How many squaresof shingles to cover 672 sq ft?

That depends on the size of the shingles.

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Size of shingles makes no difference. Shingles are sold by the bundle and the amount of bundles per square depends on the weight of the shingles. So you could have 3 bundles, 4 bundles or 5 bundles per square but they still add up to a (100 sq ft) square. The only difference would be if you are dealing with metric shingles, which is not going to make a big difference in the area covered.

A square of shingles covers 100 sq ft. So for a 672 sq ft roof you will need 7 squares.

Dave