Free 100% In-Browser Tool

Paint Calculator

Calculate paint gallons and quarts needed based on room dimensions, doors, windows, coats, and waste margins.

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How to Use the Paint Calculator – Gallons Needed & Wall Coverage

1Enter the room length, width, and ceiling height in feet.
2Specify the count of doors (standard 21 sq ft) and windows (standard 15 sq ft).
3Choose the number of coats (standard is 2 coats) and whether to include the ceiling.
4Read the recommended gallons to purchase and view the step-by-step surface area math.

Formula & How It Works

Wall Surface Area & Paint Coverage Math

Gallons = (Net Wall Area + Ceiling) × Coats × (1 + Waste%) ÷ Coverage

Gross wall area equals perimeter (2 × (Length + Width)) multiplied by wall height. Door and window openings are deducted. The net paintable area is multiplied by the coat count and augmented by a waste allowance (standard 10%) before dividing by standard coverage (350 sq ft per gallon).

Worked Examples

Standard Bedroom (14 × 12 ft, 9 ft ceilings)

Input: 14 × 12 ft room, 9 ft height, 1 door, 2 windows, 2 coats, 10% waste

Explanation: Gross wall area = 52 × 9 = 468 sq ft. Openings = 21 + 30 = 51 sq ft. Net area = 417 sq ft. 2 coats with 10% waste = 917.4 sq ft ÷ 350 = 2.63 gal → 3 gallons.

Result: Recommend: 3 Gallons (Exact: 2.63 gal, Net Area: 417 sq ft/coat)

Small Bathroom Refresh

Input: 8 × 6 ft room, 8 ft height, 1 door, 1 window, 2 coats, 10% waste

Explanation: Perimeter = 28 ft × 8 ft = 224 sq ft. Openings = 36 sq ft. Net = 188 sq ft. With 2 coats and waste = 413.6 sq ft ÷ 350 = 1.18 gal.

Result: Recommend: 2 Gallons (Exact: 1.18 gal)

Living Room with Ceiling Included

Input: 20 × 15 ft room, 10 ft height, 2 doors, 4 windows, ceiling included, 2 coats

Explanation: Gross walls = 700 sq ft. Openings = 102 sq ft. Net walls = 598 sq ft. Ceiling = 300 sq ft. Total coat = 898 sq ft. 2 coats + 10% waste = 1,975.6 sq ft ÷ 350 = 5.68 gal.

Result: Recommend: 6 Gallons (Exact: 5.68 gal)

Single Accent Wall Project

Input: 15 ft wall, 9 ft height, 0 openings, 2 coats

Explanation: Area = 135 sq ft × 2 coats × 1.1 waste = 297 sq ft ÷ 350 = 0.85 gal (1 gallon can suffices).

Result: Recommend: 1 Gallon (Exact: 0.85 gal)

Frequently Asked Questions

How many square feet does 1 gallon of paint cover?▼

One gallon of interior wall paint typically covers 350 to 400 square feet of smooth, pre-primed drywall with a single coat.

Should I always apply two coats of paint?▼

Yes, two coats provide rich color depth, uniform sheen, and durability. Even self-priming paints usually require two coats for dramatic color changes.

How much area does a standard door and window take up?▼

A standard interior door is approximately 21 square feet (3 ft × 7 ft), while an average window is about 15 square feet (3 ft × 5 ft).

Why should I add a 10% waste factor?▼

A 10% allowance covers roller nap absorption, tray residue, wall texture porosity, and leftover touch-up paint for future scuffs.

Can I use the leftover paint from walls for ceilings?▼

Ceilings typically use flat, non-reflective white paint to minimize imperfections and maximize ambient light reflection, whereas walls use eggshell, satin, or semi-gloss.

Does this calculator run client-side without collecting my room data?▼

Yes, all calculation formulas execute entirely in your web browser with 100% data privacy.

Last reviewed & updated: 2026-11-01

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