from carpentry and construction, best treated as cultural reference. Real dimensions must still meet engineering standards, building codes and practical needs; no favourable division justifies a door too narrow or a step too high.