Ordering roof shingles just based on the roof area means you will way short of what you need.
Several bundles of extra shingles are needed as starter shingles along the eaves of the roof especially to use on large roofs. Starter shingles main purpose is to shed water that accumulates between the joints of roof shingles. Sometimes you will need extra for the rakes too. If you are making your own ridge caps then order extra shingles to use on ridges and hips.
To save some money you can order 3-tab shingles to be used as starter courses or if you have left-over shingles from previous roofing jobs that can also be used. Check with your roofing contractor. Many use starter course for the rake edges too in order to create a neat looking edge. You should not use laminated shingles rake edges as they are too thick. To determine the number of shingles needed as starter courses measure the edges of the roof and rakes and divide by 3 which is length of each shingle.
Shingles need to be cut to fit to size, so waste will be generated and cannot be avoided. Roofing contractors will be cutting shingles to fit near the ridges, walls, valleys, around chimneys and other roofing features, etc. Some cut-offs are big enough to be reused others will need to be discarded as they are too small.
Simple gable roofs generate the least amount of waste. For simple roofs factor one percent as wastage, for complex roofs plan 5 percent as wastage in materials. Complex roofs are those with lot of hips, valleys, chimneys, skylights, dormers and other vents.
3-tab shingles generate the most waste while laminated shingles are nearly not as wasteful. But, as it is difficult to estimate by what factor your waste is reduced it is better to buy the same extra number of shingles as you would buy with 3-tab shingles. You can tell the supplier that you will return any extra un-opened bundles. Laminated shingles have no cutouts, so you have to just seal the shingles by pressing it down and nailing them.
When transporting shingles some of them will be damaged as they are dropped or banged against hard surfaces. Some may be salvaged.
During installation some of the shingles are damaged during the nailing process.
Newbie workers tend to waste more so do careless crew. So, factor extra bundles for these kinds of workers. While really good workers may use maximum one extra bundle for the same job.
Factor in extra shingles as shingles will be laid to overlap each other. You need to remember that on the roof at every location – ridges, valleys, on plain surfaces, starter courses, around chimneys and protruding features, around dips and skylights you need to overlap the shingles for proper sealing off all gaps. This prevents leakages. So, you need extra for overlapping and this will be usually factored in when purchasing roof shingles. Over the ridges and hips the shingles are cut and placed on top to cover the ridge tops. Some of the cut-offs shingles from previous use can also be used to be as ridge cap shingles. You will need length of all ridges and hips divided by 35 to determine the number of bundles of shingles needed.
Else, you can order ridge caps, hip caps air vents, and other companion pieces that come with roof shingles. Many companies insist that you buy three or more of these companion pieces in order for their warranty on the shingles to be valid. If you purchasing these pieces then you can reduce the number of shingles needed accordingly. For some types of shingles like patterned shingles you have to buy the accompanying pieces as otherwise the look of the roof is spoiled.