Paint Drywall Calculator

Purpose and model boundary

This calculator estimates the paint, primer and drywall for the interior of a floor plan you draw. You draw the outline, the interior walls, the windows and the doors, set one wall height for the plan, check each window's and door's height and each window's sill, enter the drywall sheet size in the drawing's Materials panel, and press Lay out sheets: the drawing lays whole drywall sheets on every wall face and ceiling and counts them, and the spreadsheet returns the paint and primer to buy, the drywall sheets to buy with the waste added, the joint compound, tape and screws for them, and each room's wall and ceiling areas, paint and sheets. One wall height applies to the whole plan; ceilings are flat; every wall face and every ceiling is laid in one orientation for the whole plan, horizontal unless it is changed in the drawing's Materials panel, each ceiling from its room's corner unless that start is moved on the drawing, and each wall face and each ceiling is laid on its own. It does not cover sloped or vaulted ceilings, trim, the doors' own finish, texture, pricing or labour.

Inputs and units

The drawing is the first input. The drawing tool finds the rooms and writes one row per room (its ID, floor area and enclosing-wall length), one row per wall side of every room, and one row per window and per door with its width and the room it belongs to. A wall between two rooms is listed once for each room, and a free-standing partition twice, once for each face. The outline is also sent as its corner points, and up to five islands or columns inside the floor as exclusions. Each exclusion has a height, set by double-clicking it on the drawing: blank is full height, as for a column or a closet, and a height at or above the wall height counts as full height. A lower one, such as a kitchen island, leaves the ceiling above it to paint and board.

The heights are set on the drawing or in the tables under it: one wall height for the whole plan, set with the drawing's Wall height button; each window's height and sill height; and each door's height. Lengths and heights are in feet under US and metres under Metric, never inches. The spreadsheet takes a wall height from 6 to 20 ft (1.80 to 6.00 m), a window at least 1 ft (0.30 m) tall on a sill of zero or more, and a door at least 5 ft (1.50 m) tall; these are the drawing tool's ranges, a product range rather than a code requirement. On the page the drawing accepts the wall heights valid in both unit systems, 6 to about 19.68 ft (1.83 to 6.00 m), so a unit switch never leaves the wall out of range; in the same way it asks for a window at least 1 ft (0.3048 m) and a door at least 5 ft (1.524 m) tall in both systems, so in Metric it refuses a 0.30 m window or a 1.50 m door that the spreadsheet alone would take. The wall height is one physical height and converts exactly on a switch (8 ft is 2.4384 m). Each unit system keeps its own opening heights; a new opening, and after a switch or when a worked example loads an opening with no height in the current system, gets the drawing's default heights: a door 6 ft 8 in or 2.03 m, no higher than the wall, and a window 4 ft or 1.20 m on a 3 ft or 0.90 m sill, the sill lowered as needed so the window fits below the wall. A blank window or door height, or a blank sill on a window that has a height, is refused.

Lay out sheets writes one row per room: the sheets laid on its wall faces and the sheets laid on its ceiling, before waste. Calculate pressed while the sheets are not laid, or after anything they depend on has changed (the drawing, a height, the sheet size, the orientation or the unit system), lays them first, so the counts sent always belong to the drawing on screen.

The drywall sheet is entered in the drawing's Materials panel, above its Lay out sheets button, in feet under US and metres under Metric, with the one orientation for the whole plan:

• Sheet length and sheet width: the drywall sheet, its length laid along the wall when it is hung horizontally, 8 by 4 ft under US and 2.4 by 1.2 m under Metric by default. For a 12 by 4 ft sheet type a length of 12.
• Orientation: Horizontal by default, which lays each sheet's length along the wall; Vertical stands every wall sheet upright and turns every ceiling's sheets a quarter turn. The one choice applies to every wall face and every ceiling.

The panel shows no counts: each room's sheets laid are returned in the results, in the Drywall card and the By room table, once Calculate is pressed.

The inputs beside the drawing are:

• Coats: finish coats of paint, 2 by default. Primer is counted separately, at one coat.
• Paint coverage: the area one gallon or litre covers in one coat, 350 sq ft per gallon under US and 10 square metres per litre under Metric by default.
• Include ceilings: Yes paints and boards the ceilings as well as the walls; No leaves them out, and the ceiling sheets the drawing laid are then not counted.
• Drywall waste: extra for cutting and fitting, 10% of the sheets laid by default, applied once.

Each unit system has its own coverage and sheet size, which are separate figures, not conversions of each other. Every default is a typical figure or an assumption to review; use the coverage on your paint's label and your supplier's board size.

