Luban rule
Check whether a door, bed or altar size lands on a good or bad segment of the Lỗ Ban rule, with the arithmetic shown. Pick what you are measuring and the matching rule is used; on a bad segment you get the nearest good sizes, or every good size in a range.
Where this comes from
The measurement and the division with remainder are arithmetic. Which sectors are lucky is a convention of the Lỗ Ban rule used in carpentry and building, in three lengths of 52.2cm, 42.9cm and 38.8cm. Sector names follow the Chinese characters cut on the rule. The Lu Ban Jing itself records only the eight-character rule (Tài, Bệnh, Ly, Nghĩa, Quan, Kiếp, Hại, Cát), the Menguang rule, one chi four cun four fen of the carpenter's foot long; the 52.2cm and 38.8cm rules are those in circulation in Vietnam and are not in the book.
The Luban rule is a traditional measuring convention with no scientific basis. Building dimensions must still follow engineering standards. Division boundaries are closed at the lower end and open at the upper.
This tool is part of Feng shui. See every tool and article on Feng shui.
Classical allusion
“Cư trần lạc đạo thả tuỳ duyên, cơ tắc xan hề khốn tắc miên.”
Translation: Dwell in the world, delight in the Way, and follow what comes: eat when hungry, sleep when tired.
Trần Nhân Tông · Rhapsody on Dwelling in the World and Delighting in the Way
Next
House direction lookup
You have a good measurement. House direction shows which directions suit the owner by the Eight Mansions method.
Open House direction lookupWhere the schools differ
Sector names on the 42.9cm rule
1 more
Length of one sector on the 52.2cm rule
What no one has shown
No study has shown that a dimension falling in a lucky sector brings anything. Real dimensions must still follow technical standards and practical need.
Read on
Ask Nhật Nguyệt AI
Learn more
How many Lỗ Ban rules are there, and how do you measure correctly?
The Lỗ Ban rule is really three rules, each with its own cycle length and set of segments. The 52.2cm thông thủy rule has 8 segments and measures openings such as doors, windows, vents and light wells, taken as the clear inner span. The 42.9cm dương trạch rule has 8 segments and measures solid pieces such as beds, cabinets, tables and steps, taken over the outside. The 38.8cm âm phần rule has 10 segments and measures altars, ritual objects and graves.
The same number lands on three different segments on the three rules, so the usual mistake is using the wrong rule. Here you pick what you are measuring and the rule is chosen to match. If a size lands on a bad segment you get the nearest good sizes with how many millimetres to add or trim; while designing, enter a range to get every good size in it.
The method: convert the size to millimetres, divide by the length of one cycle and keep the remainder, then see which segment the remainder falls in. For an 820mm door on the thông thủy rule: 820 divided by 522 leaves 298; each segment is 65.25mm long, so 298 falls in the fifth segment, Phúc Lộc. A size exactly on the line between two segments counts toward the one above.
Vietnamese sources do not fully agree. On the 42.9cm rule, one source calls the sixth and eighth segments Nạn and Mạng, another Kiếp and Bản; the site follows the original Chinese set 財病離義官劫害本, that is Kiếp and Bản. Many tables also round each segment of the 52.2cm rule to 65mm instead of 65.25mm, and the error builds up enough to push a size near a line into the next segment.
The Lỗ Ban rule is a convention of carpentry and building. Real dimensions still have to follow technical standards and actual use: a door on a good segment that is too narrow to get a wardrobe through is still a bad 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 Lỗ Ban rules are there, and which one applies?
Three. Thông thủy 52.2cm for openings such as doors, windows, vents and light wells. Dương trạch 42.9cm for solid pieces such as beds, cabinets, tables and steps. Âm phần 38.8cm for altars, ritual objects and graves.
For a door, do I measure the clear opening or include the frame?
Measure the clear opening, the space you actually walk through, without the frame. Including the frame usually adds a few centimetres, enough to move the result into another segment. Beds and cabinets are the opposite: measure the outside.
What if my size lands on a bad segment?
You get the nearest good sizes on both sides, wider and narrower, with the difference in millimetres. While designing, enter a range, say 80 to 90cm, to get every good size in it.
Why do two lookup tables disagree on the same number?
Usually for one of three reasons: the two tables use different rules for the same object, one table rounds the segment length, or they treat a size exactly on a line differently. Here, a size on a line belongs to the segment above.
Can an altar be measured with the 42.9cm rule?
By trade convention, no. Altars, ritual objects and graves use the 38.8cm âm phần rule with 10 segments. The same number gives a completely different segment on the wrong rule.
Does the Lỗ Ban rule have a scientific basis?
No. It is a traditional convention of carpentry and building, best taken as cultural reference. Real dimensions must still follow technical standards and practical needs.