Materials & quantities
Paint quantity calculator
Calculate paint for walls and optional ceilings, including coats and tin sizes.
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What a paint calculator does
A paint calculator estimates how much paint a room needs: the wall area (room perimeter × wall height), minus doors and windows, plus the ceiling if you are painting it, multiplied by the number of coats and divided by the coverage printed on the can. This one adds a waste allowance and rounds up to whole cans in the size you choose.
Interior wall paint labels commonly quote about 350 to 400 ft² per gallon per coat on smooth, primed surfaces. Two coats on the walls of a 12 × 14 ft room with 8 ft ceilings, 416 ft², need 2.38 gallons at 350 ft²/gal, or 2.61 gallons with a 10% allowance. That is three 1-gallon cans.
Coverage depends on the product and the surface, so the calculator uses the figure you enter rather than assuming one. Gallons are US gallons, and the Metric switch works in litres and square metres.
How to use the paint quantity calculator
- Choose Imperial or Metric. Imperial uses feet, square feet per gallon and US gallons; Metric uses metres, square metres per litre and litres.
- In the room table, enter each room's Length, Width and Wall height, the number of Doors and Windows, and any Other opening area. Use Add room for the next room; the result rolls up per room.
- Set the Area per standard door (20 ft² for a 3 ft × 6 ft 8 in door) and the Area per window, and set Include ceiling to Yes or No. The ceiling uses the same coverage and number of coats as the walls.
- Enter the Number of coats. Either leave Finish on 'From the product label' and enter the Product coverage per coat (350 ft²/gal by default), or pick a typical flat, eggshell or semi-gloss rate. Enter the Tin size: 1 for gallon cans, 0.25 for quarts or 5 for a 5-gallon pail.
- Set Primer coat to Yes to add one coat at its own Primer coverage per coat with an optional primer price. Set a Waste allowance and optional price per tin. Read the net surface area, paint including allowance, tins to order, purchased volume, primer and the per-room table with pro-rata cost. Export a PDF or CSV or copy the result.
Worked example
Imperial: 448 ft² of surface, two coats and 350 ft²/gal coverage require 2.56 gal before allowance.
Metric: A 6 × 4 × 2.4 m room has 48 m² of wall; two coats at 10 m²/L need 9.6 L before allowance.
How much paint do I need for a room?
The chart uses the calculator's method for common room sizes with 8 ft walls, two coats, 350 ft²/gal coverage, a 10% allowance and 1-gallon cans. No openings are deducted, which is a reasonable margin for rooms with one door and a window or two.
Notice the 16 × 12 ft room with its ceiling: 4.02 gallons rounds up to five cans. When a result lands just over a whole can, check whether the allowance you set is realistic, or buy a quart instead of a fifth gallon.
| Room | Wall area | Walls only | Walls + ceiling |
|---|---|---|---|
| 10 × 10 ft | 320 ft² | 2.01 gal (3 cans) | 2.64 gal (3 cans) |
| 12 × 12 ft | 384 ft² | 2.41 gal (3 cans) | 3.32 gal (4 cans) |
| 12 × 14 ft | 416 ft² | 2.61 gal (3 cans) | 3.67 gal (4 cans) |
| 16 × 12 ft | 448 ft² | 2.82 gal (3 cans) | 4.02 gal (5 cans) |
| 20 × 15 ft | 560 ft² | 3.52 gal (4 cans) | 5.41 gal (6 cans) |
How many square feet does a gallon of paint cover?
Most interior wall paints state a coverage of roughly 350 to 400 ft² per gallon per coat. That assumes a smooth, sealed surface and the application method on the label. Bare drywall, fresh plaster, masonry, heavy texture and rough-sawn wood absorb more and hold more paint, so real coverage can be well below the label figure.
The table shows how much the coverage figure moves the answer for the same 416 ft² of wall at two coats, before any allowance. In metric, 350 ft²/gal is about 8.6 m²/L and 400 ft²/gal is about 9.8 m²/L. The calculator's metric default of 10 m²/L is about 407 ft²/gal.
| Coverage (ft²/gal) | Coverage (m²/L) | Gallons needed |
|---|---|---|
| 250 | 6.1 | 3.33 |
| 300 | 7.4 | 2.77 |
| 350 | 8.6 | 2.38 |
| 400 | 9.8 | 2.08 |
Worked example: a bedroom with a door and two windows
A 12 × 14 ft bedroom with 8 ft walls has one 3 ft × 6 ft 8 in door (20 ft²) and two 3 × 5 ft windows (30 ft²), 50 ft² of openings. The net wall area is 366 ft². At two coats and 350 ft²/gal, that is 2.09 gallons, or 2.30 gallons with a 10% allowance: three 1-gallon cans.
