Siding Calculator

Area first, squares by definition, pieces from your spec

The worked house: 150 ft of perimeter × 8 ft = 1,200 sq ft of wall, plus two gables at ½ × 24 × 6 = 144 sq ft together, minus 120 sq ft of openings = 1,224 sq ft — 12.24 squares, since a square is 100 sq ft by the industry's ordering definition. With 10% cutting waste that is 1,346.4 sq ft, 13.46 squares; at the example profile's 5 sq ft per piece (5 in exposure × 12 ft), 270 pieces, with 3.6 sq ft of coverage bought beyond the waste-adjusted need. Computed at build time by the same engine wired to the form.

The square itself — why 100 sq ft is a definition rather than a measurement, what status every other coverage figure on this site carries, and which figures the site deliberately leaves to you — is the subject of the guide Coverage figures and where they come from. The rest of this page covers what the siding calculator does with your measurements: the pipeline it runs, why each field moves the result the way it does, where the model stops, and the entries that quietly produce the wrong order.

What the calculator computes, stage by stage

Every estimator on this site runs the five-stage pipeline documented on the methodology page: geometry, coverage, waste, package rounding, and a cost at your price. The siding tool runs all five, with two particulars. Its coverage stage has two divisors — one a definition, the other a figure only you can supply — and its rounding stage runs only when you have supplied that second figure.

1. Geometry. The walls are treated as one long rectangle unrolled around the house: perimeter × wall height. Each gable is a triangle, ½ × base × height, multiplied by the number of matching gables. Those two areas are added to give the gross area, and the openings you enter are subtracted from it: net = walls + gables − openings. In metric mode every length is divided by the exact 0.3048 m per foot and the openings figure by the exact 0.09290304 m² per square foot before any of this runs, so the arithmetic downstream is identical in both systems. Nothing is rounded.
2. Coverage. The first divisor is the square: net ÷ 100 gives the squares line, and because the square is a definition rather than a measured coverage, that line is exact arithmetic on your wall area. The second divisor exists only if you give it: the coverage of one piece, exposure ÷ 12 × piece length square feet, with both figures read from your product's spec sheet. The site keeps no profile of its own, so with those fields blank the pipeline stops at squares. With them filled, net ÷ coverage per piece is the net piece count — what the geometry needs, before any allowance.
3. Waste. The engine multiplies by (1 + waste ÷ 100) before anything is rounded, producing the waste-adjusted area and, when a profile was given, the waste-adjusted piece count — the same multiplier on both. The default of 10% is a trade convention recorded in the site's materials data, not a manufacturer figure, and the form lets you replace it. It is shown explicitly here because this tool deducts the openings; the trade's worksheet, as the FAQ explains, builds its allowance by leaving them in.
4. Package rounding. A piece is its own package, so the engine computes ceil(waste-adjusted pieces) — the only rounding in the chain, and it always rounds up, because a count rounded down leaves a wall short. The spare is pieces × coverage per piece − waste-adjusted area: siding the packaging obliges you to buy beyond the allowance. When the division lands on a whole number within floating-point noise, the engine treats it as an exact fit rather than adding a phantom piece; the garage example below shows the guard working. This stage does not run at all without a profile, which is why a squares-only result carries no round-up.
5. User-priced cost. If you type a price per piece, the cost is that price times the rounded piece count — the figure you buy, not the adjusted one. The site never supplies or averages a price; leave the field blank, or leave the profile blank, and the line does not appear.

The order of the last three stages matters. Waste is applied to the quantity, then the round-up happens once. Rounding to pieces first and then adding a margin to the count would round up twice, and the engine offers no such path.

Why each input matters

Perimeter and wall height set the bulk of the area, and it scales with each of them directly: double either and the wall area doubles before anything else happens. Both go in as decimal feet, so a tape reading in feet and inches is converted first — the feet & inches calculator does that for awkward readings. The height is the eave height of the rectangular wall, measured to the line where the gable triangle begins, not to the peak; the triangle above that line is the gable's job.

