Paint Calculator

About this calculator

This calculator estimates how many gallons or litres of paint you need to cover a room. Enter the room dimensions, number of coats, and how many doors and windows to deduct, and it gives you a paint volume plus a whole-can count to buy.

How to read your results

The headline figure is the exact paint volume required. The stacked bar chart beneath it breaks the total surface area into three segments — walls, ceiling (if selected), and openings deducted — so you can see how each component contributes to the final paintable area. The "cans to buy" stat is that volume rounded up to the next whole unit.

How it's calculated

Paintable wall area equals the wall perimeter multiplied by the ceiling height: 2 × (length + width) × height. If the ceiling is included, the floor area (length × width) is added. Standard opening deductions — 20 sq ft per door and 15 sq ft per window — are then subtracted, with the result floored at zero. Paint volume equals paintable area multiplied by the number of coats, divided by the coverage rate you enter (defaulting to 350 sq ft per gallon in imperial, 10 m² per litre in metric). The whole-can count is the volume rounded up to the nearest integer. The method follows the approach published by Inch Calculator and the coverage standards cited by Sherwin-Williams.

Worked example

15 ft long × 12 ft wide room with a 9 ft ceiling, 2 coats of paint, 1 door, and 2 windows. Ceiling not included. Coverage rate 350 sq ft per gallon.

Wall area is 486 sq ft. Deducting the door (20 sq ft) and two windows (30 sq ft) gives 436 sq ft of paintable surface. At 2 coats and 350 sq ft per gallon, you need 2.5 gallons — so buy 3 cans.

Frequently asked questions

What coverage rate does the calculator use?

The coverage rate is an editable field. It defaults to 350 sq ft per gallon in imperial mode — a standard industry mid-range figure from Sherwin-Williams and similar paint manufacturers — and 10 m² per litre in metric mode. Most cans state their own spread rate (typically 250–400 sq ft per gallon, or 6–10 m² per litre), so enter the figure from your tin for the most accurate estimate.

How are doors and windows handled?

Each door is assumed to cover 20 sq ft and each window 15 sq ft, following common estimating guides. These areas are deducted from the total wall area before calculating paint. If your openings differ significantly from these defaults, increase or decrease the count to compensate.

Should I paint the ceiling separately?

Yes. The ceiling toggle adds the floor area of the room to the paintable surface and includes it in the volume estimate. Ceilings typically take a different product (flat or matte finish) than walls, so use the total as a guide and consider buying separately if you are using a different paint.

Why does the calculator round up to whole cans?

Paint is sold in whole-unit containers, so buying the exact decimal amount is not possible. Rounding up ensures you have enough to finish the job. If the overage is large, check whether a smaller can size (quart vs gallon) would be more economical.

Does this include primer?

No. The estimate covers finish coats only. If you are priming bare drywall or a dark colour, add a separate primer calculation using the same area figure and the primer coverage rate from the product label.

Sources

• www.inchcalculator.com/paint-calculator
• www.sherwin-williams.com/en-us/color/color-tools/paint-calculator

Reviewed by the YouCalc Team · Last reviewedJune 12, 2026