What is a Lỗ Ban ruler? Three rulers and when each applies

Carpenters and builders in Vietnam often carry a tape measure printed with red and black bands, the Lỗ Ban ruler (thước Lỗ Ban; 鲁班尺 in Chinese). An owner may want the front door’s width or the altar’s height to “land on a good segment”. It is a measuring tradition in Vietnamese and Chinese culture. This guide explains how the ruler is divided, and does not say what any length will bring.

Three rulers

The tradition uses three rulers, each for a kind of object:

Ruler	Used for	One cycle	Segments
52.2 cm	doors, windows, openings	522 mm	8, each 65.25 mm
42.9 cm	counters, plinths, steps	429 mm	8, each 53.625 mm
38.8 cm	furniture, altars, graves	388 mm	10, each 38.8 mm

Sources also differ a little on which ruler goes with which object.

Which segment a length falls in

Each ruler repeats. Measure a length, count how many full cycles it covers, and see which segment the remainder lands in. On the 52.2 cm ruler, 1,200 mm covers two full cycles (1,044 mm), and the remaining 156 mm lands in the third segment.

So the practical question is usually “if I make it a little shorter or longer, which segment does it move into?” A few centimetres can move a length into the next segment.

Segment names

Each segment has a name, and the tradition counts some as favoured (usually printed in red) and the rest as not (usually black). One published set of names, in Vietnamese:

• 52.2 cm ruler: Quý nhân, Hiểm họa, Thiên tai, Thiên tài, Phúc lộc, Cô độc, Thiên tặc, Tể tướng. Favoured: Quý nhân, Thiên tài, Phúc lộc, Tể tướng.
• 42.9 cm ruler: Tài, Bệnh, Ly, Nghĩa, Quan, Kiếp, Hại, Bản. Favoured: Tài, Nghĩa, Quan, Bản.
• 38.8 cm ruler: Đinh, Hại, Vượng, Khổ, Nghĩa, Quan, Tử, Hưng, Thất, Tài. Favoured: Đinh, Vượng, Nghĩa, Quan, Hưng, Tài.

On a real ruler, each segment of the 52.2 cm and 42.9 cm rulers is split again into four parts. Segment and part names can differ between rulers on sale, so check against the one you are using.

Reading the result

Segment names are labels of the tradition, not promises or warnings. The result says nothing about health, money or luck. A door first needs to be wide enough to walk through and move furniture, and an altar needs to fit its room. Measure for what you actually need.