Gables are entered as a base, a height, and a count. The base is the width of the wall the gable sits on, usually the house width; the height is the vertical distance from the eave line to the peak. Measuring the gable height from the ground instead adds half of the rectangular wall under it a second time, and the misreadings section shows what that does to the order. Houses with several matching gables use the count field. Gables of different sizes are not a single entry: total their triangles by hand, ½ × base × height each, and enter the total as the gable base with a height of 2 ft and a count of 1 — ½ × total × 2 is the total again, so the engine sees the right area.

Openings are a single total in square feet, subtracted before the squares are computed. Two consistent methods exist, as the FAQ explains: deduct the openings and add an explicit allowance, which is what this tool does, or leave ordinary windows and doors in the area as the allowance, which is the trade worksheet's method. On the worked ranch, leaving the 120 sq ft of openings in and keeping the full 10% allowance orders 296 pieces against 270; deducting them with the allowance set to zero orders 245. The intended figure sits between the two, which is the point: pick one method, not both and not neither.

Exposure and piece length are the only figures on this page that come from a product rather than from the house, and they come only from you. The exposure is the face height one piece shows once installed, in inches; the piece length is in feet; their product, divided by 12, is the square feet one piece covers. The placeholders in the form are examples, not defaults, and the engine refuses one field without the other rather than guessing. The squares line never changes with the profile; only the pieces do, which the two-profile example below makes visible.

Waste multiplies the area, so it moves the adjusted squares smoothly but the piece count in steps: a small change is often absorbed by the round-up, and then a further change tips the count by one piece. The sweep below shows the steps on the worked ranch. What the percentage should be for your own walls is the subject of Waste factors, explained.

Price per piece touches only the last line. It never feeds back into the quantity, so you can add or change it after the fact without disturbing the count.

The units toggle changes what the fields expect and how the net area is presented, not what is computed. In metric mode meters are divided by 0.3048, square meters by 0.09290304, and centimeters by 2.54 — all exact definitions — before the engine sees them, and the result reports the net and adjusted areas in m² while the squares and pieces are the same figures the imperial path would produce from the converted dimensions.

The figures this page relies on

• Siding square, 100 sq ft — a definition, not a measured value: the Vinyl Siding Institute Installation Manual ("Getting Started / Estimating") states it as the unit for ordering purposes, and the site's materials data records it with that source. Because it is a definition, the squares line cannot be wrong by product.
• Default waste, 10% — a common trade convention recorded in the materials data, labelled a convention because no primary source publishes a universal figure; it stands for cutting at gables, corners, and openings. Adjustable in the form.
• Inches per foot, 12 — a definition, exact; it turns the exposure you enter in inches into feet so the coverage per piece comes out in square feet.
• Meters per foot, 0.3048; centimeters per inch, 2.54; square meters per square foot, 0.09290304 — exact definitions, used only in metric mode to convert what you type and to present the areas back in m².
• Coverage per piece — none. This page holds no product figure. Profiles vary far too much for a built-in constant, so the coverage of one piece is arithmetic on the two figures you read from your spec sheet, and the materials data deliberately holds nothing for it.

Worked examples

Every figure in this table was produced at build time by the same sidingEstimate call the form makes, with the wall and gable areas computed by the same geometry functions. The inputs are chosen to exercise different parts of the pipeline, not to describe any particular house or product; the metric row was entered as meters, square meters, and centimeters and converted exactly before the engine ran, which is what the metric toggle does.

Squares only. A 120 ft perimeter × 9 ft house with one 28 × 7 ft gable and 96 sq ft of openings has 1,178 sq ft gross and 1,082 sq ft net — 10.82 squares, 11.9 with the 10% allowance. No profile was entered, so the pipeline stops there: no piece count, no round-up, no spare, and no cost line even if a price had been typed. This is the figure to carry to a supplier who quotes by the square, or to compare products with before a profile has been chosen.

