Siding Calculator — Squares, Planks and Trim
Siding is quoted in squares — 100 square feet — but installed by the plank, and a plank does not cover its own width. Enter the walls and this gives you both numbers plus the courses and corner trim.
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How to work it out
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Measure the walls
Length times height for each wall, added together. Measure to the eave, not to the roof peak — the triangle above the eave is a gable and goes in its own box, because measuring it as a rectangle would overstate it by half.
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Add the gables
A gable is a triangle: half the base times the height. A house 30 ft wide with a gable rising 6 ft above the eave has 90 sq ft on that end. Two gable ends means twice that.
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Work out what a plank covers
This is the step people miss. Lap siding overlaps, so a 8 1/4 in. plank only shows about 7 inches — and 7 inches is what it covers. A 12 ft plank at 7 in. exposure covers 7 sq ft, not the 8.25 sq ft the board contains.
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Order in squares, install by the plank
Siding is priced and sold in squares of 100 sq ft. Both numbers are above: squares for the order, planks so you can check the delivery and plan the courses. Buy trim, starter strip and fasteners separately — none of it is in the square footage.
What one plank covers
Coverage is exposure, not plank width.
| Plank | Exposure | 12 ft plank | 16 ft plank |
|---|---|---|---|
| 5 1/4 in. fiber cement | 4 in. | 4.00 sq ft | 5.33 sq ft |
| 6 1/4 in. fiber cement | 5 in. | 5.00 sq ft | 6.67 sq ft |
| 7 1/4 in. fiber cement | 6 in. | 6.00 sq ft | 8.00 sq ft |
| 8 1/4 in. fiber cement | 7 in. | 7.00 sq ft | 9.33 sq ft |
| 9 1/4 in. fiber cement | 8 in. | 8.00 sq ft | 10.67 sq ft |
| 12 in. fiber cement | 10 3/4 in. | 10.75 sq ft | 14.33 sq ft |
Exposure × length ÷ 12. The inch or so of overlap does no covering, so it does not count.
Squares by house size
Rough wall area for a two-story box, 10% waste, before gables.
| Footprint | Perimeter | Wall area | Squares |
|---|---|---|---|
| 24 × 36 ft | 120 ft | 2,160 sq ft | 23.8 |
| 28 × 40 ft | 136 ft | 2,448 sq ft | 26.9 |
| 30 × 50 ft | 160 ft | 2,880 sq ft | 31.7 |
| 36 × 60 ft | 192 ft | 3,456 sq ft | 38.0 |
Perimeter × 18 ft of wall height for two stories. Add gables and deduct only the large openings.
What is a square of siding?
One hundred square feet. It is the unit siding and roofing are both sold and quoted in, and it exists so that nobody has to say "three thousand four hundred and fifty-six square feet" out loud on a job site.
What a square arrives as varies. Fiber cement usually comes as planks on a pallet; vinyl comes in cartons that are typically two squares each, though it is worth checking, because some are one. The square footage is the constant, so that is what to order by.
Exposure is the number that matters
Lap siding works by overlapping, so the visible part of each plank — the exposure — is always less than the plank itself. An 8 1/4 inch fiber cement plank is normally hung at 7 inches, which means 1 1/4 inches of every board is doing nothing but keeping water out.
Coverage is calculated on exposure. Use plank width instead and you will come up about 15 per cent short, which on a whole house is several squares and a second delivery.
Should I deduct the windows?
Deduct the big ones — a picture window, a garage door, a set of sliders. Leave the ordinary ones in.
Every opening generates cut planks, and the cut-off from one side of a window is rarely the right length for anywhere else. Deducting a 15 sq ft window and then cutting 20 sq ft of planks to fit around it leaves you short. On a plain wall the cut-offs get used; around openings they mostly do not.
Courses, and where the first one goes
Divide wall height by exposure and you have the number of courses. It almost never comes out even, and where you put the leftover is a decision you make before the first plank goes on, not after.
The usual approach is to adjust the exposure slightly across the whole wall so the courses land neatly at the window heads and the top. Tightening 7 inches to 6 3/4 over twenty courses moves the top by five inches — enough to line things up without any of it being visible.
What the square footage does not include
Trim, and it adds up. Outside corners at the height of the wall, inside corners, window and door casing, a starter strip along the entire bottom run, frieze at the top, and the fasteners for all of it.
The corner trim figure above is a starting point — corners times wall height. Measure the openings separately, and order trim in the same batch as the siding so the color matches.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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wall area = length × height, summed over every wallMeasured to the eave. Gables are added separately. -
gable area = ½ × base × heightA gable is a triangle; measuring it as a rectangle doubles it. -
coverage per plank = (exposure in. ÷ 12) × plank length ftA plank covers what it shows, not what it measures. -
planks = area with waste ÷ coverage per plankRounded up — siding is sold whole. -
squares = area with waste ÷ 100A square is 100 sq ft, the unit siding is priced in. -
courses = wall height in. ÷ exposure in.Rarely a whole number, which is why exposure gets adjusted on site.
Common questions
How much siding do I need for a 1,500 sq ft house?
Floor area does not tell you — wall area does. A single-story 30 × 50 ft house is 1,500 sq ft of floor but only about 1,440 sq ft of wall, which is 14 to 16 squares with waste and gables. Measure the perimeter and the height.
How many square feet does a square of siding cover?
One hundred, by definition. That is the whole point of the unit. What varies is how many planks or cartons make up a square, which depends on the profile.
What is exposure?
The part of a plank you can see once the one above it overlaps. An 8 1/4 in. fiber cement plank is normally set to 7 in. exposure, so 7 in. is what it covers. Manufacturers specify a minimum overlap — usually 1 1/4 in. — and going below it voids the warranty.
How many planks in a square of fiber cement?
About 14 for 12 ft planks at 7 in. exposure, since each covers 7 sq ft. At 6 in. exposure it is closer to 17. Pallet quantities vary, so check what a unit contains before you order.
Do I need house wrap underneath?
Yes — code requires a weather-resistive barrier behind siding, and every manufacturer warranty assumes one. Siding sheds most of the water; the barrier deals with what gets past it. It is not an optional layer.
How do I measure a gable?
Half the base times the height above the eave. A 28 ft wide gable rising 7 ft is ½ × 28 × 7 = 98 sq ft. Add it to the wall area rather than trying to fold it into a rectangle.
Can I put new siding over old?
Sometimes, but it is usually a bad idea. You cannot see what is behind it, the wall builds outward past the window trim, and any rot carries on rotting. Stripping it costs a day and tells you what you are actually dealing with.
How much waste should I allow?
Ten per cent on a plain rectangular house. Fifteen if there are gables, dormers, bump-outs or a lot of windows, because every one of those turns full planks into cut-offs you cannot use anywhere else.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- James Hardie installation instructions — fiber cement plank sizes and exposures
- Vinyl Siding Institute — panel profiles, coverage and carton sizes
- International Residential Code, Chapter 7 — exterior covering and weather-resistive barriers
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 15, 2026.