How to Measure for Siding

A few common oversights consistently cause material shortages or overages on siding projects:

• Forgetting to measure gable triangles separately from rectangular wall sections
• Applying a flat length-times-height calculation to walls with angled or curved features
• Overlooking the reveal - the exposed face of each siding course - which affects how many linear feet you actually need
• Failing to account for waste from diagonal cuts around windows and rakes
• Using rough builder dimensions instead of actual measured dimensions

Step 1: Sketch a simple outline of your home
Before you measure anything, draw a rough top-down outline of your home and label each wall with a letter or number. Include any bump-outs, attached garages, or additions. This sketch keeps your numbers organized and makes it easier to double-check your work later.

Step 2: Measure each rectangular wall section
For each wall, measure the total width at the base and the height from the foundation line to the roofline or soffit. Multiply width by height to get that wall's gross square footage. Record it on your sketch. Repeat for every exterior wall.

Step 3: Calculate gable end triangles
For any gable ends - the triangular sections beneath a pitched roof - measure the base width and the vertical height at the peak. Multiply base by height, then divide by two. This gives you the area of the triangle. Add it to your running total.

Step 4: Measure and subtract all openings
Measure the width and height of every window and door on each wall. Multiply each pair together to get its square footage, then subtract those totals from your gross wall area. Most siding calculators and contractors allow you to deduct full opening dimensions since that material is not installed.

Step 5: Add all wall totals together
Sum every wall section - rectangles, triangles, and any irregular shapes - after subtracting openings. This is your net square footage of siding needed.

Step 6: Add a waste factor
For straightforward installations, add ten percent to your net total. For homes with many angles, dormers, or diagonal patterns, consider adding up to fifteen percent. Multiply your net square footage by 1.10 or 1.15 accordingly.

Step 7: Convert to squares for ordering
Divide your final number by one hundred to get the number of squares. Most siding products are sold and priced by the square. Note that trim pieces, corner posts, J-channel, and starter strips are measured and ordered separately by linear foot, so walk the perimeter and record those lengths as well.

• Always use a ladder rated for your weight plus tools, and set it on stable, level ground
• Have a second person hold the base of the ladder when measuring upper wall sections
• Do not lean out to the side - reposition the ladder instead
• Avoid measuring on wet, icy, or windy days
• Wear non-slip footwear and keep both hands free whenever possible
• If a wall section is above two stories, consider hiring a professional to take those measurements

Sometimes the measuring process reveals issues that go beyond ordering materials:

• Walls that bow outward or inward may indicate structural or moisture damage behind the existing siding
• Soft or spongy spots found when pressing on older siding boards can signal rotted sheathing underneath
• Visible gaps, warping, or separation at corners suggest the substrate may need repair before new siding goes on
• Staining or discoloration on foundation areas near the base of walls can point to chronic water infiltration
• If trim boards crumble when measured around, the framing members behind them may be compromised as well