Two profiles on one house. A 180 × 8 ft house with two 30 × 5 ft gables and 140 sq ft of openings is 1,450 sq ft net, 14.5 squares, 15.95 after waste — with either profile. Suppose the first spec sheet gives 8 in of exposure over 12.5 ft: 8.33 sq ft per piece and 192 pieces, 5 sq ft spare. Suppose the second gives 10 in over 12 ft: 10 sq ft per piece and 160 pieces, 5 sq ft spare. The squares line is the stable figure to compare products on; the pieces line depends on the product, which is why it is computed only from your figures.

The priced lap board. A 96 × 9 ft house with one 26 × 6.5 ft gable and 80 sq ft of openings is 868.5 sq ft net. A board whose sheet gives 7 in of exposure over 12 ft covers 7 sq ft, so the house needs 137 pieces after the 10% allowance. Suppose you typed $20.00 into the price field as a stand-in for whatever your own supplier quoted — the site holds no siding price and suggests none. The cost line is then 137 × $20.00 = $2,740.00, in the dollars you typed, on the rounded count because the rounded count is what you buy. Change the price and only that line changes.

The garage, with and without an allowance. An 80 ft perimeter × 9 ft outbuilding with no gables and no openings entered is 720 sq ft, 7.2 squares. With the waste field set to 0 and the 8 in × 12.5 ft profile, the net piece count is 86.4 and the order is 87 pieces with 5 sq ft spare: the round-up does not go away when the allowance does. At the default 10% the same walls order 96 pieces with 8 sq ft spare — the allowance alone adds 9 pieces here.

The exact-fit guard. Give the same garage, at the default allowance, a profile of 8 in over 12 ft — 8 sq ft per piece. The net count is 90 pieces exactly, and 90 × (1 + 10 ÷ 100) is 99 in exact arithmetic, but it evaluates in binary floating point to 99.00000000000001. A naive ceiling on that value would buy 100 pieces. The engine treats a quantity within one part in a billion of a whole package as an exact fit, so the order is 99 pieces and the spare is 0 sq ft. A phantom piece is never bought for a rounding artifact.

The metric house. In metric mode, 45 m of perimeter × 2.4 m walls, 2 gables of 7.3 × 1.8 m, and 11 m² of openings reach the engine as 147.64 × 7.87 ft walls, gables of 23.95 × 5.91 ft, and 118.4 sq ft of openings. The net area is 110.14 m² (1,185.54 sq ft), 11.86 squares, and with 10% waste 121.15 m² — 13.04 squares. A profile of 20 cm over 3.66 m becomes 7.87 in over 12.01 ft, 7.88 sq ft per piece, for 166 pieces with 3.85 sq ft spare. The square stays a 100 sq ft unit in both modes, because that is the unit siding is ordered in.

How waste moves the piece count

The worked ranch, run at five waste settings with its 5 sq ft profile. The adjusted squares move smoothly; the pieces move in whole steps, and the spare column shows how much of the last piece each setting leaves unused.

Read the spare column together with the piece count. A large spare means the round-up already bought most of a piece beyond the adjusted area, so a slightly higher setting would have added nothing; a small spare means the count is sitting just above a boundary, and a slightly lower setting would have dropped it by one.

How to read the result

The headline line is the waste-adjusted squares — the figure to give a supplier who sells by the square. Beneath it, Net wall area shows the bare geometry in square feet (m² in metric mode) and the net squares it makes, so a typo in a dimension is visible before it is padded; With N% cutting waste shows the adjusted area with the percentage that produced it. If you entered a profile, Pieces at N sq ft each (your profile) repeats the coverage the engine computed from your two figures and the whole pieces to buy; check that coverage against the sheet before you order, because it is the one number on the line that came from you. A cost line appears only when both a profile and a price were entered. The result box does not print the spare; the tables on this page show it so the round-up is visible. The copy button places a one-line summary — net area, adjusted squares, and pieces — on the clipboard.

Common misreadings

The engine sees numbers, not units or intentions. Each entry below is a real run, so the consequence is a piece count rather than a warning.

