How to Figure Gallons for an Interior Room

Why this matters

Run short and the crew stops mid-wall to wait on a store run, then rolls a new can that may not match the one it finished. Run long and the customer's garage fills with gallons they paid for and never needed. A gallon count from a takeoff takes ten minutes per room, and it gives the estimator a material line, the crew a box count, and the shop a number to reconcile against when the job closes. The two places it usually goes wrong are using a house coverage number instead of the one printed for the product, and treating the first coat over a color change or new drywall as if it spreads like the second.

Start state

• A tape or laser, a pad, and the product data sheet for each coating you will use on that room: primer, wall finish, ceiling. The spread rate comes off that sheet, never from memory or another product.
• The scope settled: walls, ceiling, closet interior or not, trim or not. Trim is a separate line in linear feet; this card covers walls and ceilings.
• The existing condition read: what color is there now, what color is going on, any new or patched drywall, any stained or glossy areas. That reading decides how many lines the takeoff has.

The procedure

1. Measure the room's length, width and wall height. Wall height is floor to ceiling, not floor to the top of the baseboard; the baseboard area is small and covers handling losses. Why: every later line is built on these three numbers, and a 9 ft ceiling taken as 8 ft drops a ninth of the wall area.

2. Compute the perimeter, then the gross wall area. Perimeter is 2 x (length + width); gross wall is perimeter x height. Why perimeter first: it handles any rectangular room in one step and makes a missed wall impossible. For an L-shaped room, walk the walls and add each length rather than forcing the formula.

3. Deduct the openings you will not paint. Doors and windows at their nominal opening size. A standard 3 ft 0 in by 6 ft 8 in door is 3 x 6.67, about 20 sq ft. What to watch: some estimators skip deductions on purpose to cover waste, and some add a waste factor. Doing both buys the waste twice. This card deducts openings and adds one handling factor at step 7.

4. Compute the ceiling area. Length x width for a flat ceiling. Sloped or vaulted ceilings are triangles and trapezoids; see Area of Triangles, Gable Ends and Sloped Walls rather than guessing.

5. Split the takeoff into lines by coat condition. Each line is one product, one surface, one coat condition. A first coat over a color change, over bare drywall, or over a patched and primed surface goes on its own line because it is a different product (a primer) or the same product at a lower spread rate. Lumping it with the topcoats is where most interior shortfalls come from.

6. Divide each line's area by the data sheet spread rate. Area x coats, divided by the sheet's spread rate in sq ft per gallon. Use the low end of the sheet's range on anything less than smooth and sealed. Why: the sheet's figure assumes the recommended film on a smooth, non-absorbent surface with no losses, and a real wall falls short of that. The sheet's spread rate is also a film thickness in disguise: at 350 sq ft per gallon it is laying about 4.6 mils wet, and that math is owned by Spread Rate, Wet Film and Dry Film: The 1604 Number.

7. Add a handling factor, then round up to cans you can buy. Paint left in the roller nap, the tray, the cut bucket and the strainer is real. This card uses 10 percent as a shop default; tune it from your own reconciliations. Then round up to gallons and quarts. Why round up and not to the nearest: the shortfall costs a store trip and a possible batch mismatch, the overage costs a quart.

8. Write the box count on the ticket. Any line of more than one gallon gets boxed before rolling; see How to Box Paint for a Large Job.

Worked example: bedroom 2

A 12 by 14 ft bedroom with 8 ft walls. One entry door, a bifold closet opening 5 ft wide, two windows 3 by 5 ft. The walls are a deep brick red going to a light greige; the ceiling is sound flat white staying flat white. Closet interior is not in scope.

Values from the data sheets in hand for this job (illustrative here; read yours): the stain-blocking primer the shop uses for color changes lists 300 sq ft per gallon on previously painted surfaces; the wall finish lists 350 to 400; the ceiling flat lists 350 to 400.

The color change gets a tinted primer coat on its own line. Two finish coats over a primed, light-tinted surface is the plan, not two finish coats straight over red, which on a light color usually turns into three coats and a short order.

