Paint calculator

Gallons or litres for any room, with the surface-condition adjustment most calculators skip. A textured or unprimed wall drinks paint the spec sheet never planned for, and that's often the difference between three gallons and four.

01 How the math works

Paint is a coverage problem. Work out the paintable area, multiply by the number of coats, and divide by how far a gallon goes on that surface. Then round up to whole cans, because a wall that's 5% short still reads as unfinished.

The surface factor is the part other calculators drop. The 350–400 ft²/gal printed on the can is a manufacturer figure for a smooth, primed wall. A textured wall has more surface per square foot of wall, so it takes more paint; unprimed drywall soaks up the first coat; a repaint in the same colour glides on and covers better than spec. The factors above come from PDCA field guidance and the paint makers' own data sheets, and they move the answer more than most people expect.

02 Worked example

Take the walls of a 12 ft × 14 ft bedroom with 8 ft ceilings (3.66 m × 4.27 m × 2.44 m), ceiling left out. Wall area is 2 × (12 + 14) × 8 = 416 ft²; knock off a door and one window at about 33 ft² and the paintable area is 383 ft² (35.6 m²).

Two coats over a smooth, primed wall at 350 ft²/gal (32.5 m² per 3.79 L): 383 × 2 = 766 ft² of coverage needed, ÷ 350 = 2.19 gallons (8.3 L) of paint. Rounded up to whole cans, order 3 gallons in imperial; toggle to metric and the same job orders as 9 L.

Now change one thing. If that same room is new, unprimed drywall, coverage drops to about 250 ft²/gal, and 766 ÷ 250 = 3.06 gallons — which rounds up to 4 gallons. Same room, same two coats, one gallon more, entirely because of what's under the paint. That's the number a flat calculator misses.

03 When this calculator is wrong

Manufacturer coverage figures are for smooth, primed walls under ideal conditions, which describes almost no wall anyone is actually painting. Textured walls cut coverage by 20–30%. Unprimed new drywall cuts it by 30%+ on the first coat. Take the can figure at face value and you'll under-order by one or two gallons on a typical room. The exception runs the other way: a repaint in the same colour over a previously painted smooth wall often covers 10–15% better than spec, so there the manufacturer number is conservative.

• The unprimed penalty is really a first-coat penalty. New drywall soaks up the first coat, but once that coat seals the surface, later coats cover close to normal. Applying the reduced coverage to every coat over-orders slightly — which we'd rather do than leave you short, but if you prime the board first, the whole job reverts to the smooth-primed figure and you can drop back a gallon.
• A drastic colour change can need an extra coat the area math won't show. Going from a dark wall to white, or covering a strong colour, sometimes takes three coats where two would otherwise do. The calculator sizes the coats you tell it; it can't see the colour underneath.
• Spraying is not rolling. A sprayer lays a heavier film and loses paint to overspray, so a sprayed job runs through more than a rolled one for the same wall. If you're spraying, treat the result as a floor, not a ceiling.
• The ceiling is usually a different can. Ceilings are typically flat white ceiling paint, not the wall colour. Leave the ceiling toggle off unless you're genuinely running the same product over both.

04 What to do with the result

Order the rounded-up gallon count, and match the surface setting to the wall you actually have — not the one on the label. If the unrounded figure lands just over a whole gallon (say 2.1), a gallon plus a quart covers it: a quart goes about 100 ft², which is enough to close a small gap without buying a second full gallon. For a repaint in the same colour, that quart is often all the second "coat" a few thin spots need.

Buy the colour in one trip when it's custom-mixed. Tinted paint is matched to a batch, and a second order weeks later can come back a shade off — close enough to see in daylight on the same wall. Round up the first time so you're not chasing a match on the last 40 ft².

05 Common questions

How much paint do I need for a room?

Work out the wall area (perimeter × wall height), subtract the doors and windows, multiply by your coat count, and divide by the coverage for that surface. A 12 × 14 ft bedroom is about 383 ft² of wall — two coats on a smooth, primed surface takes 3 gallons.

How much does a gallon of paint cover?

A US gallon of standard interior latex covers 350–400 ft² (32.5–37.2 m²) per coat on a smooth, primed wall. On a textured wall, drop that by 20–30%. On new, unprimed drywall, expect 250–300 ft² per gallon on the first coat.

Do I need one coat or two?

Two is the default for a colour change or new drywall — one coat rarely reads as even. A repaint in the same colour over a sound surface can be a single coat plus touch-ups. The calculator lets you set the coats; the paint scales with them directly.

Why do textured and unprimed walls need more paint?

Texture adds surface area — the paint has to cover the peaks and the valleys, so a square foot of wall is more than a square foot of paint. Unprimed drywall and its paper face are porous and pull the first coat in. Both effects are real, both are in the numbers above, and both are why the can figure runs optimistic.

How much paint for a 12×12 room?

A 12 × 12 ft room with 8 ft walls is 384 ft² of wall before openings — about the same as the bedroom example. Two smooth-primed coats land at 3 gallons. Switch the surface setting if the walls are textured or bare.