The ceiling usually gets a different paint, so leave Include ceiling set to No and work it out on its own. The 168 ft² ceiling at two coats needs 0.96 gallons net. A 10% allowance takes it to 1.06 gallons, which the calculator rounds up to two cans. One gallon plus a quart is the practical buy.
Taller walls add up quickly. The same room with 9 ft walls has 468 ft² of wall, 12.5% more than at 8 ft.
Primer, new drywall and color changes
New drywall and patched areas are usually primed first, and primer often has a lower stated coverage than finish paint. Run the calculator once for primer, with its own coverage and one coat, and again for the finish coats.
Covering a strong color with a much lighter or darker one can take an extra coat. Follow the manufacturer's recommended number of coats, and use the Number of coats field rather than raising the waste allowance, so the estimate shows where the paint is going.
Who it is for and when to use it
Repainting a room
Buy the right number of cans for walls and ceiling before a weekend project, with doors and windows deducted.
Quoting painting work
Estimate materials per room for a painting quote and export the PDF or CSV with the inputs and coverage figure stated.
Rental turnovers
Price repaint materials for apartments or offices between tenants using the same room sizes each time with saved inputs.
Comparing products
See how a premium paint with higher stated coverage changes the number of cans compared with a cheaper product.
Method, formulas & assumptions
Per room: net surface = perimeter × height − doors × door area − windows × window area − other openings + optional ceiling. Paint = surface × coats ÷ coverage (entered, or a typical finish preset), plus waste; a primer coat uses its own coverage. Tins round up on the total.
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Using floor area instead of wall area. Walls are the perimeter times the height: a 12 × 12 ft room has 144 ft² of floor but 384 ft² of wall at 8 ft.
- Forgetting the second coat. Most estimates, and most label instructions for a color change, assume two.
- Taking the label coverage at face value on bare drywall, plaster, masonry or texture. Porous and rough surfaces use more.
- Mixing US and imperial (UK) gallons. A US gallon is 3.785 L and an imperial gallon is 4.546 L; this calculator uses US gallons.
- Buying tinted paint in separate trips. Custom colors can vary slightly between batches, so buy the full quantity at once and mix the cans together before you start.
- Counting trim and doors in the wall area. They are usually a different product and sheen, so estimate them separately.
Scope & limits
Coverage presets and door/window areas are editable examples, not product data. Porosity, texture and application method affect actual use; per-room costs are pro-rata. Gallons shown are US gallons.
Frequently asked questions
How much paint do I need for a 12x12 room?
The walls of a 12 × 12 ft room with 8 ft ceilings are 384 square feet. Two coats at 350 square feet per gallon need 2.19 gallons, or 2.41 with a 10% allowance, so buy three gallons. Adding the ceiling brings it to 3.32 gallons, or four cans.
How many square feet does a gallon of paint cover?
Interior wall paint commonly states 350 to 400 square feet per gallon per coat on smooth, primed surfaces. Porous, rough or textured surfaces can use noticeably more.
How many gallons of paint for 1,000 square feet?
At 350 square feet per gallon, 1,000 square feet needs 2.86 gallons for one coat and 5.71 gallons for two, before any allowance.
Is one gallon of paint enough for a bedroom?
Usually not for two coats. A 10 × 10 ft room with 8 ft walls has 320 square feet of wall, which takes 0.91 gallons for one coat and 1.83 gallons for two at 350 square feet per gallon.
Should I subtract windows and doors when calculating paint?
Yes for full-size doors and windows, especially in rooms with several. A 3 ft × 6 ft 8 in door is 20 square feet. For a room with one door and one small window, leaving them in gives a small built-in margin.
How many litres of paint per square metre?
At a stated coverage of 10 square metres per litre, one coat uses 0.1 litres per square metre. A 4 × 3 m room with 2.4 m walls has 33.6 square metres of wall, which needs 6.72 litres for two coats, or 7.39 litres with a 10% allowance.
How much does a quart of paint cover?
A quart is a quarter of a US gallon, so at 350 to 400 square feet per gallon it covers about 87 to 100 square feet per coat.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
Further reading
- Interior design projects in Harth
- Renovations and extensions in Harth
- All materials & quantities tools
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- Harth resource library
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