Buying too little paint means an extra trip to the store and a potential color mismatch between batches. Buying too much wastes $30–60 per unused gallon. The math to get it right is simple once you know the formula, and this guide walks through every variable: room dimensions, surface texture, number of coats, and the areas you need to subtract for doors and windows.
Gallons Needed = (Total Wall Area − Unpainted Areas) × Number of Coats ÷ Coverage Rate
Standard coverage rates for interior latex paint on smooth, previously painted surfaces: 350–400 square feet per gallon. Use 350 sq ft for conservative estimates and 400 sq ft for ideal conditions.
Measure the perimeter of the room (sum of all wall lengths) and multiply by the ceiling height. A 12 × 14 foot room with 8-foot ceilings: perimeter = (12 + 14 + 12 + 14) = 52 feet. Wall area = 52 × 8 = 416 square feet. The Square Footage Calculator can help with irregular rooms.
| Feature | Standard Size | Area to Subtract |
|---|---|---|
| Standard door | 3′ × 7′ | 21 sq ft |
| Double door | 6′ × 7′ | 42 sq ft |
| Standard window | 3′ × 4′ | 12 sq ft |
| Large window | 4′ × 5′ | 20 sq ft |
| Sliding glass door | 6′ × 7′ | 42 sq ft |
For our 12 × 14 room with two windows and one door: 416 − 24 − 21 = 371 sq ft of paintable surface.
At 350 sq ft per gallon with two coats: 371 × 2 ÷ 350 = 2.12 gallons. Round up to 3 gallons (paint is sold in quarts and gallons, so you would buy 2 gallons plus 1 quart, or simply 3 gallons for touch-up reserve).
Pro tip: Always add 10–15% to your calculated amount. Paint gets absorbed into rollers and brushes, textured surfaces drink more paint than smooth ones, and having leftover paint for future touch-ups saves the hassle of color matching later. Store leftovers in a cool, dry place and they will last 5–10 years.
| Surface | Coverage (sq ft/gallon) | Notes |
|---|---|---|
| Smooth, previously painted drywall | 350–400 | Best case scenario |
| New (primed) drywall | 300–350 | Primer reduces absorption |
| Bare drywall (unprimed) | 250–300 | Always prime first |
| Textured walls (knockdown, orange peel) | 250–300 | Texture increases surface area |
| Stucco or masonry | 150–250 | Very porous — prime and use more paint |
| Wood trim and doors | 300–350 | Multiple thin coats recommended |
Two coats is the standard for most paint jobs. Two coats ensure even color, hide roller marks, and provide the durability the manufacturer tested for. Single-coat claims on paint cans refer to ideal conditions that rarely exist in real rooms.
Three coats are necessary when painting light colors over dark (especially white over deep red, navy, or black) or when painting dark colors over light surfaces. The underlying color bleeds through with fewer coats.
One coat may be sufficient when recoating the exact same color and sheen, using a premium self-priming paint on a clean, smooth surface with no repairs or patches.
| Room Size | Wall Area | After Subtractions | Gallons (2 coats) |
|---|---|---|---|
| Small bedroom (10×10) | 320 sq ft | ~265 sq ft | 2 |
| Average bedroom (12×12) | 384 sq ft | ~330 sq ft | 2–3 |
| Large bedroom (14×16) | 480 sq ft | ~415 sq ft | 3 |
| Bathroom (8×10) | 288 sq ft | ~230 sq ft | 2 |
| Living room (16×20) | 576 sq ft | ~490 sq ft | 3–4 |
| Kitchen (12×14) | 416 sq ft | ~310 sq ft | 2–3 |
Assumes 8-foot ceilings, standard doors and windows. Use the Paint Calculator for precise results.
For ceilings, calculate length × width. A 12 × 14 room ceiling = 168 sq ft, requiring about 1 gallon for two coats. Ceiling paint is typically flat white and covers slightly better than wall paint because smooth ceilings have no texture.
For trim (baseboards, crown molding, door frames, and window frames), a quart of trim paint covers about 75–100 linear feet. Most rooms need 1–2 quarts of trim paint.
Calculate your paint needs. Use the free Paint Calculator to get exact gallon counts for any room, and the Square Footage Calculator for wall measurements — no signup required.
Related tools: Paint Calculator · Square Footage Calculator · Flooring Calculator