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Siding Calculator

Find how much siding to order for your house's walls and gable ends, after doors and windows. You get squares with a waste allowance, plus a box count.

Level: Grades 6–7

  • Common Core standard 6.G.A.1Find the area of right triangles, other triangles, special quadrilaterals, and polygons by composing into rectangles or decomposing into triangles and other shapes; apply these techniques in the context of solving real-world and mathematical problems.
  • Common Core standard 7.G.B.6Solve real-world and mathematical problems involving area, volume and surface area of two- and three-dimensional objects composed of triangles, quadrilaterals, polygons, cubes, and right prisms.
Printable worksheet

Siding Calculator

Know the answer you need? Pick an input to work backward.

Inputs

The triangles of wall under the roof slope at each end of a gable roof.

Usually the width of the house.

10% is common for simple walls. Use 15% or more for many corners, gables and openings.

Check the carton. Many vinyl siding cartons hold 2 squares.

Result

Siding needed (with waste)

14.06 squares

Cover 1,278 ft² of wall: about 14.06 squares with waste, or 8 boxes.

Boxes or cartons
8
Net area to cover
1,278 ft²
Area including waste
1,405.8 ft²
Gable area
180 ft²
Doors and windows subtracted
162 ft²
Starter strip (house perimeter)
140 ft

See the step-by-step solution ↓

Step-by-step solution

  1. Find the wall area: the distance around the house times the wall height

    2×(40+30)×9=1,260 ft²

  2. Add the gables. Each one is a triangle, so its area is half the base times the height

    2×0.5×30×6=180 ft²

  3. Add the walls and gables, then take away the doors and windows you won't cover

    1,260+180−2×3×7−10×3×4=1,278 ft²

  4. Add 10% for waste, then divide by 100 because each square covers 100 ft²

    1,278×1.1100=14.058 squares

  5. Divide by 2 squares per box, and round up since you can't buy part of a box

    14.0582=7.029=rounded up:8

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how much siding to buy for your house. Siding is sold in squares, a trade unit equal to 100 square feet of wall. To find how many you need, add up the walls around the house. Add the gable ends too: the triangles of wall under a sloped roof. Take away the doors and windows. Then add extra for the scraps you cut off each row (course) of siding.

This calculator does that for a rectangular house. It multiplies the distance around the house (the perimeter) by the wall height. It adds each gable as half its base times its height. Then it takes away the openings you list and adds your waste percentage. Dividing by 100 gives squares. It also counts cartons. And it gives the perimeter as the length of starter strip, the strip that runs along the bottom.

Where it's used

Why this math matters outside the classroom:

  • Gable ends. A gable 30 ft wide and 6 ft tall is a triangle of ½ × 30 × 6 = 90 ft² of siding.
  • Contractor quotes. Siding contractors convert 1,278 ft² of net wall to 12.78 squares, then add waste before pricing per square.
  • Composite figures. Students find a house's siding area by adding rectangles and triangles and subtracting the windows.
  • Paint versus siding. Homeowners comparing repainting with re-siding use the same net wall area for both estimates.
  • Carton counts. At 2 squares per carton, 14.06 squares of siding means ordering 8 cartons rather than 7.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.G.A.1, 7.G.B.6. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Enter the house's length and width, and the wall height up to the eaves (where the roof meets the wall).
  2. Enter the number of gable ends, with their width at the base and their height from the eaves to the peak. Use 0 for a hip roof, which slopes down on all four sides.
  3. Enter the doors and windows to subtract, with a typical size for each.
  4. Choose a waste allowance and the squares per carton. Then read the squares and boxes to order.

What the inputs mean

Number of gable ends
The triangles of wall under the roof slope at each end of a gable roof.
Gable width at the base
Usually the width of the house.
Gable height (eaves to peak)
Measure from the level of the eaves up to the peak. You can also use the roof pitch (how many inches the roof rises for every 12 inches across). Take half the gable width, times the rise, ÷ 12. For example, a 6-in-12 roof over a 30 ft gable rises 7.5 ft.
Waste allowance
Covers cuts around openings, at corners and on gable angles, plus damaged pieces. Houses with dormers (windows that stick out of the roof) or many corners need more.
Squares per box or carton
Siding cartons often hold 2 squares, but panel shapes differ. The label on the carton gives the coverage.

Formula and method

wall area = 2 × (length + width) × wall height
gable area = gables × ½ × base × height
net area = wall area + gable area − doors − windows
squares = net area × (1 + waste ÷ 100) ÷ 100
boxes = squares ÷ squares per box, rounded up

All lengths are in feet and areas in square feet. The perimeter, 2 × (length + width), times the height covers all four walls of a rectangular house. Each gable is a triangle, so its area is half the base times the height. Openings are taken away at their full size.

Squares are left as a decimal, so you can compare them with the coverage on the product. Boxes are rounded up, because you can't buy part of a carton.

Worked example

A 40 × 30 ft house with 9 ft walls and two 30 ft gable ends rising 6 ft, with two 3 × 7 ft doors and ten 3 × 4 ft windows, 10% waste and 2 squares per carton.

  1. Walls: 2 × (40 + 30) × 9 = 1,260 ft².
  2. Gables: 2 × ½ × 30 × 6 = 180 ft².
  3. Net: 1,260 + 180 − 42 − 120 = 1,278 ft².
  4. Squares: 1,278 × 1.1 ÷ 100 = 14.06.
  5. Boxes: 14.058 ÷ 2 = 7.03, rounded up to 8.

Result: The house needs about 14.06 squares of siding, including waste. That's 8 cartons.

Understanding the result

Suppliers price siding by the square. Trim pieces aren't included. You order these separately by length: J-channel around openings, outside and inside corner posts, soffit and fascia.

Say the result lands just above a whole box. On a simple house, a slightly lower waste allowance may be reasonable. But keep a few spare panels for future repairs, since colors get discontinued.

Assumptions and limitations

  • The house is a rectangle with walls of one height. Every wall gets siding.
  • All doors share one size and all windows share one size.
  • Gable ends are simple triangles.
  • Dormers, bump-outs, garage walls and chimneys must be added separately.
  • Trim pieces, house wrap and fasteners aren't counted.
  • How much of each panel shows (its exposure) and its shape don't change the square count. They do change the number of panels per square.

Frequently asked questions

What is a square of siding?

A square covers 100 ft² of wall. Roofing uses the same unit. A wall area of 1,278 ft² is 12.78 squares before waste.

Should I subtract windows and doors?

Yes, for large openings. Some installers take away only half of each opening, to cover the extra cutting around it. A higher waste percentage does the same job.

How do I measure a gable end?

Measure its width at the eaves and its height from the eaves to the peak. The triangle's area is half of width times height. A hip roof has no gable ends.

How much starter strip do I need?

Starter strip runs along the bottom of every wall, so the house perimeter is a good estimate. Openings that reach the ground, such as garage doors, cut it down a little.

How this calculator is checked

Method: Rectangular wall area plus triangular gables, less openings, with a waste factor; 1 square = 100 ft². The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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