Why isn't gable area just width × height × 0.75?

That shortcut bundles two different things into one constant: 0.75 is 0.5 (the true triangle area) times 1.5 (an implicit 50% waste allowance). A gable end is a triangle — ½ × width × height — and this calculator keeps that area separate from waste, which you set explicitly as its own percentage instead of having it hidden inside a geometry formula.

How do I handle two gables of different sizes?

Run the calculator once for each distinct gable size and add the results together. This calculator assumes matching gables from one width, height, and count, which covers the common case of a simple gable roof's two identical ends.

How does the exposure helper work?

Enter your plank's length and your exposure (how much of each plank stays visible after the course above overlaps it), and it computes coverage per plank as length × exposure ÷ 12, filling that into the main coverage field. It's optional — if you're buying vinyl by the box, skip it and enter the box's stated coverage directly.

What exposure should I use for lap siding?

That's governed by the IRC's maximum exposure for lap siding and your product's own installation instructions — exceeding the manufacturer's stated maximum is both a code violation and can void your warranty. This page doesn't suggest a number; check the current code, your local building department, and your product's instructions for what applies to your siding.

What waste percentage should I use for siding?

This page starts at 10%, but says plainly that's an unverified starting point, not a sourced figure — contractor sources commonly cite 10% for a simple wall and 12-15% once gables, corners, and cuts add up, but none of them is a source this site will cite. Ask your installer, or check the manufacturer's own application instructions, for a real number.

Does vinyl siding really cover two squares per box?

That's what's commonly seen, not a specification — coverage varies by manufacturer and style. CertainTeed's own installation guide points installers to its materials estimating worksheet rather than a single universal figure; check your actual box's stated coverage.

Does this calculator include trim, house wrap, or fasteners?

No. Starter strip, J-channel, corner posts, trim, house wrap, and fasteners are all separate line items with their own figures this page doesn't estimate yet. This calculator counts siding units only — budget for the rest separately.

Siding Calculator

This calculator asserts almost nothing of its own: wall area and gable area are pure geometry, computed from dimensions you provide, and both your waste percentage and your product's coverage per unit are numbers you bring with you. Enter your wall's dimensions (or a total area you already have), subtract any doors and windows, add gables if your wall has them, and enter your product's actual coverage — a plank's coverage for lap siding, a box's coverage for vinyl. An optional helper computes plank coverage from plank length and exposure if you'd rather work from those than a single coverage number. See the guidance below for a widely-repeated gable-area shortcut that's wrong as geometry, why exposure is a code-governed number this page won't suggest, and what's deliberately left out.

Your wall

LengthHeightUnit

Doors & windows to subtract

Gables (optional)

Width (ft)Height (ft)Count

Area = ½ × width × height per gable, not width × height × 0.75 — see the guidance below for why that shortcut is wrong. Differing gables? Run this calculator once per size and add the results.

Waste (%)10% is a starting point, not a sourced figure — contractor sources commonly cite 10% for a simple wall and 12-15% once cuts, corners, and gables add up, but none of them is a source this site will cite. What actually drives waste is complexity: a plain rectangular wall needs very little, while gables, dormers, corners, and many openings push it up. Your installer, or the siding manufacturer's own application instructions, is where a real number for your specific job lives.

Coverage per unit (sq ft)Read this directly off your product — a box's stated coverage for vinyl, or use the plank helper below for lap siding.

Compute from plank length & exposure (optional)

Plank length (ft)Exposure (in)

Exposure has no default and no presets on purpose — the IRC sets a maximum for lap siding, and this page won't suggest a number. Check your product's instructions and local code for the exposure that applies to your siding.

Enter your wall's dimensions above to see results.

How it's calculated

The calculator resolves net wall area from either dimensions (length × height) or a direct entry, minus any doors and windows you list. If you add gables, each one contributes ½ × width × height, and matching gables multiply by however many you enter — that gets added to the net wall area for a total. A waste percentage is added on top (10% by default — see the guidance below for why that's a starting point, not a sourced figure), and the adjusted area is divided by 100 to get squares, and independently by your coverage per unit to get a unit count, rounded up since a partial plank or box still has to be bought. The exposure helper, if you use it, only ever computes a number and fills it into the coverage field — it never touches this calculation directly.

