Estimating metal roofing supplies is a task that is often left to the professionals or even ignored and replaced with the eye to thumb rule  if it fits on the end of your thumb, then it will fit on the roof. The problem with guess estimating is that it can cost you thousands in extra materials, or it can slow your project when you have not estimated enough for the job. The average doityourselfer can complete a thorough estimating job on a metal roofing project in one to two hours, depending on the size and scope of the project.
Things You'll Need
 Tape Measure
 Calculator
 Writing Pad
 Pencil

Measure the length of the entire roof. This will include measuring the overhangs or rakes of the roof, as well. You will use this number to multiply with the width of the roof to get a square footage estimate. Be sure to measure twice and write the number down onto your pad.

Measure the width of the roof. Since most roofs are not flat and are either gabled or hipped, you will need to gather the width of all roof sections. Measure the width of each side of a gabled roof and record the measurements. A hip roof is basically the same as a gabled roof but with more sides. Just record each side onto your writing pad. The only difference is that you will need to record each of these sides and number them so as to keep the length and widths together. For example, a hip roof may have four sides, so record "hip side 1  length and width" and keep them together. A gabled roof only has two sides, so there will be no problems with this record. In fact, if you flattened a gabled roof, then you would just have a square of length and width.

Measure any dormers or extensions from your roof and record them accordingly. Keep the length and widths of each section together. You will combine these eventually.

Multiply each subsection times its length and width and record these numbers beside the subsection. Once you have all sections multiplied, then you will add these numbers together to get a total square footage. For example, if you have a dormer that has a gabled top on it that measures 2 feet on each side and is 3 feet long, then your total square footage will be 12 square feet. If you also have, on that same roof, a gabled roof that is 20 feet long and each side of the gable measures 10 feet, then your roof square footage is 400 square feet. Combine these numbers together for a total.

Multiply the total number of square footage by 125 dollars. As of 2010, this is a national average number that will cover all metal roofing costs per square foot. However, regional costs may vary, so you may need to call a few metal roofing suppliers and get a square footage cost before completing this step.
Tips & Warnings
 Always measure and calculate twice.
 Keep you measurements and calculations recorded and organized.
References
 "Construction Estimating Reference Data;"Ed Sarviel; 1993
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