20 mm = 0.020 meters
18 square meters * 0.020 meters = 0.36 cubic meters
I think you should check your units, bro...
meters times meters will give you square meters, not cubic meters. You can't convert from square meters to cubic meters.
You can't convert between square meters and cubic meters.
The area 90 square meters times 0.075 meters gives a volume of 6.75 cubic meters.
3 x 3 x 3.5 = 31.5 cubic meters.
A cubic yard of gravel will cover 81 square feet at 4" of thickness. Multiply the width of the patio by the length of the patio to get your square footage. Gravel is usually sold by the cubic yard.
2.924
1 cubic meter of grave contains 50 square meters at 20mm depth (1000mm / 20mm = 50). 2860 / 50 = 57.2 So you need 57.2 cubic meters of gravel.
744 Square Feet
It depends how thick the gravel is being laid - at 4 inches thick, a cubic yard will cover 81 square feet.
1 cubic yard equals 27 cubic feet. At a six inch depth 1 cubic foot will cover 2 square feet, so 27 cubic feet will cover 54 square feet.
9.26 cubic yards for every inch of depth.
Ten cubic feet will do the job!
92.593 cubic yards for every inch deep.
You will need about 3.7 cubic yards (around 4.75 tons) of gravel.
You'll need about 28.704 cubic yards (37 tons) of gravel.
Plan on about 2.2 cubic yards (2.8 tons) of gravel.