Worked example

A 40 ft long, 9 ft high wall (360 sq ft gross), with one 3×7 ft door (21 sq ft) and one 3×4 ft window (12 sq ft) subtracted: net wall area = 360 − 21 − 12 = 327 sq ft. Two matching gables, each 20 ft wide and 5 ft high: each gable is 0.5 × 20 × 5 = 50 sq ft, and two of them add 100 sq ft — total area = 327 + 100 = 427 sq ft. Adding the 10% waste default: 427 × 1.10 = 469.7 sq ft. In squares, that's 469.7 ÷ 100 = 4.70 squares. Using the exposure helper — a 12 ft plank at 7-inch exposure covers 12 × 7 ÷ 12 = 7 sq ft per plank — units needed = 469.7 ÷ 7 = 67.1, rounded up to 68 planks. Change any of these numbers to your actual project's; every one of them is yours to set, not this page's.

Gables, exposure, waste, and what's left out

Net wall area comes from your wall's dimensions (or a total area you already have) minus any doors and windows you list — the same subtraction every area-based calculator on this site uses. Gables are the part that's easy to get wrong.

A gable end is a triangle, and its area is exactly ½ × width × height — nothing more. A widely-repeated shortcut online instead uses width × height × 0.75, and it's worth being precise about why that's wrong: 0.75 is 0.5 × 1.5 — the true triangle area (0.5) multiplied by an implicit 50% waste allowance (1.5), smuggled into a single constant. That conflates two things this calculator keeps separate on purpose: area is area, and waste is a percentage you set yourself, applied on top, visible as its own number rather than hidden inside a geometry formula. Compute the true triangle, then apply your waste percentage like everywhere else — if 50% waste happens to be right for your gable, set it explicitly and see that number, rather than trusting a constant that assumes it silently.

Gables genuinely do generate more waste than a straight wall, which is part of why the 0.75 shortcut persists — it's not entirely making something up, just hiding a real effect inside the wrong formula. Every course of siding on a gable meets the sloped rake at an angle, and each of those angled cuts produces an offcut that's often too short to be reused on that same course — though the offcut from one side of a gable sometimes matches what the other side needs, so cutting both sides from the same batch can recover some of that loss. This calculator assumes two matching gables from one width and height, which covers the common case of a simple gable roof's two ends. If your gables differ — an addition with a different pitch, an L-shaped roofline — run the calculator once per distinct gable size and add the results together rather than trying to average them into one number.

Coverage per plank isn't a fixed product spec for lap siding — it depends on exposure, how much of each plank stays visible after the course above overlaps it, and exposure is set by your product's width and how you install it. The formula is plank length (ft) × exposure (in) ÷ 12: a 12 ft plank at 7 in exposure covers exactly 7 sq ft, and at 5 in exposure the same plank covers only 5 sq ft — 40% more planks for the same wall. That's pure geometry and needs no citation, the same reason roofing's pitch multiplier and 27 cubic feet in a cubic yard don't need one. James Hardie's own product data states HardiePlank lap siding is milled in 12 ft lengths, with nominal widths producing exposures from about 4 in to 10¾ in — real product facts, which is why this page's exposure helper defaults plank length to 12 ft. Exposure itself is different: the IRC sets a maximum exposure for lap siding, and exceeding it is both a code violation and grounds to void the manufacturer's warranty. This page states no exposure number, not even as a default — check the current IRC, your local building department, and your product's own installation instructions for the maximum that applies to your siding, and enter whatever real exposure you're installing at into the helper.

Waste percentage on this page starts at 10%, and like roofing's default, that number deserves more scrutiny than most defaults on this site get. Contractor sources commonly cite 10% for a simple wall and 12-15% once cuts, corners, and gables add up, but none of them is a source this site will cite secondhand. What actually drives waste is complexity: a plain rectangular wall with no obstructions needs very little, while gables, dormers, corners, and many openings push it up — the gable discussion above is part of why. Your installer, or the siding manufacturer's own application instructions, is where a real number for your specific job lives.

Coverage per unit is this calculator's other editable, no-default-that-pretends-to-be-standard field — the same treatment sod's pallet coverage, paver-patio's pallet coverage, insulation's bag coverage, and roofing's bundle coverage all got. Vinyl siding is commonly sold in boxes covering about two squares (200 sq ft), and fiber cement lap siding is sold by the plank rather than the box — that's what's typically seen, not a specification, and CertainTeed's own installation guide directs installers to its materials estimating worksheet rather than a single universal number. The actual coverage for the specific product you're buying — whether that's a box's stated coverage or a plank's coverage computed from its length and your exposure — is the number to enter here.

