Hardie Siding Calculator

14.3 planks of the 8.25 in board at its standard 7 in exposure, because each 12 ft plank covers 12 × 7 ÷ 12 = 7 sq ft and a square is 100 sq ft. The narrow 6.25 in board at 5 in takes 20 per square. Add your waste allowance on top before you order.

Exposure is the part of each plank that stays visible after the plank above overlaps it, so it is always less than the full width. With a 7-inch exposure, each row shows 7 inches of face even though the board is 8.25 inches wide. The hidden strip is what keeps water out.

James Hardie lists 4 in for the 5.25 in plank, 5 in for the 6.25 in plank, 6 in for the 7.25 in plank, 7 in for the 8.25 in plank, 8 in for the 9.25 in plank, 10.75 in for the 12 in plank. Each width leaves the same 1.25 inches of lap. Picking a width in the calculator sets the matching exposure.

12 feet, which is the calculator's default. Some product lines and regions carry other lengths, so check what your supplier stocks before you order. Plank length goes straight into the count, so the wrong length throws off the whole number.

An outside corner is usually built from two trim boards, one on each face, and trim boards come 12 ft long. A one-story wall up to 12 ft tall takes 2 boards per corner, so a plain rectangle with 4 corners needs 8. A two-story wall around 18 ft tall needs two lengths stacked on each face, which is 16 boards for the same 4 corners. Window and door trim is extra and is counted opening by opening.

A gable is a triangle, so its area is half the base times the height. A 30 ft wide gable rising 6 ft from the eave to the peak is 0.5 × 30 × 6 = 90 sq ft, about 15 more 8.25 in planks once 10% waste is added. Enter the number of gables and their height in the calculator and it adds them in. Gables produce more cutting waste than square walls, so use closer to 15% waste on a house with tall gables.

With a narrower exposure, more of each board is hidden under the overlap, so each plank covers less wall and you need more of them. A wider exposure covers the wall with fewer planks and costs less, but gives a bolder, less traditional look. The choice comes down to appearance versus cost.

Published 2026 cost guides put fiber cement at $6 to $13 per square foot of wall installed, materials and labor together. The 40x30 ranch in the table above has 1,077 sq ft of wall, which puts it at $6,462 to $14,001. Tearing off old siding adds $1 to $2 per sq ft.

Fiber cement lap siding comes as individual planks of a fixed length, installed at a set exposure, so this calculator counts it by the piece. The siding cost calculator works in squares of 100 sq ft and compares installed cost across vinyl, fiber cement, wood, and aluminum. Use that one if you have not picked a material yet.