Exterior Wall Square Footage Calculator

This calculator works out the total exterior wall area of a rectangular building from its length, width, and height, minus any openings you subtract. You enter the building’s outside dimensions and the total area of doors and windows, and it returns the net wall square footage you need for siding, painting, or insulation estimates.

Formula

The calculator uses two simple formulas. First, the perimeter of the building footprint:

Perimeter = 2 × (Length + Width)

Then the gross exterior wall area:

Gross Wall Area = Perimeter × Wall Height

Finally, subtract the total area of all openings (doors, windows, vents, etc.):

Net Wall Area = Gross Wall Area − Total Openings Area

All dimensions are in feet (or meters, as long as you stay consistent). The result is in square feet (or square meters).

Worked Example

Using the default values: a building 40 ft long, 30 ft wide, with walls 10 ft high, and 120 sq ft of doors and windows.

1. Perimeter = 2 × (40 + 30) = 2 × 70 = 140 ft
2. Gross wall area = 140 × 10 = 1,400 sq ft
3. Net wall area = 1,400 − 120 = 1,280 sq ft

The calculator shows a perimeter of 140 ft, gross area of 1,400 sq ft, and net wall area of 1,280 sq ft.

Why the Perimeter Method Works

Most buildings are roughly rectangular, so multiplying the perimeter by the wall height gives the entire surface area of the exterior walls. This includes all sides, even if some walls have different features. It is the same method used by painters and siding contractors for quick estimates. For a simple rectangular house, the formula is exact. For L-shaped or U-shaped homes, you can still use it by measuring each straight wall segment separately and adding them, or by using the overall bounding rectangle and then adjusting for the missing sections.

How to Measure Your Building

Accurate input is the key to a useful result. Here are the steps:

• Length and width: Measure the outside of the building from corner to corner, not the interior rooms. If you only have interior measurements, add the wall thickness (typically 6–8 inches for wood frame, 8–12 inches for masonry).
• Wall height: Measure from the top of the foundation (or the bottom of the siding) to the top of the wall plate, just below the roof overhang. For a two-story house, measure each floor separately and add them.
• Openings: Measure each door and window. For a standard door, that is roughly 21 sq ft (3 ft × 7 ft). A typical window might be 12–15 sq ft. Add them all up.

If you are measuring in inches, convert to feet by dividing by 12. For example, a window 36 inches wide and 48 inches tall is 3 ft × 4 ft = 12 sq ft.

Common Mistakes to Avoid

• Mixing units: If you measure length in feet and height in inches, the result is wrong. Convert everything to the same unit first.
• Forgetting gable ends: This calculator only covers the rectangular walls. If your house has triangular gable ends under the roof, add their area separately: (base × height) / 2 for each gable.
• Including the foundation: Wall height should stop at the top of the foundation, not include the concrete below grade.
• Rounding too early: Keep full precision until the final step, then round to the nearest square foot.

What the Result Tells You

The net wall area is the amount of siding, paint, or insulation you need to cover. For paint, one gallon typically covers about 350 sq ft with one coat, so you would divide the net area by that. For siding, you usually order about 10% extra for waste and cutting. For insulation, the same net area applies, but you may need to add for rim joists or other details. Always check with your material supplier for specific coverage rates.

Limitations of This Calculator

This tool assumes flat, vertical walls with no complex features. It does not account for:

• Gable ends, dormers, or bay windows
• Wall thickness (the formula uses exterior dimensions, which is correct for siding)
• Overhangs, soffits, or trim
• Curved or irregular walls

For a house with many angles, it is better to measure each wall section individually and add them. For a simple rectangle, this calculator is accurate to within a few percent.

Frequently Asked Questions

Do I include the foundation in the wall height?

No. The wall height should be from the top of the foundation to the top of the wall plate. The foundation is usually concrete and not covered by siding or paint, so including it would overestimate the area.

How do I calculate the area of a gable end?

Measure the width of the gable (same as the building width) and the height from the top of the wall to the peak. Multiply them and divide by 2. Add that to the net wall area from the calculator.

What if my house is L-shaped?

Break it into two rectangles. Calculate the wall area for each rectangle separately, then add them. Make sure you do not double-count the wall where the two rectangles meet.

How much extra material should I order?

For siding, add 10% for waste and cutting. For paint, buy enough for two coats, and add a little extra for touch-ups. The calculator gives you the exact surface area, but real-world installation always has some waste.

Can I use this for interior walls?

No. This calculator is for exterior walls only. Interior walls have different dimensions and you would need to measure each room separately, subtracting door openings and closets.

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in length | Building length | ft | 40 | 0
in width | Building width | ft | 30 | 0
in height | Wall height | ft | 10 | 0
in openings | Doors + windows total | sq ft | 120 | 0
out perimeter | Wall perimeter | ft | 2 * (length + width) | 1
out gross | Gross wall area | sq ft | perimeter * height | 1
out net | Net wall area (to cover) | sq ft | gross - openings | 1