From

www.mcmaster.com
1) Surface area of enclosure (total sq ft. of walls, floor, ceiling)

2) Heat loss factor. 25% well sealed and insulated, 75% sealed but not well insulated, 125% neither well sealed or insulated

3) desired change in temperature fahrenheit

Multiply for BTU/hr

In your case 936 sq ft x 25% x 40 degrees (pretty toasty) = a little over 9000 BTU

From another HVAC site if you are forced air heating and ventilating:

1) Cubic feet of enclosure

2) Air changes per minute

3) 1.08 (constant factor for energy to heat air and time)

4) desired change in temperature fahrenheit

Multiply for BTU/hr

Second formula more useful if you are painting, heating a public space that requires ventilation due to cooking, smoking, etc.

Hope this helps