This site has many: http://www.its-canada.com/reed/insul/in-struc.htm
The unit resistance value of walls (less the insulation) is approximately R4. Add the R of the insulation batt and board to the construction of the separation and the value is simple to calculate. This is a fast way to estimate without calculating drywall, siding, etc. as the estimate comes close. Adding up each layer of wall is more precise, but close. Don't forget to compensate for the R-value of the stud if you use the detailed method.
A Flash4 heat loss calculator is on-line at: http://www.hydronic.net