How much fiber cement siding do I need?

Area divided by the exposure gives linear feet. A 2,455 sq ft wall at 7 in exposure needs 4,208 linear feet of board, or 4,629 with 10% waste. You will buy 2,893 sq ft of board to cover 2,455 sq ft of wall, because the lap is bought and hidden.

Is fiber cement lap or panel?

Both exist and Table R703.3(1) treats them separately. Lap is a horizontal board laid to an exposure, which is what this calculator counts. Panel is a sheet, typically 4 by 8, fastened at edges and intermediate supports with no exposure at all. Their waste behaves completely differently and they are priced apart.

Can I choose the exposure on fiber cement?

Not really. Unlike wood, the minimum lap is set by the manufacturer and is usually part of the warranty terms, so the exposure comes with the product rather than being an installer decision. That makes it the one number to confirm before ordering anything.

Is cutting fiber cement dangerous?

It produces respirable crystalline silica when cut dry with an abrasive blade, which is a regulated OSHA exposure rather than a nuisance dust. Score and snap, shears, or a dust-collecting saw with a vacuum are the ordinary controls. Cutting outdoors is not a control on its own.

How thick does fiber cement have to be?

Table R703.3(1) gives 5/16 in nominal for panel siding, along with the fastener type and spacing. That table is the prescriptive schedule and Table R703.3.2 is what limits when it may be used at all.

Does a smaller exposure cost more?

Yes, in direct proportion. The board bought is the area times face over exposure, so dropping from 7 in to 6 in on the same 8 1/4 in board takes the ratio from 1.179 to 1.375, which is nearly 17% more material for the same wall.

Does fiber cement need to be painted?

It is sold both primed and pre-finished, and that is a cost and warranty question rather than a code one. What matters for the takeoff is that a pre-finished product usually has a shorter list of acceptable site cuts, because every cut end has to be sealed and a factory finish cannot be matched on site.

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Fiber Cement Siding Calculator

R703.2 barrier laps and the R703.3.2 wind limits

You buy 18% more board than you have wall, and no better cutting reduces it. The lap is what makes lap siding work, and it is the part of every plank you pay for and never see.

Fiber cement takeoff

Building length

Building width

Wall height per storey

Storeys

Gable ends

no.

Roof pitch, rise per 12

Doors

no.

Windows

no.

Board face width (measure one)

Exposure per course (the maker's figure)

Plank length (0 to skip plank count)

Ultimate wind speed

Wind exposure category (the ground, not the weather)

Design wind pressure

psf

Waste allowance (ours)

Priced as (the source's own row labels)

Board face and exposure are the manufacturer's. The barrier laps come from R703.2 and the wind limits from Table R703.3.2, both quoted from the 2018 Seattle Residential Code.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Table R703.3.2, and where it stops answering

Prices for exterior wall finishes

The published figures for exterior wall finishes are on the siding calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.

This site publishes no price of its own. Why, and what that means for these figures.

Worked example

A 40 ft by 28 ft two-storey house in 8 1/4 in fiber cement lap at 7 in exposure.

2,455 square feet of wall, and 2,893 square feet of board. The 1 1/4 in lap is 15.2% of every plank, which compounds to 17.9% more board than wall. That is 438 square feet bought and covered up.
4,208 linear feet of board, which is the area divided by the exposure. Lap siding is ordered by the foot, and this is why: the area tells you nothing until you know the exposure.
31 courses up an 18 ft wall, and the last one is almost never full.
At 115 mph in Exposure B the attachment is a table lookup. Move the same house to 140 mph in Exposure C and Table R703.3.2 says Design Required, and no calculator can give you a fastening schedule.
Plus the barrier. R703.2 wants it behind the cladding lapped 2 in horizontally and 6 in at vertical joints, so roughly 2,823 square feet of it for 2,455 of wall.

Exposure is the number that matters and it is the manufacturer's, not the board's. Everything else follows from it.

The formula

Lap siding is counted on exposure. Panel siding is not counted this way at all:

linear feet = area × 12 / exposure board bought = area × face / exposure

exposure: what one course shows. On fiber cement this comes with the product
face: the full board width, which is what appears on the invoice
face / exposure: the material multiplier, 1.179 on an 8 1/4 in board at 7 in

None of this applies to panel siding, which is a sheet rather than a course: a panel covers what it measures less its joint treatment, and its waste comes in whole offcuts wherever a wall is not a multiple of four feet. Using the lap arithmetic on panel gives an answer that describes neither product.

What the code sets and what the manufacturer sets

The code sets the barrier and the wind gate. The manufacturer sets the exposure. That division runs through this whole group of pages and it is worth stating: R703.2 requires a water-resistive barrier behind the cladding, lapped 2 in horizontally and 6 in at vertical joints, and Table R703.3.2 decides whether prescriptive attachment applies at all.

