it depends how thickly you lay it.
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about 71 cubic metres of gravel i think
Gravel is bought in cubic quantities, or by weight. You ask "to fill" but you only give a square measure. The volume of gravel required wil depends on the depth you wish the gravel to be. To fill 81 square metres to a depth of 5 cm would require 4.05 cubic metres of material.
16 cubic yards
218.75 cubic feet or 8.1 cubic yards
You will need about 65.185 cubic yards (83.6 tons) of gravel for this area.