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The standard allowance most suppliers and estimating guides assume for ordinary work.
The face area of the wall being boarded, measured on one side only.
Measure the elevation of the face that receives board. For a shaftwall this is the occupied side; the shaft side is closed with liner panel between the studs, which is counted by length rather than by area and is a separate order.
The coverage area of one board as supplied.
Fire-rated board comes in longer lengths than standard board precisely so a wall of any real height can be run without a horizontal joint, and a longer board changes this figure. Confirm the size actually being delivered rather than the size usually stocked.
How many layers of board the tested design puts on this one face.
Read it off the listed design, not off the rating. Two assemblies with the same hourly rating routinely differ in layer count because they differ in stud, cavity, liner and fastener schedule, and a layer count assumed from the rating is a guess about a tested system.
Total area of access doors, louvres and openings you will not board over.
Deduct only openings large enough that the offcut is genuinely reusable. A run of small penetrations produces waste rather than saved board, and deducting them makes the order short.
Allowance for cuts, damage and offcuts that cannot start the next run.
Multi-layer work needs more than single-layer work, because the joints in each layer have to be offset from the layer beneath and the offset is paid for in cut ends. Tall shafts with few openings need less; heavily penetrated corridor walls need more.
Type X boards needed
22 boards
This counts one face only, which is the point of the page: a shaftwall, a party-wall closure and a one-hour corridor lining are all boarded on a single side, and doubling them for a second face is a straight 100% over-order. Take the layer count from the tested design listing for your exact assembly.
- Boards in one layer
- 11 boards
- Net area after opening deductions
- 320 ft²
- Area including the waste allowance
- 352 ft²
They open the calculator with your figures already in it
Single-Sided Fire-Rated Gypsum Board Calculator: 22 boards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- ASTM C840, Standard Specification for Application and Finishing of Gypsum Board, which governs how the layers are laid up, offset and fastened once the count is known
- The number of layers required for a given fire-resistance rating comes from the tested UL or GA design listing for the exact assembly, and cannot be derived from a rating alone
- Arithmetic only: boards per layer = ceil((wall area − openings) × (1 + waste) ÷ board area), multiplied by the number of layers on the single finished face
Inputs used
- Wall Area on the Finished Face
- 320 sq ft
- Board Area
- 32 sq ft
- Layers on the Finished Face
- 2
- Opening Area to Deduct
- 0 sq ft
- Waste Allowance (%)
- 10
Intermediate steps
- Boards in one layer
- 11 boards
- Net area after opening deductions
- 320 ft²
- Area including the waste allowance
- 352 ft²
Confidence note: This counts one face only, which is the point of the page: a shaftwall, a party-wall closure and a one-hour corridor lining are all boarded on a single side, and doubling them for a second face is a straight 100% over-order. Take the layer count from the tested design listing for your exact assembly.
What this calculation does not cover
- Boards only. Screws, joint tape, setting compound, corner bead and the firestopping at the head and the penetrations are separate quantities.
- Does not verify the assembly. Whether a given layer count achieves the rating you need is settled by the listed design, not by a count.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations3
- ASTM C840, Standard Specification for Application and Finishing of Gypsum Board, which governs how the layers are laid up, offset and fastened once the count is known
- The number of layers required for a given fire-resistance rating comes from the tested UL or GA design listing for the exact assembly, and cannot be derived from a rating alone
- Arithmetic only: boards per layer = ceil((wall area − openings) × (1 + waste) ÷ board area), multiplied by the number of layers on the single finished face
Which documents these citations point at
Standards referenced: ASTM C840 (ASTM International, United States).
Cite this page
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