• Exposure in feet. The exposure field takes inches; the piece length takes feet. Enter the 8 in profile as 8 ÷ 12 = 0.6667 — the same measurement in feet — and the engine computes 0.6944 sq ft per piece instead of 8.33: the 180 × 8 ft house orders 2,297 pieces instead of 192 — 11.96 times the real count, the factor of 12 between inches and feet blurred only by the round-up. The squares line is untouched, which is the tell: a piece count wildly out of proportion to the squares means a profile field, and this is the field to check first.
• Piece length in inches. The mirror image. Type the 12.5 ft piece as 150 and each piece is credited with 100 sq ft: the same house orders 16 pieces, roughly a twelfth of the real figure, with no warning, because a 150 ft piece is a valid entry.
• Waste as a fraction. The waste field takes a percentage. The ranch at 10% orders 270 pieces; typed as 0.1 it is read as 0.1%, the adjusted area falls to 1,225.22 sq ft, and the order drops to 246 — 24 pieces short of the intended allowance, because a fraction of a percent is a valid input.
• Gable height from the ground. The gable height is measured from the eave line to the peak. Measure the ranch's gables from the ground instead — 8 ft of wall plus 6 ft of gable, 14 ft — and each triangle grows from 72 to 168 sq ft: the net area becomes 1,416 sq ft and the order 312 pieces instead of 270. The wall under the gable has already been counted in perimeter × height; the triangle must start where the rectangle stops.
• Openings handled twice, or not at all. Leaving ordinary windows and doors in the area is one method; deducting them and adding the allowance is this tool's. The ranch worked under Why each input matters shows both figures, 296 and 245, against the intended 270.
• Metric figures in the imperial fields. The metric house above is 45 × 2.4 m with a 20 cm × 3.66 m profile. Type those same numbers with the toggle still on Imperial and the engine sees a 45 × 2.4 ft wall and a 20 in × 3.66 ft piece: 110.14 sq ft net, 1.21 squares, and 20 pieces, against 166 the metric mode computes. The error is not a simple factor, because the wall shrank and the piece grew; a count implausibly small for the house usually means the toggle.
• An area typed as the perimeter. The first two fields want a length around the house and a height, not a wall area. A wall area you already know should go in as the perimeter with a height of 1; the engine multiplies the two fields, so any pair with the right product gives the right area, and the gables and openings then apply as usual.

Where the model stops

This tool estimates siding quantities only. It determines nothing about fastening, the water-resistive barrier, clearances, or wind ratings — those are installation and code matters for the manufacturer's instructions and your local requirements, as the note under the form says — and nothing about the structure the siding hangs on. Within the quantity itself, the model counts the field pieces that cover the wall and nothing else: J-channel, starter strip, corner posts, and utility trim are linear-foot items that depend on your opening and corner count, and the linear feet calculator measures them. Fasteners are not modelled, in line with the site-wide omissions on the methodology page, nor are regional product variations or delivery minimums.

The geometry is deliberately simple. The walls are one rectangle of uniform height, so a house whose eave height changes from wall to wall is either several runs or a wall area settled first with the square footage calculator and entered as the perimeter with a height of 1. Gables are matching triangles; dormers, bays, and anything that is not a rectangle or a triangle enter only through the area you measure yourself. Openings are a single total, and the waste allowance is one flat percentage for the whole house — real cutting waste varies with the layout, the profile, and the number of corners and openings, and the percentage is the one input you are given to fold that into. The square is treated as exactly 100 sq ft, which it is by definition; the coverage of a piece is treated as exactly exposure ÷ 12 × length, which is only as right as the two figures you read from the sheet. Treat the output as a well-organised starting point for a purchase, not as a bill of materials.

The rest of the exterior

Measuring the wall area a different way? The square footage calculator adds arbitrary rectangles, and painted exteriors size their coats with the paint calculator from the same perimeter-×-height area. Roof squares — the same 100 sq ft convention pitched upward — live in the roofing shingle calculator, and trim runs in the linear feet calculator. For where the square comes from and what the site leaves to you, read Coverage figures and where they come from; for the percentage, Waste factors, explained; for the unit toggle, Metric and imperial in the estimators. The full list is on the guides index.

Frequently asked questions

Dimensions never leave your browser. Sources and the estimating pipeline are documented on the methodology page.