Multiply the number of therms by 100,000. There are 100,000 British thermal units in one therm. For example, if there are .036 therms, then .036 * 100,000 is 3,600 BTU.
Find the heat content per cubic feet of the material. This value can be found online and in many scientific references. In this example, natural gas produces 1,030 BTU per cubic foot.
Divide the value from step 1 by the value from step 2. In this example, 3,600 BTU / 1,030 BTU/ft^3 yields 3.5 ft ^3.
Divide the value from step 3 by 35.3. There are 35.3 feet cubed in one meter cubed. In this example, 3.5 ft ^3 / 35.3 yields .0099 m^3.