Understanding your result
The headline is the wall paint for the house, and the sub-line gives the can list for all three products. The four figures show the total wall area, the ceiling paint, the trim enamel and either the total bought or, if you entered prices, the paint cost.
The breakdown goes room type by room type, with the wall area each type contributes, then the hallway, then the three products with their areas, quantities and cans, then the house total and the cost. The arithmetic panel shows how each room type’s wall area was derived from its template size, so you can see whether the templates fit your house or whether a room should be counted as the next size up.
Twenty-six gallons across three products is a job to plan, not just to buy. The painting time calculator turns the same wall area into hours, the project planner into a calendar, and the paint cost calculator adds supplies and tax to the paint figure. If a hired painter is quoting, the painter cost estimator uses the same areas.
How we calculate this
Each room type is a template: a floor size and a count of doors and windows, from which the wall area, ceiling area and trim area follow by the same arithmetic the wall paint, ceiling paint and trim and baseboard calculators use for a single room. A small room is 10 × 10 ft with one door and one window: 320 sq ft of wall at 8 ft, 285 after the openings, 100 sq ft of ceiling, and about 41 sq ft of trim (37 ft of 5 in baseboard plus a door casing and a window casing). The templates are listed on the standard sizes page.
The allowances are the same as the single-room calculators: 5% on walls and ceilings, 10% on trim because it is brushed. Each product is rounded to its own cans, so a 17.54 gallon wall figure becomes three pails, two gallons and three quarts. The master guide walks through a whole room by hand if you want to check one of the templates against a room of your own.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Room templates | small 10 × 10, medium 12 × 14, large 15 × 18, bathroom 6 × 8 ft | Site conventions chosen to bracket common US room sizes; listed on the standard sizes page |
| Openings per template | 1/1, 1/2, 2/2, 1/1 doors/windows; hallway one door per 10 ft | Site conventions; a door is 20 sq ft, a window 15 |
| Baseboard and casings | 5 in baseboard; 16 sq ft per door casing, 10 per window | Derived on the trim calculator page |
| Wall coverage | 350 sq ft per gallon | Interior wall paint data sheets (Sherwin-Williams, Benjamin Moore, Behr): 350–400 |
| Ceiling coverage | 400 sq ft per gallon | Flat ceiling paint data sheets, smooth ceilings; lower for texture |
| Trim coverage | 400 sq ft per gallon | Trim enamel data sheets: 350–400 |
| Coats | 2 | Manufacturers’ recommendation for a colour change |
| Allowances | 5% walls and ceilings, 10% trim | Roller and brush loss; round figures |
Assumptions last reviewed October 7, 2026.
The estimator does not model textured ceilings (set the ceiling coverage lower, or run the ceiling calculator), primer (new drywall needs it everywhere, the primer calculator gives the quantity), different colours per room (the total holds, but each colour is rounded separately so the can count rises), or rooms far from the templates. The measuring guide explains how to handle open-plan spaces, alcoves and sloped ceilings room by room.
Two worked examples
A three-bedroom house, everything, with prices
Two small rooms, three medium, one large, two bathrooms, a 30 ft hallway, 8 ft ceilings, ceilings and trim included, two coats. Wall paint $38 a gallon, ceiling paint $25, trim enamel $45.
- Walls: 2 × 285 + 3 × 366 + 458 + 2 × 189 + 420 = 2,924 sq ft
- Wall paint: 2,924 ÷ 350 × 2 × 1.05 = 17.54 gallons, buy 3 pails + 2 gallons + 3 quarts
- Ceilings: 1,175 sq ft; ceiling paint 6.17 gallons, buy 1 pail + 1 gallon + 1 quart
- Trim: 471 sq ft; trim enamel 2.59 gallons, buy 2 gallons + 3 quarts
- All products: 26.3 gallons needed, 26.75 bought
- Cost: 17.75 × $38 + 6.25 × $25 + 2.75 × $45 = $954.50
Nearly a thousand dollars of paint for a house that a contractor would quote at several times that, which is the usual proportion: the DIY versus hire guide works out whether the difference is worth your weekends.
