I would place it in a container full of liquid after measuring the liquid.
I would measure the amount of liquid displaced by the object.
That amount is equal to the volume of the irregular object if it is fully submerged in the liquid.
To find the density of a regular shaped object, you can calculate it by dividing the mass of the object by its volume. For irregular shaped objects, you would measure the mass of the object using a scale, then measure its displaced water volume when submerged in a graduated cylinder. The density can then be calculated by dividing the mass by the volume of water displaced.
Displacement
For an irregular shaped object that cannot be measured and then volume calculated with a formula.
I would place it in a container full of liquid after measuring the liquid. I would measure the amount of liquid displaced by the object. That amount is equal to the volume of the irregular object if it is fully submerged in the liquid.
It depends on what object you are trying to measure. Displacement is appropriate for a medium sized, non-soluble object, of any shape. It would be pretty useless if you wanted to measure the volume of a concert hall or a needle! And it would not work for a sugar cube either. A micrometre would be appropriate for small objects with simple geometry, such as a needle or possibly a sugar cube. But is would be no use for a concert hall nor an irregular object. A ruler would be appropriate for large objects with a simple geometry such as a concert hall. But it would be no use for a needle, nor for an irregular object.
To find the volume of an irregular solid using the overflow can method, you would first fill the overflow can with water and measure the initial volume. Next, you would submerge the irregular solid in the overflow can, causing the water to overflow. Measure the new volume of water in the overflow can with the solid submerged. Finally, you would subtract the initial volume from the final volume to find the volume of the irregular solid.
That sounds too difficult. Instead, if the item can handle being submerged in liquid, you could just measure how much liquid it displaces when submerged.
To find the density of an irregular shape, you would first measure its mass using a scale. Then, you would need to measure its volume using a displacement method or by using water displacement. Once you have both the mass and volume, you can calculate density by dividing the mass by the volume.
You place it in water to see the volume of water it displaces. Fill a large, graduated measuring cylinder to about halfway with water (say to 50mL) Put the irregular solid in, and measure the volume it reads (solid + water). (say it reads 80mL) So the volume of the irregular solid will be: volume(solid+water) - volume(water). For example, the volume of the water was 50mL, and when the solid was added, the volume increased to 80mL. The volume of the solid would be 80mL - 50mL. So it would be 30mL.
If possible, fully immerse the object in water, and measure the volume of displaced water.
Measure the volume of the object which is same as the volume of the displaced liquid in which that sinks. And measure the mass of the same object from a weighing machine. then use density = mass/volume
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.