TAKEOFF  BEDROOM 2   12 x 14 ft, 8 ft walls
WALLS
  Perimeter    2 x (12 + 14)          =  52 ft
  Gross        52 ft x 8 ft           = 416 sq ft
  Less door    3 x 6.67               = -20 sq ft
  Less closet  5 x 6.67               = -33 sq ft
  Less 2 win   2 x (3 x 5)            = -30 sq ft
  Net wall     416 - 83               = 333 sq ft
CEILING        12 x 14                = 168 sq ft
LINE 1  wall primer, 1 coat, PDS 300
  333 / 300                           = 1.11 gal
  x 1.10 handling                     = 1.22 gal
  Buy                                 = 1 gal + 1 qt
LINE 2  wall finish, 2 coats, PDS low 350
  2 x 333 = 666 / 350                 = 1.90 gal
  x 1.10 handling                     = 2.09 gal
  Buy                                 = 2 gal + 1 qt
LINE 3  ceiling flat, 1 coat, PDS low 350
  168 / 350                           = 0.48 gal
  x 1.10 handling                     = 0.53 gal
  Buy                                 = 1 gal

Checking the work from the other direction: line 2 buys 2.25 gallons, which at 350 sq ft per gallon covers 787 sq ft against 666 sq ft of wall, two coats. That is 121 sq ft of slack, about 18 percent of the need, which covers the 10 percent handling allowance with a little left for a touch-up reserve. Line 1 buys 1.25 gallons, 375 sq ft at 300, against 333 sq ft, 42 sq ft of slack, about 13 percent; tighter, and acceptable for a primer that will be covered.

Line 3 is the decision worth looking at. 0.53 gallon is just over the 0.50 gallon that two quarts hold. Two quarts would leave the crew short by a few square feet of ceiling, which is the worst possible place to run out because ceiling flat touch-ups flash. The ticket buys a gallon.

What the ticket now says: 1 gallon and 1 quart of primer, 2 gallons and 1 quart of wall finish (boxed into one pail), 1 gallon of ceiling flat. If the shop had used a single house number of, say, 400 sq ft per gallon for everything and skipped the primer line, the order would have been two coats of finish only, 666 / 400 = 1.67 gallons, rounded to 2. The crew would have found out on the second coat that red does not hide under light greige in two passes, with no primer on the truck.

What changes the count

• New drywall instead of a color change. Line 1 becomes the drywall primer the board and mud call for, at that primer's own sheet rate. Bare paper and joint compound absorb unevenly, so the primer line uses the sheet's low end. See Priming New Drywall (PVA vs Level 5 Skim Coat Sealer) before choosing the product.
• Same color, sound surface. A refresh in the same color over a clean, sound, same-sheen finish may need only one coat, and the takeoff drops to one line at the finish rate. Decide that after reading the wall, not before.
• The closet comes into scope. The customer decides at the walkthrough that the closet gets painted too. A 2 ft deep by 5 ft wide closet with 8 ft walls carries a back wall of 5 x 8 = 40 sq ft and two sides of 2 x 8 = 16 sq ft each, 72 sq ft of wall (the front is the opening, already deducted). Over red it needs the primer too: line 1 becomes 333 + 72 = 405 sq ft / 300 = 1.35 gallons, x 1.10 = 1.49, so 1 gallon and 2 quarts. Line 2 becomes 2 x 405 = 810 sq ft / 350 = 2.31 gallons, x 1.10 = 2.55, so 2 gallons and 3 quarts, which is the point at which most crews buy 3 gallons and box them. A closet looks like nothing at the walkthrough and adds over a fifth to the wall lines, 72 against 333 sq ft.
• Textured walls. Knockdown and orange peel carry more true surface than the flat area you measured, and they hold more paint in the roller. Use the sheet's low end or below, and let your reconciliation history set how far below.
• Taller walls or a stair hall. The area formula does not change, but a 16 ft stairwell wall on a ladder or plank runs more handling loss from cutting in at height. Some shops raise the handling factor for those lines; record what you do so the reconciliation can test it.

How to verify the count was right

Close every job with the reconciliation the film-build SOP describes: measured area times coats, divided by gallons actually used, for each line. If line 2 on this bedroom used 1.8 gallons, the crew spread 666 / 1.8 = 370 sq ft per gallon, inside the sheet's 350 to 400 range. If it used 1.5 gallons, 666 / 1.5 = 444 sq ft per gallon is above the top of the range, and a spread rate above the range means the film is thin somewhere, whatever the wall looks like on handover day. See Film Build and Wet Film Thickness Verification for what the crew does about it.

Over a run of jobs, the reconciliation tells the estimator where the takeoff is consistently short or long. That is how a shop earns the right to tune the handling factor, and it is the only honest source for a texture or stairwell adjustment.

References

• See related: Spread Rate, Wet Film and Dry Film: The 1604 Number
• See related: How to Box Paint for a Large Job
• See related: Area of Triangles, Gable Ends and Sloped Walls
• See related: Film Build and Wet Film Thickness Verification
• Coating manufacturer product data sheets for spread rate on the surface in question