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Painting a Room: How to Calculate Exactly How Much Paint to Buy

June 4, 2026 • 6 min read
Freshly painted modern room with neutral walls and natural light

Painting should be one of the cheaper, easier ways to make a room look better. In theory, anyway. In practice, plenty of people end up making two trips to Bunnings, buying one random extra tin in a panic, and then storing half a litre of mystery beige in the garage until the heat death of the universe.

The good news is the maths is not hard. If you can measure a wall and divide by the coverage rate on the tin, you can get very close to the right amount of paint. The trick is knowing what to include, what to subtract, and where to leave yourself a bit of breathing room.

This guide walks through the simple formula, a worked example, and the most common mistakes Australians make when estimating paint for a room.

The basic formula

For most rooms, the starting point is:

Paint needed = paintable area ÷ coverage rate × number of coats

That sounds almost too neat, but it works well if your measurements are decent and you use the coverage rate printed on the product you are actually buying.

Step 1: Measure the wall area

Measure the width and height of each wall in metres, then multiply them to get square metres.

Wall area = wall width × wall height

If your room is a simple rectangle, you can either calculate each wall one by one or use the room perimeter:

Total wall area = room perimeter × wall height

Example room:

Perimeter = 4.0 + 3.5 + 4.0 + 3.5 = 15.0 m

Total wall area = 15.0 × 2.4 = 36.0 m²

That is your raw wall area before subtracting windows and doors.

Step 2: Subtract doors and windows

You usually do not need full wall paint for glass, and doors often get a different enamel or trim product anyway, so subtracting openings gives you a more realistic estimate.

Using rough examples:

If our room has one door and one window:

Paintable wall area = 36.0 − 1.67 − 2.16 = 32.17 m²

If you are doing a very quick estimate for a basic spare room, you can skip this step and rely on a buffer. But if you want a more accurate number, especially for a room with big windows or sliding doors, subtract them properly.

Step 3: Add the ceiling if you are painting it too

Ceilings are easy:

Ceiling area = room length × room width

For our example room:

4.0 × 3.5 = 14.0 m²

If you are painting walls and ceiling the same weekend, keep them as separate estimates if the products differ. Ceiling paint often has a different finish and sometimes a different coverage rate. One tin does not magically become two because you believed in yourself.

Step 4: Check the coverage rate on the tin

This matters more than people think. Coverage rates vary by product, finish, brand, surface texture, and whether the wall is already sealed. So do not use a random internet number if the actual tin in front of you says something else.

For example only, let us say your wall paint states 12 m² per litre per coat.

Wall paint for one coat = 32.17 ÷ 12 = 2.68 litres

If, again just as an example, your ceiling paint says 10 m² per litre per coat, then:

Ceiling paint for one coat = 14.0 ÷ 10 = 1.4 litres

Step 5: Multiply by the number of coats

For most repaint jobs, planning on two coats is a sensible starting assumption unless the product label or your surface condition points you another way.

For our room:

Walls: 2.68 × 2 = 5.36 litres

Ceiling: 1.4 × 2 = 2.8 litres

So you would want roughly:

Step 6: Round up to real tin sizes

This is where theory meets retail packaging. Paint is sold in practical sizes, not in beautiful little 5.36 litre containers that exist only in spreadsheets.

If you need 5.36 litres for the walls, buying exactly 5 litres is asking for trouble. A better move is rounding up to the next sensible combination, such as:

Always allow for a small buffer because roller sleeves, trays, touch-ups, textured walls, and cutting in around trims all chew through paint in the real world.

What about trims, skirting boards, and doors?

These usually use a separate trim product, often semi-gloss or gloss, so estimate them separately.

For skirting boards, architraves, and frames, you can estimate the surface area by measuring their total length and multiplying by the visible width. It does not need to be obsessive, just reasonable.

Example:

That is a simple estimating shortcut and does not capture every edge or decorative profile. Add architraves and door faces if painting those too. Because trim jobs are usually small and fiddly, many people simply buy a 1 L or 2 L trim tin rather than chasing perfect decimal accuracy.

Common mistakes that blow out the estimate

Ignoring the second coat

This is the classic one. People estimate for one coat, then act shocked when the tin empties halfway through round two. Paint was innocent. Maths was innocent. The problem was optimism.

Using the wrong coverage rate

If the tin says 10 m² per litre and you budgeted using 16, you are going to have a bad afternoon.

Not accounting for texture

Heavily textured walls, rough plaster, brick, or rendered surfaces can use more paint than smooth walls. They have more actual surface area, even if the room dimensions are the same.

Forgetting primer

If the wall needs primer or sealer, that is a separate product and quantity. Do not assume your top coat will do the whole job beautifully out of pure loyalty.

Buying too little custom tint

If you are using a mixed colour, running out can be annoying because the next batch may not be a perfect match. Close is not always close enough once it dries on the wall.

A quick shortcut for standard bedrooms

If you just want a rough sanity check for a typical bedroom with standard ceiling height, one door, and one window, plenty of rooms can land somewhere in the ballpark of:

That is a rule of thumb, not a replacement for measuring, but it is a decent sense-check. If your calculator says a tiny bedroom needs 17 litres, something has gone sideways.

Use a calculator if you want the easy version

If you would rather skip the scribbling on the back of a receipt, use the Paint Calculator to estimate walls, ceilings, coats, doors, and windows properly.

And if your project spills into other weekend-DIY chaos, SmartKoala also has a Concrete Calculator for slabs and posts, plus a Soil & Mulch Calculator for garden beds that somehow become part of the same job. They always do.

Simple checklist before you buy

  1. Measure each wall in metres.
  2. Add the wall areas together.
  3. Subtract doors and windows if you want a more precise estimate.
  4. Calculate the ceiling separately if needed.
  5. Use the coverage rate printed on your specific paint tin.
  6. Multiply by the number of coats.
  7. Round up to practical tin sizes and keep a small buffer.

Final word

Buying the right amount of paint is mostly about being methodical for ten minutes so you do not waste money for the next ten days. Measure properly, trust the coverage rate on the tin, assume two coats, and round up like a sensible person.

That gets you close enough to avoid the two worst outcomes: running out halfway through, or storing a depressing amount of leftover eggshell white forever.

FAQ

How do I calculate how much paint I need for one wall?

Multiply the wall width by the wall height to get square metres, then divide by the paint coverage rate shown on the tin. Multiply by the number of coats after that.

Should I subtract windows and doors?

Yes, if you want a more accurate estimate. In larger rooms the difference may be small, but in compact rooms or spaces with large windows it can meaningfully reduce the paintable area.

Do I need a buffer?

Usually yes. Cutting in, roller absorption, textured walls, and future touch-ups all make a small buffer worthwhile. Rounding up to the next sensible tin size is usually smarter than buying the exact calculated minimum.