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Wall areaOpenings deductedFree

Paint Area
Calculator

Estimate net paintable wall area and how much paint to buy. Enter the room dimensions and wall height, deduct doors and windows, choose your number of coats, and get the gallons (or liters) needed at your paint's coverage rate.

Who it's for: painters, builders, and homeowners pricing a job or a weekend repaint who want a quick, honest gallon count before bidding a job or a trip to the paint store.

Net area = walls − openings
Wall area = perimeter × height. Built-in estimating assumptions: door = 21 ft² (1.95 m²), window = 15 ft² (1.4 m²). Paint = net area × coats ÷ coverage. The editable coverage default is 350 ft²/gal (8.5 m²/L); use the rate printed on the selected paint.
Paint Calculator
Net area · gallons

How to use this calculator

1
Choose units
Pick feet and gallons or metres and litres. The tool switches its default coverage rate and opening sizes to match, so a metric room reads litres and a US room reads gallons.
2
Enter room length, width, and wall height
Type the two floor dimensions and the floor-to-ceiling height. The tool walls the room from these: gross area = perimeter × height = 2 × (length + width) × height.
3
Count doors and windows
Enter how many doors and windows the walls hold. Each door subtracts about 21 ft² (1.95 m²) and each window about 15 ft² (1.4 m²) from the area you actually paint.
4
Set the number of coats
Most repaints need two coats for even color and full hide. Bare or patched drywall often wants a primer coat plus two finish coats. The tool multiplies net area by this count.
5
Enter your coverage rate
Read the spreading rate off the paint can, often near 350 ft²/gal (8.5 m²/L). Leave it blank to use that default. Lower it for rough, porous, or thirsty surfaces.
6
Read the paint to buy
The panel returns net paintable area, total area across all coats, and the volume needed. Round up to whole cans, since stores sell paint by the gallon or litre.

The formula

The amount of paint needed is the net paintable area — walls minus openings — multiplied by the number of coats, divided by the paint's spreading rate.

Paint volume = (perimeter × height − openings) × coats ÷ coverage

perimeter = 2 × (length + width)
openings = (number of doors × 21 ft²) + (number of windows × 15 ft²); these are editable only by adjusting the counts and are estimating assumptions, not standard dimensions
coats = number of finish coats applied to each wall surface
coverage = spreading rate from the paint label; the editable field defaults to 350 ft²/gal (8.5 m²/L) when left blank

Worked example

Example
A 16 × 12 ft room with 9 ft walls has a gross wall area of 2 × (16 + 12) × 9 = 504 ft². Deducting 1 door (21 ft²) and 2 windows (30 ft²) leaves a net 453 ft². Over 2 coats that is 906 ft², and at 350 ft²/gal the job needs 2.6 gallons — buy 3 gallons and keep the remainder for touch-ups.

When this estimate will be off

  • Assumes rectangular walls — alcoves, angled ceilings, bay windows, or irregular room shapes need each section calculated separately.
  • Coverage rate varies by surface texture and porosity; a rough, unprimed, or previously dark-painted wall can consume 20–30% more paint than the default suggests.
  • Opening sizes use estimating assumptions (21 ft² per door, 15 ft² per window) — unusually large or small openings should be measured separately.
  • Excludes ceiling, trim, baseboards, and closet interiors; plan a separate quantity if any of those surfaces are painted a different color.

Frequently asked questions

A common mistake: Assumes rectangular walls — alcoves, angled ceilings, bay windows, or irregular room shapes need each section calculated separately.

Paint volume is net paintable area times the number of coats, divided by the coverage rate. For 453 ft² of wall over 2 coats at 350 ft²/gal, the math is 453 × 2 ÷ 350 = 2.6 gallons. Round up to whole cans, since paint sells by the gallon or litre.

You only buy paint for surfaces you actually cover, and openings are not painted with wall paint. The tool uses estimating assumptions of 21 ft² (1.95 m²) per door and 15 ft² (1.4 m²) per window. Measure unusual openings separately because these values are not standard dimensions.

Two coats is the standard for a clean, even finish on a previously painted wall. Plan a primer plus two coats on bare drywall, new patches, or a strong color change. Each extra coat multiplies the paint needed, so coats matter as much as area in the total.

Use the spreading rate printed on the selected paint label. The calculator falls back to 350 ft² per gallon, or 8.5 m² per litre, only as a planning assumption. Rough, textured, or unprimed surfaces can reduce actual coverage, so the product-specific rate and substrate condition should control the purchase estimate.

Sources

  • Calculation basis — Elementary rectangle-area and division arithmetic implemented on this page; no controlling building standard applies.
  • Product-specific coverage — Coverage varies by coating and substrate. Replace the built-in 350 ft²/gal (8.5 m²/L) estimating assumption with the spreading rate printed on the selected paint label.

This free Paint Area Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.