• British standard trays (internal measurements 344 x 214 x 52 mm)
• 20 x standard size seed trays without drainage holes
• The manufacturer states these are "formed from 75 mm thick recycled high impact polystyrene". 
• These are not the solid rigid trays that you can buy. There is a degree of flexibility.

With a little careful handling, these can be used over and over again - wash them after use & keep out of direct sunlight when storing them. All in all an excellent economy buy.