Walls & Insulation
Drywall Calculator
Enter your room dimensions in feet to get the number of drywall sheets to buy, with standard opening deductions, a wastage allowance and an optional cost estimate.
Formula tested · Local units · No sign-up
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Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Wall area = 2 × (room length + room width) × wall height. A 12 ft × 10 ft room with 8 ft ceilings has 2 × (12 + 10) × 8 = 352 ft² of wall. Including the ceiling adds length × width on top.
Doors and windows are deducted using editable standard sizes — 20 ft² per door and 15 ft² per window — so you don't buy sheets for openings you'll cut out anyway.
The net area gets a wastage allowance (10% is a common default for cuts and breakage), is divided by the sheet area (a 4 × 8 ft sheet is 32 ft²; a 4 × 12 ft sheet is 48 ft²), and rounds up to whole sheets.
Real-world example
Worked example: 12 ft × 10 ft room, 8 ft walls, 1 door, 2 windows
- Wall area: 2 × (12 + 10) × 8 = 352 ft² (walls only, no ceiling).
- Deduct openings: 1 door × 20 + 2 windows × 15 = 50 ft².
- Net area: 352 − 50 = 302 ft².
- Add 10% wastage: 302 × 1.10 = 332.2 ft².
- Divide by a 4 × 8 sheet (32 ft²): 332.2 ÷ 32 = 10.38 → round up to 11 sheets.
Buy 11 sheets. At an example price of $12 per sheet with 8% sales tax, that's $132.00 + $10.56 = $142.56.
Before you start
How to measure
- Measure room length and width at floor level in feet, and wall height from floor to ceiling.
- For a single wall or an odd-shaped space, switch to 'single wall / known area' mode and enter the area in ft² directly.
- Count every door and window opening; if a door or window is much larger than standard (double doors, picture windows), adjust the counts to approximate the real opening area.
Local guidance
Notes for United States
- US drywall is stocked in 4 × 8 ft sheets as the default, with 4 × 12 ft (and other lengths) available for taller or longer walls — longer sheets mean fewer butt joints but are heavier to handle.
- Common thicknesses are 1/2 in for walls and 5/8 in for ceilings and fire-rated assemblies; local code and the assembly rating govern — check before ordering.
- Joints are typically finished with paper or mesh tape and joint compound ('mud') in multiple coats — tape and compound are not included in this estimate.
- Sales tax varies by state and locality and is entered manually; supplier quotes usually exclude it.
Quick reference
Common US drywall sheet sizes (planning values)
| Sheet size | Area | Typical use |
|---|---|---|
| 4 × 8 ft | 32 ft² | Standard walls, easiest to handle |
| 4 × 12 ft | 48 ft² | Long walls, fewer butt joints |
Stock sizes vary by supplier — match the sheet your store actually carries.
Good to know
Common mistakes to avoid
- Forgetting to include (or exclude) the ceiling — it can add a third or more to the sheet count.
- Skipping the wastage allowance: cuts around outlets, corners and openings almost always consume extra board.
- Deducting oversized openings at the standard size — a patio slider is far more than 20 ft².
- Buying 4 × 12 sheets for a room you can't carry them into — check stairways and door swings first.
Need help?
Frequently asked questions
How many 4×8 sheets do I need for a 12 ft × 12 ft room with 8 ft ceilings (walls only)?
Wall area is 2 × (12 + 12) × 8 = 384 ft². With no openings and 10% wastage that's 422.4 ft², and 422.4 ÷ 32 = 13.2, so buy 14 sheets.
How much does drywalling the ceiling add?
The ceiling adds length × width. For the 12 × 12 room above it adds 144 ft²: (384 + 144) × 1.10 = 580.8 ft², and 580.8 ÷ 32 = 18.15, so 19 sheets instead of 14.
Should I use 4×8 or 4×12 sheets?
4 × 12 sheets cover more per sheet — the 384 ft² room needs 422.4 ÷ 48 = 8.8 → 9 sheets instead of 14 — and leave fewer butt joints, but they are heavier and harder to move through a finished house. Choose what you can safely handle.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- United States
- Spotted an error?
- Report a correction
Methodology
- The area to board is walls (plus the ceiling if included). In room mode, wall area = 2 × (length + width) × height; the ceiling adds length × width. All lengths are converted to metres internally before any arithmetic to avoid unit drift.
- Openings are deducted using clearly-labelled editable standard sizes (door 1.9 m² / 20 ft², window 1.4 m² / 15 ft²) multiplied by the counts you enter; the net area is floored at zero.
- Sheets = net area × (1 + wastage%) ÷ sheet area, rounded UP to a whole sheet, because sheets are only sold whole. Exact multiples are not bumped up an extra sheet.
- The cost estimate simply multiplies the sheet count by the price you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Sheet areas: 4 × 8 ft = 32 ft² and 4 × 12 ft = 48 ft² (exact); 1 ft² = 0.09290304 m² (exact definition).
- Opening deductions: Editable planning standards of 20 ft² per door and 15 ft² per window — adjust to your actual openings.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not account for fire, moisture or acoustic board requirements, fixings, jointing materials or labour. Confirm quantities and board specification with your supplier before ordering.