Paint calculator — how much paint do I need?
Work out how much paint a room needs from wall dimensions, coats and coverage, with doors and windows deducted automatically.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
Area
416 sq ft
Measured area
416 sq ft
Openings deducted
51 sq ft
1 doors, 2 windows
In square metres
35.61 m²
In square yards
42.58 sq yd
Estimated cost
$0.00
enter a price to see this
| Area | Shape | Dimensions | Sq ft |
|---|---|---|---|
| Bedroom | Room (four walls) | 14' 0" × 12' 0" × 8' 0" | 416 |
Formula and working
Area of each shape
Room (four walls): 2 × (length + width) × wall height Bedroom: 14.00 × 12.00 × 8.00 ft = 416.00 sq ft = 416 sq ft total
Paintable area
net = wall area − doors − windows + ceilings = 416.00 − 51.00 + 0.00 = 365 sq ft
Paint required
gallons = net area × coats ÷ coverage per gallon = 365.00 × 2 ÷ 350 = 2.086 gallons, rounded up to 3 = 3 gallons (7.9 litres)
Metric conversion
1 sq ft = 0.09290304 m² exactly (1 ft = 0.3048 m) 365.00 × 0.09290304 = 33.91 m²
Feet-and-inches entries such as 12′ 6″ are read exactly, and the conversion to metres uses the international foot of 0.3048 m, which is exact by definition.
Assumptions
- Measure the longest and widest points of each space, and split awkward rooms into two rectangles rather than trying to describe them with one shape.
- A door is deducted at 21 sq ft (3 × 7 ft) and a window at 15 sq ft (3 × 5 ft). Coverage of 350 sq ft per US gallon is typical for a smooth, previously painted wall; bare drywall, textured surfaces and deep colour changes all use more.
- Quantities are always rounded up to whole units — cans, boxes and bags are not sold in fractions.
- Square footage is a plan area. It does not account for skirting, thresholds, or the direction a plank or tile will run.
- Nothing is uploaded. Saving is off by default and writes only to this browser, with a delete button next to the switch.
From room dimensions to tins
Wall area is the perimeter multiplied by the ceiling height:
wall area = 2 × (length + width) × wall height
paint = wall area × coats ÷ coverage per tinA 14 × 12 ft room with 8 ft ceilings has 416 square feet of wall. Deduct one door and two windows (51 sq ft) and you are painting 365. Two coats at 350 sq ft per gallon is 2.09 gallons — so three tins, because paint is not sold in fractions.
Coverage: the number that actually decides the answer
"350 square feet per US gallon" is the manufacturer's figure for a smooth, sound, previously painted wall. Real surfaces are frequently not that.
| Surface | Sq ft per gallon | m² per litre |
|---|---|---|
| Smooth, previously painted | 350–400 | 10–11 |
| New plaster or drywall (mist coat) | 250–300 | 7–8 |
| Lightly textured | 250–300 | 7–8 |
| Heavy texture, brick, render | 150–250 | 4–6 |
| Ceilings (roller, matt) | 350–400 | 10–11 |
A US gallon is 3.785 litres. Metric tins are typically 2.5 or 5 litres, so a 5 L tin is about 1.32 gallons — set coverage to match whichever unit your tin is sold in.
How many coats
- Same colour, sound wall: one coat, occasionally.
- Normal repaint: two. This is the honest default and what the calculator starts at.
- Light over dark, or a strong colour: a tinted primer plus two topcoats. Reds and deep yellows are the worst offenders and can need three.
- Bare plaster: a thinned mist coat first — it soaks in and does not count as a topcoat — then two.
Doors and windows
Each door is deducted at 21 square feet (a standard 3 × 7 ft opening) and each window at 15 (3 × 5 ft). Those are the conventional trade allowances. If your openings are unusually large — patio doors, a picture window — set the counts to zero and subtract a wall row instead, or leave them in as extra safety margin.
Many decorators deliberately do not deduct anything for a small room, on the grounds that the allowance is smaller than the wastage. The calculator lets you choose either way.
Ceilings, trim and the things paint calculators forget
Set the ceiling field to 1 to add the floor-plan area of each room row to the paintable total — length × width, once per coat. Trim is a different product and a different calculation altogether: skirting, architrave and door frames are usually estimated by length rather than area, and a litre of gloss covers a surprising amount of it. Doors themselves are roughly 20 square feet per side.
Buy the whole job at once
Two practical reasons. Tinted colours are mixed in batches and two batches of the same colour can differ visibly on a large wall. And running out three-quarters of the way along a wall leaves a wet edge that dries before you get back, which shows up as a stripe under raking light forever.
Round up, always. This calculator does it for you, because the difference between 2.09 gallons and three tins is a few units of currency, and the difference between finishing and not finishing is an afternoon.
A rough order of work
- Measure each room: length, width and ceiling height.
- Set coats and coverage honestly for the surface you actually have.
- Count doors and windows.
- Read the tin count, add the ceiling if you are doing it.
- Add primer separately if you are covering bare plaster or a dark colour.
- Export the list and take it to the shop.
Works in an empty house with no signal
There is no server behind this page and nothing is uploaded. Once it has loaded it runs entirely on your device, which is useful when you are standing in an unfurnished room with a tape measure and one bar of signal. Saving is off by default and writes only to this browser when you turn it on.
Questions
How much wall does a gallon of paint cover?
About 350 square feet on a smooth, previously painted surface — roughly 10 square metres per litre. Bare drywall, textured render and a big colour change all soak up more, so drop the coverage figure for those.
How many coats do I need?
Two is standard. One coat only works when repainting the same colour on a sound surface; covering a dark colour with a light one, or painting bare plaster, often needs a primer plus two topcoats.
Does this deduct doors and windows?
Yes. Each door is deducted at 21 square feet (3 × 7 ft) and each window at 15 square feet (3 × 5 ft). Set the counts to zero if you would rather buy on the gross wall area.
How is the wall area worked out from room dimensions?
Perimeter times height: 2 × (length + width) × wall height. A 14 by 12 foot room with 8 foot ceilings has 416 square feet of wall before openings are deducted.
Should I round up?
Always. The calculator rounds to whole cans because paint is sold that way, and running out mid-wall with a batch-matched colour is the one mistake that cannot be fixed cheaply.