A ruler has length and therefore measures lengths not volumes.
A metric ruler measure a legth not a volume.
Ruler
you would use a ruler.
Volume of a rectangular solid. V = lwh
using a cubic ruler
You don't
A ruler is used to measure length and does not account for the height or width of an object, which are necessary to calculate its volume. To find the volume of a paperclip, you need to measure its dimensions in three dimensions using a tool such as a caliper or by using a water displacement method.
A ruler or tape measure would be the best tool. You cannot measure the volume directly. First, measure the length, width and height with the ruler and use the formula of volume of cubiod (L x W x H)
Because most rocks are not made up of common geometric shapes, it would be difficult or impossible to find the volume of a rock using a ruler; there would be no easy way to measure the rock's irregular volume. However, by measuring the volume of liquid a rock displaces, its volume can easily be determined.
When the object is a regular, geometric shape.
l times width times height