Understanding your result
The headline is the total number of 5/8 in type X sheets for the assembly, and the line under it shows how that total is built: sheets per layer per side, times the layers, times the sides. A one-hour wall boarded on both sides is 1 × 2; a two-hour wall is 2 × 2; a garage ceiling is 1 × 1. The tiles give the per-layer count, the screws for all layers, the weight of the order at 2.2 lb per square foot, and the pails of compound for fire taping.
The table lists the assembly by name with the GA-600 or IRC family it comes from, the face area after openings, the sheet arithmetic, the board area and weight, and the screws by layer: base-layer screws long enough to reach the framing through one sheet, face-layer screws long enough to reach through two. The joint length is for the face layer only; the base layer’s joints are not taped in most two-layer listings, only staggered.
The notes are the part that matters most. A rating is a property of a tested assembly, board, framing, fasteners, joints and sometimes insulation together, and the calculator counts materials for the assembly family you named. For the materials of a specific sound-and-fire wall, the fire-rated assembly calculator works from a chosen listing; for the garage walls that go with a garage ceiling, use the garage calculator. The assemblies reference page lists the common listings with their ratings and sources.
How we calculate this
The layers and sides come from the assembly: one layer each side for one-hour walls, two for two-hour walls, one layer below for the ceiling assemblies and the garage ceiling. Each layer per side is rounded up to whole sheets on its own, because each layer is hung separately. Screw spacing is the Gypsum Association’s single-layer figure (16 in on walls with 16 in framing, 12 in with 24 in framing, 12 in on ceilings); the listing you build to may require closer spacing, and that number wins. The screw calculator gives any other pattern in pounds and boxes.
Compound is the Level 2 fire-taping figure, 0.42 gallons per 100 sq ft of the exposed face, pro-rated from USG’s complete-finishing coverage; the coverage table has the levels side by side. The weight uses 2.2 lb per sq ft for 5/8 in type X; lightweight type X boards are about 1.75 lb per sq ft, and the sheet weight calculator compares them. Every size and board type is on the sheet sizes and weights table.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| 1-hour wall | One layer of 5/8 in type X each side | Gypsum Association GA-600 wood-stud wall listings such as WP 3660 |
| 2-hour wall | Two layers of 5/8 in type X each side, joints staggered | GA-600 two-hour wood-stud partitions such as WP 4135 |
| 1-hour floor-ceiling | One layer of 5/8 in type X or type C below; many listings add resilient channel | GA-600 FC listings |
| Garage ceiling under living space | One layer of 5/8 in type X | IRC Table R302.6 (prescriptive) |
| Screw spacing per layer | 16 in walls (12 in with 24 in framing), 12 in ceilings | GA-216 single-layer spacing; the listing governs |
| Screw lengths | 1-5/8 in base layer, 2 in face layer | At least 5/8 in penetration into wood framing through 5/8 in board, or through two layers |
| Board weight | 2.2 lb per sq ft | Manufacturer data sheets for 5/8 in type X |
| Fire taping | Level 2, 0.42 gal per 100 sq ft | GA-214 Level 2; USG coverage pro-rated |
| Waste | 10% | Rated walls are plain and long; cut-up walls need 15% |
Assumptions last reviewed October 8, 2026.
The calculator does not pick the listing for you, does not model the framing, insulation, sealant or rated doors that are part of a rated assembly, and does not cover shaft walls, area separation walls or steel-stud assemblies, which have their own listings and board types. The guide to fire-rated assemblies explains how to read a GA-600 listing; the garage fire-separation guide covers the prescriptive garage rules, which are simpler.
Two worked examples
A 2-hour wall, 30 ft long and 9 ft tall, with a rated door
Both sides boarded, 16 in framing, 4 × 8 sheets, 10% waste.
- Face area: 30 × 9 − 20 = 250 sq ft
- Sheets per layer per side: 250 ÷ 32 × 1.1 = 8.59, rounded up to 9
- Sheets: 9 × 2 layers × 2 sides = 36 sheets of 5/8 in type X, 1,000 sq ft, about 2,534 lb
- Screws: 28 per sheet; 504 base-layer screws of 1-5/8 in and 504 face-layer screws of 2 in
- Fire taping: 2.1 gallons of compound, one pail; 188 ft of face-layer joints to tape
Thirty-six sheets at 70 lb each is a delivery, not a pickup load. The second layer’s joints are offset from the first, horizontally by half a sheet and vertically so no joint lines up.
