If the density of pound per cubic foot is known, then one formula is to multiply the density by 0.0135 (27/2000). Another rule of thumb is cubic yards x 1.4 = tons. The formula varies according to the material type and density.
It is a difficult conversion to calculate, because cubic yards are volumetric measurements and tons are weight measurements. The bulk density will determine the weight per cubic yard. Calculators can be found on the web to assist in completing this type of conversion, such as the one available at unitconversion.org.