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Paint Calculator | Estimate Paint Quantity and Cost

Estimate how many gallons or litres of paint you need from room dimensions, doors and windows, coats, and the coverage rate on the can — plus the total paint cost at your price per gallon. The result rounds up to whole cans so you buy exactly what the job takes.

Last updated: August 2026 · Free · No sign-up required

A paint calculator estimates gallons or liters of paint from wall dimensions, doors, windows, coats, coverage, and waste, then calculates the expected project cost.

How this calculator works

How much paint do you need?

The calculator estimates paintable surface area from the room perimeter and wall height, subtracts doors and windows, multiplies by the number of coats, and divides by the coverage rate printed on the paint can. A waste allowance is then added.

Coverage and assumptions

Coverage varies by product, surface texture, color change, primer, and application method. Enter the manufacturer’s coverage rate when available. The result is an estimate; round up to the next purchasable container size rather than relying on a fraction of a can.

Worked example

Scenario: A 12 ft by 10 ft room has 8 ft walls, 40 sq ft of openings, two coats, and paint rated at 350 sq ft per gallon.

Result: The calculator subtracts openings, accounts for both coats and waste, and rounds the purchase quantity upward.

Frequently asked questions

How much paint does one gallon cover?

Coverage depends on the product and surface. Use the coverage rate printed on the can; the calculator provides a default only as a starting estimate.

Should I include a waste allowance?

Yes. A modest allowance helps account for roller absorption, spills, texture, touch-ups, and container rounding. Adjust it for your project.

Does the calculator include ceilings?

The default calculation estimates walls. Add ceiling area separately if you plan to paint it, or use the room-area result as a separate surface.

How do I calculate paint for multiple coats?

Enter the number of coats required by the paint manufacturer or your project plan. The calculator multiplies paintable area by that coat count before applying coverage and waste.

Are doors and windows subtracted?

Yes. Enter the combined area of doors and windows when you want to exclude those openings from the paintable wall area.

Does the paint calculator estimate cost?

Yes. If you enter a price per gallon, the calculator multiplies the rounded gallon requirement by that price. Actual prices and container sizes vary by product and retailer.

From our guides

Quick reference

Paint TypeCoverage (sq ft/gal)Coats for best finish
Latex flat (walls)350–4002
Latex eggshell/satin300–3502
Semi-gloss (trim)300–3502
Primer300–4001
Textured/stucco150–2502–3

format_paint Subtract openings

layers Plan for coats

shopping_cart Estimate cost

lightbulb Worked example

Painting a 12x14 ft bedroom with 8 ft ceilings, one door and two windows, two coats.

Result: 3 gallons of paint; cost = 3 x price per gallon.

Formula & methodology

Formula: paintable area = total wall area − openings; paint volume = paintable area × coats ÷ coverage × (1 + waste)

How much paint do you need?

The calculator estimates paintable surface area from the room perimeter and wall height, subtracts doors and windows, multiplies by the number of coats, and divides by the coverage rate printed on the paint can. A waste allowance is then added.

Coverage and assumptions

Coverage varies by product, surface texture, color change, primer, and application method. Enter the manufacturer’s coverage rate when available. The result is an estimate; round up to the next purchasable container size rather than relying on a fraction of a can.

Authoritative source: Sherwin-Williams Paint Quantity Guide

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