Walls only, one coat, 9 ft ceilings
A same-colour refresh: three small rooms, two medium, one large, two bathrooms, a 20 ft hallway, 9 ft ceilings, one coat, ceilings and trim not included, wall paint at $40.
- Walls: 3 × 325 + 2 × 418 + 524 + 2 × 217 + 320 = 3,089 sq ft
- Wall paint: 3,089 ÷ 350 × 1 × 1.05 = 9.27 gallons, buy 2 pails (10 gallons)
- Cost: 10 × $40 = $400
The 9 ft ceilings add an eighth to every wall figure. A single coat is honest only because nothing is changing colour; the result’s note says so.
Where to find your inputs
Room counts. Walk the house with a tape and sort each room into the nearest template by floor size. A 11 × 12 room is medium; a 14 × 20 is large; a walk-in closet you are painting is small. Open-plan living and kitchen areas: count the floor as one or two large rooms, then subtract a large room’s worth of missing walls by lowering the count, or run the wall paint calculator for the space.
Hallway. Total length of corridors with both sides painted. Stairwells are not hallways; the stairwell calculator handles tall walls.
Ceiling height. Floor to ceiling; 8 ft in most older US houses, 9 ft in many newer ones. If it varies, use the most common height and run the exceptions separately.
Prices. Per gallon, from the store’s shelf or website. Pails are usually cheaper per gallon; the can size optimizer finds the cheapest combination for each product.
Common mistakes
- Dividing floor area by 400. The walls of a 2,000 sq ft house are 4,000 to 5,000 sq ft at 8 ft ceilings before openings.
- Counting the hallway doors twice. The hallway template already subtracts a door per 10 ft; the rooms subtract their own.
- Buying one product for everything. Flat wall paint on trim stays soft; trim enamel on walls is a waste of money.
- Forgetting ceilings are a separate colour and can. They need their own rounding and their own pail.
- Treating the total as a single colour. Every colour rounds up separately; five colours mean five lots of leftover.
- Skipping primer on new drywall. It is a whole house of primer, and it is not in this figure.
Questions people ask
- How much paint does a whole house interior take?
- A house with two small rooms, three medium, one large, two bathrooms and a 30 ft hallway, 8 ft ceilings, has about 2,920 sq ft of wall. Two coats at 350 sq ft per gallon come to 17.5 gallons of wall paint, 6.2 gallons of ceiling paint and 2.6 gallons of trim enamel; a little over 26 gallons in all.
- Can I estimate from the floor area instead?
- Roughly. Wall area is typically two to two and a half times the floor area at 8 ft ceilings, but it depends on how the floor is divided, since many small rooms have more wall than one open plan of the same size. Counting rooms by size, as this estimator does, is closer, and the room-by-room calculators are closer still.
- What room sizes does it assume?
- Small rooms 10 × 10 ft with one door and one window; medium 12 × 14 ft with one door and two windows; large 15 × 18 ft with two doors and two windows; bathrooms 6 × 8 ft with one door and one window; hallways with both sides painted and one door per 10 ft. They are listed on the standard sizes page.
- Why are the three products separate?
- Because they are bought separately. Wall paint comes in 5-gallon pails for a job this size, ceiling paint is flat and cheaper, trim enamel is harder and sold in quarts and gallons. Each quantity is rounded to its own cans, so the house total is honest about the leftover.
- How much does the paint cost for a whole house?
- Enter your prices per gallon and the estimator multiplies them by the cans bought. At $38 for wall paint, $25 for ceiling paint and $45 for trim enamel, the example house comes to about $955 of paint. Labour, if you hire it, is several times that.
- What is not included?
- Doors (the door paint calculator counts them), kitchen cabinets, closets, primer, and any room that is not one of the four sizes. For an unusual house, run the wall paint calculator room by room and add the results.