Where the drawn values come from

Each room's floor area and enclosing-wall length come from the drawing tool's room detection, and each wall's length and each window's and door's width and room come from the drawing. The spreadsheet measures the outline's perimeter and area from the corner points itself, and checks that the room areas add up to the floor area, that the walls listed go all the way round every room and the outline, and that every wall, window and door belongs to a listed room.

Governing relationships

Every room is taken off at the plan's wall height:

Wall run = the sum of the room's wall lengths

Gross wall = wall run × wall height

Openings = the sum of width × height of the room's windows and doors

Net wall = gross wall − openings

Ceiling = the room's floor area, when ceilings are included, otherwise 0

Paintable area = the sum of every room's net wall and ceiling, plus, when ceilings are included, the ceiling above every exclusion lower than the walls (the sum of their areas; a full-height exclusion has no ceiling)

Paint = paintable area × coats ÷ coverage, rounded up

Primer = paintable area ÷ coverage, rounded up

Sheets laid = the sum of every room's wall sheets, plus its ceiling sheets when ceilings are included

Drywall sheets = sheets laid × (1 + waste), rounded up

Compound buckets = drywall sheets ÷ 12, Tape rolls = drywall sheets ÷ 25 and Screw boxes = drywall sheets ÷ 30, each rounded up

A window's sill decides whether it fits and where its cut-out sits in the sheets; it never changes the area taken off. A window fits when its sill plus its height is no higher than the wall, and a door when its height is no higher than the wall.

Nothing else is rounded: the shown figures are the display of the full values. Every count is taken at six decimals first, so a figure that is whole on paper stays whole. The By room table rounds each room's paint and sheets up on their own, so its Paint and Sheets columns need not add up to the totals and are usually more; with an island or column drawn and ceilings included, the ceiling above it is in the total paint but in no room's row, so the Paint column can also add up to less.

How the sheets are counted

The drawing tool lays whole sheets on each wall face, from the top of the wall down and from the left end as seen inside the room, with the joints of each row offset by half a sheet, cutting out each window and door; offcuts are reused on the same face. It lays each ceiling with whole sheets over the room less its full-height exclusions, so over an exclusion lower than the walls, such as an island, too, from the room's corner unless that room's start is moved on the drawing. Every wall face and every ceiling is laid in the one orientation chosen for the whole plan in the drawing's Materials panel, Horizontal by default; the counts follow what is laid. It sends the number of sheets laid per room; the spreadsheet checks that they cover at least the area they are laid on, allowing for the drawing tool's contact tolerance, adds the waste and rounds up.

The check holds each room's wall sheets × the sheet's area to at least its net wall, and, with ceilings included, its ceiling sheets × the sheet's area to at least its floor area, each less the contact allowance: the drawing tool leaves out a sheet that would meet a surface only along a sliver no wider than its contact tolerance, 0.00001 in (0.0000254 cm), so a surface may be covered short of its area by at most that sliver for each sheet it examines, at most 10,000 per surface. For an 8 by 4 ft sheet the allowance is 10,000 × 0.00001 × 96 ÷ 144 = 0.0667 sq ft per surface; for a 2.4 by 1.2 m sheet, 0.006096 square metres. Each wall row is one surface, and each ceiling one. With ceilings included the ceiling sheets together must also cover the rooms plus the exclusions lower than the walls, less one allowance a room, because the drawing lays each ceiling over the room less its full-height exclusions.

A laid count is higher than the wall area divided by a sheet, because every face starts with whole sheets.

Calculation sequence

1. The spreadsheet checks the drawing, the tables, the heights, the inputs and the sheet layout, in the order the status list below gives.
2. For each room it takes the wall run at the wall height, takes off the windows and doors, and adds the ceiling when ceilings are included.
3. It adds the rooms' paintable areas and the ceiling above any exclusion lower than the walls, and works out the paint and the primer.
4. It adds the sheets the drawing laid, the ceiling sheets only when ceilings are included, adds the waste once and rounds up, then counts the compound, tape and screws from the sheets to buy.

The default plan works through as follows: one 20 by 15 ft room with 8 ft walls and no openings has a 70 ft wall run, the wall height 8 ft × the 70 ft wall run = 560 sq ft of wall, and a 300 sq ft ceiling, so 860 sq ft to paint: 860 × 2 ÷ 350 = 4.91 → 5 gallons of paint and 860 ÷ 350 = 2.46 → 3 of primer. The drawing lays 18 wall sheets and 10 ceiling sheets: 18 wall sheets + 10 ceiling sheets = 28 laid; 28 × 1.10 = 30.8 → 31 to buy, with 3 buckets of compound, 2 rolls of tape and 2 boxes of screws.

