Paint Calculator

How Much Paint Do I Need?

Paint Calculator Formula

Formula

\[ A = L \times H - O \\\ T = n \times A \\\ V_p = \frac{T \times V_c}{C} \\\ N = \left\lceil \frac{T}{C} \right\rceil \]

Where:

\(L\) — total length of all walls to be painted (room perimeter)
\(H\) — wall height
\(O\) — total area of openings (doors, windows) that are not painted
\(n\) — number of coats of paint
\(A\) — paintable area for one coat
\(T\) — total area to cover with all coats
\(C\) — area one full container covers with a single coat (printed on the can)
\(V_c\) — volume of one container (its size)
\(V_p\) — total paint volume needed
\(N\) — number of containers to buy, rounded up to a whole container

The formula subtracts the openings from the gross wall area, multiplies by the number of coats, and then converts the total area into paint volume and whole containers using the coverage stated by the manufacturer.

Paint Calculator – Calculation Example

Example: a room with a total wall length of 12 m, a wall height of 2.5 m, and 6 m² of doors and windows. You want 2 coats, and one 2.5 L container covers 25 m² per coat.

\[ A = 12 \times 2.5 - 6 = 24\ \text{m}^2 \]

\[ T = 2 \times 24 = 48\ \text{m}^2 \]

\[ V_p = \frac{48 \times 2.5}{25} = 4.8\ \text{L} \]

\[ N = \left\lceil \frac{48}{25} \right\rceil = 2\ \text{containers} \]

Result: the paintable area is 24 m² per coat, you need 4.8 L of paint in total, and you should buy 2 containers of 2.5 L each.

How to Use the Paint Calculator

Measure the length of every wall you plan to paint, add the numbers together and enter the sum in Total wall length, choosing m or ft as the unit.
Enter the Wall height from floor to ceiling, in the matching unit.
Add up the areas of all doors and windows on those walls and enter the total in Openings area (a rough guide: a standard door is about 2 m², a typical window about 1.5 m²).
Enter the number of Coats of paint — most interior jobs need at least 2.
Check the label on the paint can and enter the Coverage per container for a single coat, in m² or ft².
Enter the Container size in litres or gallons, then read the results: the paintable area per coat, the exact paint volume needed, and the number of containers to buy.

What the Paint Calculator Does

The calculator answers one of the most common questions of any decorating project: how much paint do I actually need? Buying too little means an extra trip to the store in the middle of the job, and colours from different batches can differ slightly. Buying far too much wastes money, since opened cans of tinted paint are usually not returnable.

The calculation starts from the gross wall area — the perimeter of the room multiplied by its height. Openings such as doors and windows are subtracted, because they will not be painted. The remaining paintable area is multiplied by the number of coats, because every coat consumes paint over the entire surface. Finally, the total area is converted into litres using the coverage figure printed on the can, and into whole containers by rounding up — you cannot buy 1.3 cans.

Interpret the Paint volume needed as the theoretical minimum: it is exact for ideal conditions. The Containers to buy figure is the practical shopping number, and it already includes the rounding-up to full containers. In the example above, 4.8 L of paint is needed but two 2.5 L cans are required, leaving a small reserve for touch-ups later — which is usually a good thing.

Assumptions, Limitations and Practical Tips

Coverage varies by paint. The figure on the can assumes smooth, sealed, primed surfaces. Rough plaster, bare drywall, textured walls or a drastic colour change (for example white over dark red) can reduce real coverage by 20–30%, so consider buying one extra container in such cases.
Ceiling and trim are not included. The calculator covers walls only. If you are painting the ceiling too, measure its length and width and treat it as an extra "wall" in the total length.
Two coats are the standard. Even with high-quality paint, one coat rarely gives an even finish on a previously painted wall. Dark or bold colours sometimes need three coats or a tinted primer.
Primer is separate. If the surface needs priming, that is a different product with its own coverage — run the calculation again with the primer's coverage figure.
Keep some paint for touch-ups. Leftover paint stored with a tight lid stays usable for years and is invaluable for repairing scuffs without repainting the whole wall.
Metric and imperial units mix freely. The calculator converts internally, so you can enter wall dimensions in feet and coverage in m² — the results will still be correct.

If you are planning a larger household project, you might also find our Laundry Load Weight Calculator and the Long-Term Food Storage Calculator useful for estimating other everyday quantities.

Frequently Asked Questions

How do I calculate paint by hand without a calculator?

Multiply the perimeter of the room by the wall height, then subtract the area of doors and windows to get the paintable surface. Multiply by the number of coats, then divide by the coverage printed on the can. For example, a 4 m × 5 m room with 2.5 m ceilings has a 18 m perimeter, so \( 18 \times 2.5 = 45\ \text{m}^2 \). Subtract 6 m² of openings, multiply by 2 coats, and divide by the coverage figure to get litres.

How many square metres does 1 litre of paint cover?

Most interior wall paints cover about 10–12 m² per litre per coat, so check your can — figures range from roughly 8 m² for thick matte paints to 14 m² for thinner ones. A typical 2.5 L can covers 25–35 m² in one coat. If the label only states coverage per litre, multiply by the container size to get the per-can figure used in this calculator.

How do I convert between litres and gallons for paint?

One US gallon equals about 3.79 litres, and one imperial (UK) gallon equals about 4.55 litres — check which one your store uses. In practice, 1 gallon of typical interior paint covers roughly 350–400 ft² per coat. The calculator handles conversions automatically, so you can enter dimensions in feet and container sizes in litres and still get correct results.

Should I subtract doors and windows from the wall area?

Yes, subtracting them gives a more accurate estimate, but the savings are small — a standard door is only about 2 m² and a window about 1.5 m². Many people simply skip this step and accept slightly more paint, which becomes the touch-up reserve. Never subtract skirting boards or trim from wall area; trim is painted separately and needs its own paint.

Why did my paint run out even though the calculator said it was enough?

The coverage figure on the can assumes smooth, sealed surfaces. Rough plaster, bare drywall, textured walls or unprimed new plaster absorb more paint and can cut real coverage by 20–30%. Painting a light colour over a dark one also needs extra coats. If your surfaces are porous or the colour change is drastic, round your result up by one container.

How do I calculate paint for the ceiling too?

Measure the ceiling's length and width, multiply them to get its area, and treat it as an extra wall: add its length to the total wall length with the ceiling's width as its 'height', or simply run the calculator twice. Remember that ceilings often use a different paint formulation (flat white), so keep the two calculations separate. Stairwells and sloped ceilings need their actual surface dimensions, not floor area.

Is 1 gallon of paint enough for a room?

One US gallon (3.79 L) covers roughly 33–40 m² (350–430 ft²) in a single coat, or half that with two coats. That is enough for a small bedroom about 3 m × 3.5 m with 2.4 m ceilings, but a standard 4 m × 5 m room with two coats usually needs 5–6 litres. Run the numbers with your actual coverage figure rather than guessing, and remember that opened, tinted cans usually cannot be returned.