Every Carrier, including the Post Office, cube all parcels these days.
Dead or cubic weight? ... whichever the greater.
To cube your parcel multiply:
width (m) x depth (m) x height (m) x 250 = weight in kg.
Allow about 8cm each way to the product/s for packing material and box thickness.

Example:
The A42 Art Box below measures 38cm x 48cm x 13cm. It's dead weight is 2.7 kg. 
Add packing and cardboard and it becomes...
0.46m x 0.56m x 0.21m x 250 = 13.52 kg.

Some products, like stretcher bars, require no extra packing. Some, like paint and fine brushes, require three times more packing... (We will rather use more packing than have your order damaged in transit.)