How Much Paint Do I Need? A Room-by-Room Guide to Buying the Right Amount

Standing in the paint aisle doing arithmetic in your head is a quick way to either run short halfway up a wall or carry home a tin you'll never open. The good news is that the math behind "how much paint do I need" is simple once you break it into four parts: wall area, openings to subtract, coverage per can, and number of coats. Get those four right and the can count falls out on its own.

This guide walks through each step the way I'd do it before a real job, with a worked example you can copy. If you'd rather skip the longhand, the Paint Calculator does the whole thing live and rounds up to whole cans for you.

Step One: Measure the Wall Area, Not the Floor

Floor area tells you nothing useful about paint. What you paint is the walls, so you measure the perimeter of the room and multiply by the wall height.

Take a rectangular room. Add up all four wall widths to get the perimeter, then multiply by the ceiling height:

Perimeter = (length + width) × 2
Wall area = perimeter × height

For a 12 ft × 12 ft room, the perimeter is (12 + 12) × 2 = 48 ft. With 9 ft walls, the gross wall area is 48 × 9 = 432 square feet. That's the number to start from — before anything gets subtracted.

If your room isn't a clean rectangle, measure each wall on its own (width × height) and add them up. An L-shaped room or one with a chimney breast is just two or three rectangles stacked together.

Step Two: Subtract Doors and Windows

You don't paint glass, and you don't paint a door slab when you're rolling walls. So the openings come straight off the gross area before you do anything else.

Rough sizes worth memorizing:

A standard interior door is about 21 sq ft (roughly 1.9–2.0 m²).
A typical window runs 13–17 sq ft (about 1.2–1.6 m²).

Back to the 12 × 12 room. Say it has one door (21 sq ft) and two windows (15 sq ft each, so 30 sq ft). Total openings are 21 + 30 = 51 sq ft.

Net paintable area = 432 − 51 = 381 sq ft.

One thing people get wrong here: they fuss over light switches and electrical sockets while ignoring the door. Skip the tiny stuff — a few square inches is rounding noise — but always deduct full-size doors and windows. On a small room, two openings can be a meaningful slice of a whole can.

Step Three: Know Your Coverage Per Gallon

Coverage is the variable that makes or breaks the estimate. It's how much area one unit of paint covers in a single coat.

A useful, conservative rule: one US gallon covers roughly 350–400 sq ft per coat on a smooth, primed interior wall (the metric equivalent is about 10 m² per litre). That's the safe end of what Dulux, Nippon, and Benjamin Moore print on their data sheets.

But "smooth and primed" is doing a lot of work in that sentence. Real surfaces drink more paint:

Bare or freshly patched drywall is porous and pulls coverage down.
Textured render or knockdown finish has more surface area than a flat wall.
A dark colour going under a light one needs extra paint to bury the old shade.

Any of those can cut real-world coverage by 30–40%. When the wall is thirsty, drop your coverage figure to 300 sq ft per gallon or lower rather than trusting the "up to" number on the label. The label quotes the best case; your wall might not be it.

Step Four: Add Coats and Primer

Coverage gets you one coat. Almost nobody paints one coat.

Two coats is the honest default for a repaint over a similar colour. Go to three coats when you're covering a much darker shade, painting bare drywall, or using a thin budget paint that doesn't build well.

Primer is the trap that catches people every time. A primer or undercoat is a separate coat — it is not one of your two finish coats. If you're priming bare drywall and then doing two top coats, that's three trips around the room with a roller, not two. Either bump your coat count to 3 or run the primer as its own calculation. Forget this and you'll be exactly one can short on the day.

The Worked Example, End to End

Let's finish the 12 × 12 room with 9 ft walls, one door, two windows, and two coats.

Gross wall area: 48 ft perimeter × 9 ft = 432 sq ft.
Subtract openings (door 21 + windows 30 = 51): net 381 sq ft.
Multiply by two coats: 381 × 2 = 762 sq ft of paint to apply.
Divide by coverage (use 380 sq ft per gallon): 762 ÷ 380 ≈ 2.0 gallons.

So you need almost exactly two gallons of paint. In practice you'd buy two 1-gallon cans, or — if the colour is sold that way — a single 5-quart pail, which is 1.25 gallons short of a true two-gallon need, so two 1-gallon cans is the cleaner buy here. The leftover from rounding becomes your touch-up stock.

Here's where I'll admit my own painting history. The first time I repainted a bedroom, I eyeballed it, grabbed "a gallon, that looks like enough," and ran dry with one corner still in primer-grey. The second can was a slightly different batch, and in raking afternoon light you could see the seam for years. Now I always measure, subtract the door and windows, and buy the full amount in one trip — one batch, one can count, no surprise visit back to the store. Five minutes of math saved me a wall that never quite matched.

Buying the Right Amount: Round Up, Then Stop

Paint is sold in sealed tins, so your shopping quantity is always rounded up to whole cans. Needing 1.2 gallons doesn't mean buying 1.2 gallons — it means two 1-gallon cans, or one gallon plus a quart top-up if the maths is close.

A few buying tips that save money and frustration:

Mind the leftover. If you need 2.1 gallons, a small quart can for that last 0.1 might be cheaper than jumping to a third full gallon.
Buy one batch. Cans mixed in the same batch match exactly; buying a fifth can three weeks later risks a visible colour shift. Estimate the whole job up front.
Keep some back deliberately. A little leftover in a labelled tin is your touch-up insurance for scuffs and nail holes down the line.

If your paint is labelled in one unit system and your room is measured in another — an imported tin that says "400 sq ft per gallon" over walls you measured in metres — convert before you calculate. A unit converter handles the gallon-to-litre (1 gal = 3.785 L) and square-foot-to-square-metre (1 m² = 10.764 sq ft) jumps so you're not doing fiddly conversions at the counter.

Putting It Together

The whole method is four numbers: wall area, minus openings, times coats, divided by coverage — then rounded up to whole cans. Measure honestly, treat primer as its own coat, lower your coverage figure when the wall is bare or you're making a big colour change, and buy the full amount in one batch. Do that and you walk into the store with a real shopping list instead of a guess, and you walk back out with enough paint to finish the wall the first time.

Made by Toolora · Updated 2026-06-13