This calculator counts siding units only — planks or boxes, whichever you're working from. Starter strip, J-channel, corner posts, trim, house wrap, and fasteners all get ordered separately, and each has its own coverage or quantity figure this page doesn't estimate — not because they're minor, but because no sourced figure for any of them exists here yet, and a guessed number would be worse than an honest gap. Budget for all of them as separate line items when pricing a job from this page's unit count.

Common mistakes

Using the width × height × 0.75 gable-area shortcut. That constant is 0.5 (the true triangle area) × 1.5 (an implicit 50% waste allowance) smuggled together — compute the true ½ × width × height triangle, then apply your own waste percentage on top, visible as its own number.
Averaging two different-sized gables into one width and height instead of running the calculator once per distinct gable and adding the results. This calculator assumes matching gables; differing ones need separate runs.
Guessing at exposure instead of checking your product's installation instructions and the current IRC. Exposure has a code-specified maximum, and exceeding it is both a code violation and grounds to void the manufacturer's warranty.
Treating this page's 10% waste default as a sourced figure. It's an admittedly unverified starting point — your installer or the siding manufacturer's own application instructions are where a real figure for your specific job's complexity comes from.
Guessing coverage per unit instead of using your product's actual figure. Vinyl boxes commonly cover about two squares and fiber cement planks vary by length and exposure — read the number off your actual product, or compute it with the exposure helper for lap siding.
Forgetting that starter strip, J-channel, corner posts, trim, house wrap, and fasteners are separate line items. This calculator only counts siding units; budgeting off this number alone will come up short on the rest of the material list.
Skipping the doors-and-windows subtraction, or subtracting an opening twice by also shrinking the wall dimensions by hand. Enter each opening once in the list and let the calculator do the subtraction.

Where the figures come from

James Hardie's own product data states HardiePlank lap siding is milled in 12 ft lengths, with nominal widths from 5 1/4 in to 12 in producing exposures from 4 in to 10 3/4 in — that range is what makes the exposure helper's 12 ft plank-length default a real, citable product fact rather than an arbitrary placeholder. The exposure you actually use is governed by code and your product's own instructions, not this range, and this page states no exposure number as guidance. CertainTeed's Vinyl and Polymer Siding Installation Guide includes a materials estimating worksheet and states it's based on ASTM D4756, the standard practice for installing vinyl siding and soffit — cited here for the general estimating framework this page follows (area, waste, coverage), not for a specific coverage number. Both documents are manufacturer publications hosted on buildsite.com, a construction submittal aggregator — the host isn't the manufacturer, the same distinction drawn for Mohawk's Home-Depot-CDN-hosted flooring instructions elsewhere on this site.

FAQ

Why isn't gable area just width × height × 0.75?: That shortcut bundles two different things into one constant: 0.75 is 0.5 (the true triangle area) times 1.5 (an implicit 50% waste allowance). A gable end is a triangle — ½ × width × height — and this calculator keeps that area separate from waste, which you set explicitly as its own percentage instead of having it hidden inside a geometry formula.
How do I handle two gables of different sizes?: Run the calculator once for each distinct gable size and add the results together. This calculator assumes matching gables from one width, height, and count, which covers the common case of a simple gable roof's two identical ends.
How does the exposure helper work?: Enter your plank's length and your exposure (how much of each plank stays visible after the course above overlaps it), and it computes coverage per plank as length × exposure ÷ 12, filling that into the main coverage field. It's optional — if you're buying vinyl by the box, skip it and enter the box's stated coverage directly.
What exposure should I use for lap siding?: That's governed by the IRC's maximum exposure for lap siding and your product's own installation instructions — exceeding the manufacturer's stated maximum is both a code violation and can void your warranty. This page doesn't suggest a number; check the current code, your local building department, and your product's instructions for what applies to your siding.
What waste percentage should I use for siding?: This page starts at 10%, but says plainly that's an unverified starting point, not a sourced figure — contractor sources commonly cite 10% for a simple wall and 12-15% once gables, corners, and cuts add up, but none of them is a source this site will cite. Ask your installer, or check the manufacturer's own application instructions, for a real number.
Does vinyl siding really cover two squares per box?: That's what's commonly seen, not a specification — coverage varies by manufacturer and style. CertainTeed's own installation guide points installers to its materials estimating worksheet rather than a single universal figure; check your actual box's stated coverage.
Does this calculator include trim, house wrap, or fasteners?: No. Starter strip, J-channel, corner posts, trim, house wrap, and fasteners are all separate line items with their own figures this page doesn't estimate yet. This calculator counts siding units only — budget for the rest separately.

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