Worked example 11.4.1.2 takes every room off at its 9 ft wall height: the living room's 56 ft run is 56 × 9 = 504 sq ft of wall, and the 40 ft run of each smaller room 40 × 9 = 360. Worked example 11.4.1.3, in Metric at a 2.50 m wall height, needs 16 litres of paint and 50 sheets to buy.

Outputs and interpretation

• Paint and primer: the gallons or litres to buy, and the paintable area they cover.
• Drywall: the sheets to buy (the sheets laid plus the waste), the sheets laid, and the compound buckets, tape rolls and screw boxes for the sheets to buy.
• From the drawing: the net floor area, the gross area and the area of each island or column.
• By room: one row per room with its height (the wall height), its wall run, its gross wall, the window and door area taken off, its net wall, its ceiling, the paint for it, and the sheets laid and the sheets to buy. The headings carry the units of the system the drawing was sent in. The ceiling above an exclusion lower than the walls is in the paintable area and the paint, not in this table's Ceiling and Paint columns; a full-height exclusion has no ceiling. A room's Sheets laid and Sheets are its own counts, which include the ceiling sheets laid over the room less its full-height exclusions.

A laid count is the drawing's own: it is usually higher than the area divided by a sheet, because each face and ceiling starts with whole sheets and a face's offcuts are reused on that face only.

The 3D view draws the walls you drew at the wall height, with the door and window openings cut at their sills and heights, each exclusion as a block of its height (a full-height one reaches the wall tops), and no ceiling or roof. While the sheets are laid, it draws their joints on the wall faces; the ceiling sheets show on the plan only. It is for checking the drawing; it shows no figures, sends nothing and changes nothing.

Validation and status logic

The status reads OK, CHECK with what to look at, or NOT VALID with what to fix. While it is NOT VALID every quantity reads zero and the By room rows are blank; the drawn areas still show. CHECK shows every figure. The first problem found is reported, in this order:

1. NOT VALID: choose US or Metric
2. NOT VALID: draw at least three outline points
3. NOT VALID: every outline point needs both coordinates
4. NOT VALID: every point of exclusion 1 needs both coordinates
5. NOT VALID: every point of exclusion 2 needs both coordinates
6. NOT VALID: every point of exclusion 3 needs both coordinates
7. NOT VALID: every point of exclusion 4 needs both coordinates
8. NOT VALID: every point of exclusion 5 needs both coordinates
9. NOT VALID: each exclusion needs zero or at least three complete points
10. NOT VALID: outline and exclusion points must fill consecutive rows, with no blank row between them
11. NOT VALID: the floor outline encloses no area
12. NOT VALID: the exclusions leave no floor area
13. NOT VALID: room, wall, window and door rows must be complete, with lengths above zero and no negative area, and fill consecutive rows with no blank row between them
14. NOT VALID: the room areas do not add up to the floor area drawn
15. NOT VALID: every room must be listed once, and every wall, window and door must belong to a listed room
16. NOT VALID: the walls listed do not go all the way round every room and the outline drawn
17. NOT VALID: wall, window and door IDs must each be listed once, and no ID or room ID may contain *, ? or ~ or begin with <, > or =
18. NOT VALID: the wall height must be from 6 to 20 ft (US) or from 1.80 to 6.00 m (Metric)
19. NOT VALID: every window needs a height of at least 1 ft (US) or 0.30 m (Metric)
20. NOT VALID: every window needs a sill height of zero or more
21. NOT VALID: every door needs a height of at least 5 ft (US) or 1.50 m (Metric)
22. NOT VALID: a window or door reaches above the wall height
23. NOT VALID: an exclusion height must be greater than zero; leave it blank for full height
24. NOT VALID: an exclusion height is given for an exclusion that is not drawn
25. NOT VALID: the windows and doors in a room cover more than its walls
26. NOT VALID: coats must be a whole number of at least one
27. NOT VALID: paint coverage must be greater than zero
28. NOT VALID: choose Yes or No for including ceilings
29. NOT VALID: drywall sheet length and width must be greater than zero
30. NOT VALID: enter a drywall waste between 0% and 100%
31. NOT VALID: the sheet layout must list every room once, by its ID, in consecutive rows with no blank row between them: press Lay out sheets on the drawing
32. NOT VALID: every room's wall and ceiling sheet counts must be whole numbers of 0 or more: press Lay out sheets on the drawing
33. NOT VALID: a room has fewer sheets than its wall or ceiling area needs: press Lay out sheets again
34. CHECK: the walls follow the rooms only, so add the sides of any column drawn as an exclusion; with ceilings included, the ceiling over an exclusion lower than the walls is in the paintable area, not the By room table; a full-height one has no ceiling

