Headpans are not a standard measurement because they are available in various size and shape. You can fill the head pan 100% to 140%. But to answer you, a medium head pan is approximate 0.70 ft.³
One cubic meter volume is equal to 35.31 ft.³
So this will depend upon the volume of the headpans how much you fill into one head pan.
Better way would be to make 1‘ x 1‘ x 1‘ standard box of any wood or MS sheet with the handles and you can measure correct quantity.
අන්තර්ජාලය ඇසුරෙන්
https://www.quora.com/How-many-headpans-can-make-one-cubic-metre-of-sand
හැබැයි ලංකාවේ තාච්චියක 0.7 cuft අල්ලන්නේ නැහැ.