A plumb bob is a weight on a string that always hangs perfectly vertical (plumb) due to gravity. It is used to check whether a wall, stud or other vertical element is straight. The term “plumb” literally originates from the Latin word for lead — the material traditionally used for the weight.
How does it work?
The principle is simple: gravity pulls the weight straight down. The string always hangs perfectly vertical, regardless of the conditions. By hanging the plumb bob alongside a wall, you can see whether the wall is straight or leans forward or backward.
How to use
1. Hold the string at the top against the wall or stud
2. Let the weight hang freely (not touching the wall)
3. Wait until the weight is completely still
4. Measure the distance from the string to the wall, at the top and bottom
5. Equal distance = plumb. Difference = out of plumb.
Plumb bob vs. spirit level
| Plumb bob | Spirit level | |
|---|---|---|
| Checks | Vertical (plumb) | Horizontal and vertical |
| Accuracy | Very high (gravity does not lie) | Good (if properly calibrated) |
| Over great heights | Better (long string) | Limited by instrument length |
| Wind | Sensitive (string sways) | Not sensitive |
Nowadays the plumb bob has largely been replaced by the spirit level and the laser level, but it remains the most accurate instrument for checking vertical lines.
Related terms
- Spirit level
- Diagonal check