A 22 × 22 ft garage ceiling under a bedroom, 24 in trusses
Garage ceiling assembly, one layer, 24 in framing.
- Face area: 22 × 22 = 484 sq ft
- Sheets: 484 ÷ 32 × 1.1 = 16.64, rounded up to 17 sheets of 5/8 in type X, about 1,197 lb
- Screws: 5 trusses × 5 = 25 per sheet, 425 screws of 1-5/8 in
- Fire taping: 2 gallons of compound, one pail
The note reminds you that this is a prescriptive board requirement from the IRC, not an hourly rating, and that rated ceilings on 24 in framing usually need type C board or channel in the listing. A 4 × 8 sheet of type X is 70 lb; rent the panel lift.
Where to find your inputs
Assembly. Your building department, the plans, or the listing number on the drawings (a GA file number such as WP 3660, or a UL design number). If you only know “one-hour wall”, the one-hour wall option counts the most common wood-stud listing.
Length and height, or area. Measure the wall from the framing, floor to the underside of the deck above, because a rated wall runs to the structure, not to a dropped ceiling. For a ceiling, enter length and width.
Openings. Rated doors and fire-rated glazing only. Pipes, ducts and boxes are penetrations, not openings; they are sealed, not subtracted.
Framing spacing. Measure centre to centre. Rated listings specify it; a wall framed at 24 in may match a different listing from the same wall at 16 in.
Common mistakes
- Regular 5/8 in board instead of type X. Same thickness, no rating. The sheet is stamped “Type X” on the back.
- Treating the board as the rating. The screws, the spacing, the joints and the sealant are all part of the listing. Build the whole listing.
- Stopping at a dropped ceiling. A rated wall runs to the structure above. Board above the ceiling grid as well.
- Skipping the tape in an unfinished space. Untaped joints are gaps. Fire-tape every joint and cover every screw head.
- Face-layer screws that stop in the base layer. Two layers of 5/8 in need 2 in screws to reach the framing.
- Lining up the joints of the two layers. Stagger them, or the wall has a straight path for fire through both layers.
Questions people ask
- How many layers of drywall for a 1-hour fire rating?
- The common one-hour wood-stud wall in the Gypsum Association's GA-600 manual is one layer of 5/8 in type X on each side, with screws at the spacing in the listing and all joints taped. A two-hour wall is two layers of 5/8 in type X on each side with staggered joints. Floor-ceiling assemblies vary more; many one-hour ceilings need type C board or resilient channel.
- What is type X drywall?
- Gypsum board with glass fibres in the core so it holds together longer in a fire; 5/8 in type X is the board most rated assemblies specify. Type C is a further improved core used where the listing calls for it, mostly on ceilings. Regular 5/8 in board without the X designation does not carry the rating.
- How many sheets for a 2-hour wall?
- A 30 × 9 ft wall with a 20 sq ft rated door opening has 250 sq ft per face. At 10% waste that is 9 sheets of 4 × 8 per layer per side, and a two-hour wall has two layers on two sides, 36 sheets, about 2,500 lb of board. The base layer takes 1-5/8 in screws and the face layer 2 in screws, 504 of each.
- Does the garage ceiling need a rated assembly?
- Under the IRC it needs 5/8 in type X board on the garage side when there is living space above, which is a prescriptive requirement rather than an hourly rating. Choose "Garage ceiling under living space" and the calculator counts one layer of type X with the joints and screw heads fire-taped; the garage calculator adds the walls.
- Do I have to tape a fire-rated wall in a garage or attic?
- Yes. Every rated assembly in GA-600 includes taped joints and covered fastener heads, and the IRC's garage separation requires the joints to be taped or sealed. Fire taping is a Level 2 finish: tape embedded in compound and one coat over the joints and screw heads, no sanding or finish coats needed.
- Can I use adhesive or wider screw spacing on a rated wall?
- Only if the listing you are building to allows it, and most do not. Fastener type, length and spacing are part of the test, and a wider spacing than the listing voids the rating. The calculator uses the Gypsum Association's single-layer spacing for each layer as a starting figure; the listing governs.