Real Good Toys:    Calculate Shingles

Easiest: check the instructions

To calculate the number of shingles for a roof measure the roof height and length.
Add 1½” to the height for the starter course at the bottom (¼ - ⅓" shingle) and the Boston lap at the top (¾ shingle).
Add ¾” to the length for the cut shingle that starts and finishes each course
Add one inch for every valley (so if you have a gable on the front roof with two valleys, add two inches).
Round up each of the numbers to the next higher full inch – multiply the two numbers together and divide the result by the width of the shingle (the coverage of one shingle):   .75 for import or pine shingles or .84 for cedar shingles (using 1” guidelines), and 1.68 for Playscale shingles (using 1½” guidelines).

Using a different "Reveal": Divide the above calculation by the new shingle spacing measurement. For instance, if using a ¾" reveal (as in the "Cottage Style" example), and your calculation above gave 800 shingles for a 1" reveal (1" guideline spacing), you would divide by ¾"
= 800/.75
= 1066

… that’s how many shingles to buy.  Remember, it is always better to have some left over than to run short, so get extra and keep the leftovers for the next project.