How to Measure for Siding

Measuring for siding looks daunting because a house is an awkward shape, but it comes down to one trick: break every wall into rectangles and triangles, measure each, and add them up. Do that and the order is just arithmetic.

Skip the math

Measure each wall and punch the sizes into the siding calculator - it adds the walls and gables, subtracts the openings, and returns the squares, the boxes or boards, and the trim. This guide is the measuring method behind it.

Step 1: measure each wall as a rectangle

Take the length and the height of each wall (eave to ground, or to the bottom of the gable) and multiply them. Do every wall of the house. A tape on a pole or a laser measure makes the heights far easier and safer than a ladder.

wall area = length × height

Step 2: add the gable ends as triangles

The triangular wall under a pitched roof is a gable. Its area is half the base times the height to the peak:

gable area = ½ × gable width × height to the ridge

Measure the gable width along the eave line and the height straight up to the ridge. In the calculator, add the wall and switch its shape to Gable / triangle. A wall with a gable on top is just a rectangle plus a triangle - enter them as two walls.

Step 3: subtract the doors and windows

You are not cladding over the openings, so take them off. Multiply each opening’s width by its height, add them up, and subtract from the gross wall area:

net area = gross wall area − (all door & window areas)

For a tight estimate deduct every opening; if you would rather build in a cushion, leave the smallest ones in. Either way the waste allowance covers the offcuts around each opening.

Step 4: total it into squares

Add the net area of every wall and gable, then divide by 100 to get siding squares - the unit siding is ordered in. Add 10% waste for simple walls or 15% for cut-up ones, and round up to whole boxes of vinyl (about 2 squares each) or whole lap boards. See how much siding do I need for the box and board conversions.

Step 5: measure the trim

The field area does not cover the trim, which is measured off the geometry:

• Corner posts run the full wall height at each outside corner - count the corners and multiply by the height.
• J-channel wraps the perimeter of every door and window, plus the runs along the eaves and soffits.
• Starter strip runs along the bottom of each wall, and finish trim along the top.

The siding calculator works the J-channel out from the openings you enter and the corner posts from the corner count and height. Trim is sold in fixed sticks, so round each run up to whole lengths - and if you are choosing the material first, weigh it up in vinyl vs fibre-cement siding.