The height limits are the drawing tool's own: the spreadsheet holds a direct caller to the drawing tool's range for each unit system (items 18 to 21), and the page narrows the wall and the openings to the ranges valid in both systems. A room ID must match its room's ID exactly, case included, and two IDs that differ only in case count as one ID listed twice. The sheet counts are the drawing's: on the page Lay out sheets writes every room once with whole counts that cover their surfaces, and Calculate lays them again whenever anything they depend on has changed. Items 31 to 33 refuse a layout that leaves out a room or lists one twice, a count that is not a whole number of 0 or more, and a count whose sheets cover less than its wall or ceiling area, less the contact allowance, such as counts laid for walls too low to cover them. Item 33 is a lower bound only: a count laid for another drawing that still covers the area, or any count higher than the drawing would lay, is not detected, so a direct caller must send the counts laid for this drawing. CHECK means an island or column is drawn: the walls follow the rooms only and leave out its sides, and the By room table's Ceiling and Paint columns leave out the ceiling above an exclusion lower than the walls, which the paintable area and the paint include; a full-height exclusion has no ceiling. Items 23 and 24 refuse an exclusion height of 0 or less and a height given for an exclusion that is not drawn; the page never sends either. The spreadsheet checks the rows' completeness, IDs, room areas, wall coverage, heights and sheet counts; it does not check the wall and opening widths or an opening's room against any real plan, which the drawing tool writes on the page.

Assumptions and limitations

• One wall height applies to the whole plan; per-room and per-wall heights are not used.
• A column's sides are not counted. Each exclusion has a height (blank is full height): the ceiling over an exclusion lower than the walls, such as an island, is in the paintable area and the paint but not in the By room table's Ceiling and Paint columns, and a full-height one, such as a column or a closet, has no ceiling. The ceiling sheets are laid over the room less its full-height exclusions and are in its room's Sheets laid and Sheets. The status reads CHECK while an exclusion is drawn.
• An interior door is cut from the wall of the room it opens into only; the face on the other side of the doorway stays wall.
• One orientation applies to the whole plan: every wall face and every ceiling is hung horizontally, or every one vertically; one room or one wall cannot be hung differently from the rest. Offcuts are reused only on the face they were cut from, never across faces or onto a ceiling.
• The compound, tape and screws are typical ratios of one bucket for every 12 sheets, one roll for every 25 and one box for every 30, to review.
• Nothing is saved between visits. Export keeps the drawing but not the heights, and the room areas and enclosing lengths come from the drawing tool's room detection.

Restrictions and non-computing states

The spreadsheet computes nothing while the status is NOT VALID: every quantity reads zero and the By room rows are blank. Edits on the page stay local until Calculate is pressed, and Calculate lays the sheets first whenever they are not laid or are out of date.

Errors and warnings

Each NOT VALID line above names the input to fix, and the CHECK line names what to look at. A height outside the drawing's limits, a blank opening height or a blank sill on a window that has a height is refused, because the opening must still fit the wall. If the drawing cannot lay the sheets (for example with a window or door that has no height, a window that would reach above the wall, or a sheet size of zero), Calculate sends nothing and shows the drawing tool's own message, which names the opening or the setting to fix.

Sources and further reading

• National Institute of Standards and Technology, Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B.8. nist.gov. The foot is exactly 0.3048 m, the square foot 0.09290304 m² and the U.S. gallon 3.785412 L: the basis for the two unit systems, and for saying the Metric defaults are separate figures, not conversions.
• Eric W. Weisstein, "Polygon Area," MathWorld, Wolfram Research. mathworld.wolfram.com. The shoelace formula the spreadsheet uses for the outline's area.
• Gypsum Association, Application and Finishing of Gypsum Panel Products, GA-216-2010. gypsum.org. Section 1.1, its scope: the application and finishing of gypsum panel products on interior walls and ceilings, the basis for boarding both.
• Sherwin-Williams, Duration Home Interior Latex Flat, product data sheet 101.78A. paintdocs.com. The manufacturer's technical data: a coverage of 350 to 400 sq ft per gallon at 4 mils wet, and two coats over drywall.

350 sq ft per gallon is the low end of the manufacturer's stated range (cited); the default heights and ranges are the drawing tool's; the coats, sheet sizes, waste and consumable ratios are the calculator's own typical figures; each system has its own. Use the coverage on your paint's label and your supplier's board size. These sources describe units, geometry, practice and a product's data and certify no particular result.