Squares, waste and what to measure
Siding is sold and quoted by the square, and one square covers 100 square feet of wall. The arithmetic is length × height for each wall, added together, minus the area of the doors and windows you are not covering, then multiplied by a waste allowance and divided by 100. Ten percent waste suits a plain rectangular house with long uninterrupted runs; push it toward 15% or more if you have gables, dormers, lots of small windows, or a course width that forces awkward rips at every opening. Diagonal and staggered patterns waste more again.
Measure wall by wall rather than trusting a floor plan, and handle gable ends separately: a gable triangle is roughly half its base times its height, so a 30 ft wide gable rising 8 ft adds about 120 ft² per end. Only deduct openings that are genuinely large — subtracting every small window tends to leave you short, because the offcuts around a narrow window rarely get reused. Remember that the siding panels themselves are only part of the order: starter strip, J-channel, corner posts, soffit and trim are bought by the linear foot and are not included in a per-square price, so add them to your material list before you order.
Box coverage differs by product and profile, and a price per square may be materials only or fully installed. Confirm both with your supplier or contractor before ordering.