Free calculator

Paint Calculator

Estimate paint quantity, coats, waste allowance and material cost for interior wall projects.

Your estimate

Enter measurements, then calculate.

Planning estimate only. Confirm packaging, coverage, site conditions, and local building requirements before purchasing.

How to measure

Measure the length of every wall that will be painted, then multiply each wall length by the wall height.
Add the wall areas together. If you are painting a ceiling, measure its length and width separately and include it as another surface.
Subtract doors, windows and large built-in openings only when they are large enough to change the purchase decision.
Choose the number of coats based on surface condition: one coat for touch-ups, two coats for most repainting, and primer plus paint for strong color changes.

Input guide

Length and width describe the main wall surface in the simple rectangular estimate. For several walls, enter the total equivalent wall length and the wall height.
Coverage should come from the paint can or technical sheet. Matte, textured, porous or new plaster surfaces often cover less than the label maximum.
Waste covers roller loading, tray loss, touch-ups and small measurement errors. Ten percent is a practical starting point for normal rooms.
Enter the price per litre or gallon. If you do not know the price yet, leave it at zero and use the quantity result first.

Formula

Paint = (surface area − openings) × coats × (1 + waste) ÷ coverage

The calculator first finds net paintable area after subtracting openings. That area is multiplied by the number of coats because each coat uses material.

Waste is added before dividing by coverage so the final amount includes extra paint for realistic application losses.

Metric projects return litres. Imperial projects convert the same internal volume to gallons so both unit systems remain consistent.

Worked example: Two-coat bedroom repaint

A 5 m by 4 m bedroom with 2.7 m walls has about 48.6 m² of wall surface.
Subtract 4 m² for a door and window, leaving 44.6 m² of paintable area.
With two coats and 10% waste, the adjusted paint area is 98.12 m².
At 10 m² per litre, the estimate is 9.82 L, rounded up to 10 L for purchasing.

If paint costs 22 USD per litre, the material estimate is 10 L and 220 USD before primer, tools or delivery.

Buying advice

Buy from the same batch number when color consistency matters across visible walls.
Round up to a practical can size after checking the available packaging. A 10 L result may be cheaper as one large can than several small cans.
Keep a small labelled amount for later touch-ups, especially in rentals, children’s rooms and high-traffic corridors.

Common mistakes

Using floor area as wall area without multiplying by wall height
Forgetting the second coat when changing color or covering marks
Assuming the best-case coverage printed on the can applies to rough or absorbent walls
Subtracting every small opening and then running short for touch-ups
Mixing metres and feet in the same estimate

Adjust the estimate for the actual wall

The coverage printed on a paint can is usually measured on a prepared, relatively smooth surface. New plaster, unsealed drywall, masonry, textured finishes, and repaired patches can absorb more paint. A strong color change may also need an extra finish coat even when the area calculation is correct.

Use the calculator result as the measured baseline, then compare it with the primer and finish-coat instructions for the exact product. If the surface is chalky, damp, peeling, or contaminated, correct that condition before treating paint quantity as the main planning problem.

Paint purchasing checklist

Record wall measurements separately from ceilings, trim, doors, and cabinets because they often use different products.
Check whether the stated coverage is per coat and whether the surface requires a dedicated primer.
Buy finish paint from the same product line, sheen, color code, and production batch when possible.
Round up to available container sizes and keep a labeled amount for future touch-ups.
For spraying, allow for masking, overspray, and material left in the hose or cup.

What this paint result does not decide

The calculator estimates liquid quantity from area, coats, coverage, and waste. It does not diagnose moisture, choose a primer, confirm coating compatibility, or determine whether lead paint or another hazardous coating is present.

For exterior work, waterproofing systems, industrial coatings, or damaged substrates, follow the manufacturer’s written system and obtain qualified advice. Those decisions can change preparation, coat count, and actual consumption more than ordinary rounding.

Frequently asked questions

Should I include primer?

Include primer as a separate paint calculation because primer coverage and price may differ from finish paint.

How much waste should I add for paint?

Use 5% for a simple touch-up, 10% for normal rooms and 15% or more for textured walls, deep colors or inexperienced application.

Do ceilings use the same calculation?

Yes. Treat the ceiling as another rectangle, but use the ceiling paint coverage and price if the product is different.

Can I subtract doors and windows?

Subtract large openings, but do not over-subtract small trim details because extra paint is useful for edges and touch-ups.

Why does the result round up?

Paint is purchased in containers, and rounding down can leave the last wall unfinished or create a visible batch mismatch.