assuming the sandbox is a circle isnce you gave diameter
volume=area of base times depth
area of base=circle
circle=pi r^2
r=d/2
d=14
r=14/2
r=7
circle=pi7^2
circle=49ipi
volume=2 times 49pi
volume=98pi ft^3
if we aprox pi to 3.14 then
98 times 3.14=279.46 ft^3