Exposure is not in the IRC. It comes from the product, and it varies with the board width and with the manufacturer's own minimum lap. Two 8 1/4 in boards from different makers can carry different exposures, and the difference lands directly on the order.

Fiber cement also has a minimum nominal thickness in Table R703.3(1), 5/16 in for panel siding, along with the fastener and spacing schedule. That table is what R703.3.2 is limiting the use of.

And cutting it is a health question rather than a code one. Fiber cement is largely silica and cutting it dry produces respirable crystalline silica. That is an OSHA matter, it is not in Chapter 7, and it is the single most important thing about working with the material that a quantity page cannot tell you.

Panel and lap are two different products with two different rules

Fiber cement comes in both, they are priced differently, and Table R703.3(1) treats them as separate entries.

Lap siding is a horizontal board, laid in courses with an exposure, and it is what this calculator counts. Everything above is a function of the exposure, which is why the exposure is the only number that really matters on the order.

Panel siding is a sheet, usually 4 by 8 or 4 by 10, fastened at panel edges and intermediate supports rather than lapped. Its minimum nominal thickness in Table R703.3(1) is 5/16 in, and it has no exposure at all: a sheet covers what it measures, less the joint treatment.

Which means the waste behaves completely differently. Lap siding wastes at the ends of runs and around openings; panel siding wastes in whole offcuts when a wall is not a multiple of four feet. A wall that suits one may be poor for the other.

The published price rows below separate them for the same reason: planks and panels are quoted apart, and mixing the two figures on one wall gives an answer that describes neither.

Cutting it is a health question the code does not cover

This is the most important thing about working with fiber cement and it is not in IRC Chapter 7, because it is not a building question.

Fiber cement is largely silica, and cutting it dry with an abrasive blade produces respirable crystalline silica. That is a regulated exposure under OSHA, not a nuisance dust, and the difference between controlled and uncontrolled cutting is very large.

The controls exist and they are ordinary. Score and snap for straight cuts, shears for volume, or a saw with a dust-collecting blade and vacuum. Cutting outdoors is not a control on its own; the dust is fine enough to stay airborne and to reach the person holding the saw first.

It also affects the plan rather than just the day. A house with many cuts, lots of openings or a complicated gable is a house with a lot of cutting, and that is a scheduling and equipment decision made before the material arrives.

Nothing on this page substitutes for the manufacturer's own cutting guidance, which is specific to the product and is the document that matters here.

Frequently asked questions

How much fiber cement siding do I need?: Area divided by the exposure gives linear feet. A 2,455 sq ft wall at 7 in exposure needs 4,208 linear feet of board, or 4,629 with 10% waste. You will buy 2,893 sq ft of board to cover 2,455 sq ft of wall, because the lap is bought and hidden.
Is fiber cement lap or panel?: Both exist and Table R703.3(1) treats them separately. Lap is a horizontal board laid to an exposure, which is what this calculator counts. Panel is a sheet, typically 4 by 8, fastened at edges and intermediate supports with no exposure at all. Their waste behaves completely differently and they are priced apart.
Can I choose the exposure on fiber cement?: Not really. Unlike wood, the minimum lap is set by the manufacturer and is usually part of the warranty terms, so the exposure comes with the product rather than being an installer decision. That makes it the one number to confirm before ordering anything.
Is cutting fiber cement dangerous?: It produces respirable crystalline silica when cut dry with an abrasive blade, which is a regulated OSHA exposure rather than a nuisance dust. Score and snap, shears, or a dust-collecting saw with a vacuum are the ordinary controls. Cutting outdoors is not a control on its own.
How thick does fiber cement have to be?: Table R703.3(1) gives 5/16 in nominal for panel siding, along with the fastener type and spacing. That table is the prescriptive schedule and Table R703.3.2 is what limits when it may be used at all.
Does a smaller exposure cost more?: Yes, in direct proportion. The board bought is the area times face over exposure, so dropping from 7 in to 6 in on the same 8 1/4 in board takes the ratio from 1.179 to 1.375, which is nearly 17% more material for the same wall.
Does fiber cement need to be painted?: It is sold both primed and pre-finished, and that is a cost and warranty question rather than a code one. What matters for the takeoff is that a pre-finished product usually has a shorter list of acceptable site cuts, because every cut end has to be sealed and a factory finish cannot be matched on site.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 7 Wall CoveringSeattle Department of Construction and Inspections, page 439. Read 2026-09-08. Source of R703.2 for the water-resistive barrier and its laps, and of Table R703.3.2 and Section R703.3.2 for the wind limits on prescriptive attachment.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Exposure comes with the product on fiber cement. The minimum lap is the manufacturer's and is usually tied to the warranty, unlike wood where the installer sets it. Where Table R703.3.2 gives Design Required, or the design wind pressure exceeds 30 psf, no calculator can give you a fastening schedule. Cutting fiber cement produces respirable crystalline silica, which is an OSHA matter, outside IRC Chapter 7 and outside this page.