the tool of length is a RULER!
The answer depends on what characteristic the tool is used to measure.
a ruler
Ruler.
Ruler. Micrometers. Caliper.
ft
a ruler is something that measures the length of an object
With a handy tool called tape measure :)
units of length
a tape measure
A ruler can be very helpful.
kilometer/ odometer
You take it and roll it out and use a length measuring tool to measure it's approximate length in yards feet inches centimeters etc... :)