Calculate how many gallons or liters of paint you need. Accounts for doors, windows, and number of coats.
Step 1: Calculate total wall area. For a rectangular room: Wall area = 2 x (Length + Width) x Height
Step 2: Subtract openings. Standard door = ~21 sq ft (3 x 7 ft). Standard window = ~15 sq ft (3 x 5 ft).
Step 3: Multiply by number of coats. Most interior paint jobs require 2 coats for proper coverage.
Step 4: Divide by paint coverage rate. Standard interior latex paint covers approximately 350 sq ft per gallon.
Formula: Gallons = (Wall area - openings) x coats / coverage per gallon
Interior latex paint: 350-400 sq ft per gallon on smooth surfaces.
Exterior paint: 250-350 sq ft per gallon depending on surface texture.
Primer: 200-300 sq ft per gallon. Always prime new drywall and when making drastic color changes.
Textured walls: Reduce coverage by 15-25% for rough or textured surfaces.
A standard room measuring 12x12 feet with 8-foot ceilings has approximately 384 square feet of wall space. After subtracting a standard door (21 sq ft) and two windows (30 sq ft), you have about 333 square feet of paintable surface. With two coats at 350 sq ft per gallon coverage, you need approximately 1.9 gallons -- so buy 2 gallons.
For a larger living room (15x20 feet, 9-foot ceilings), the total wall area jumps to 630 square feet. After subtracting openings, two coats require about 3.2 gallons. Buy 4 gallons to have enough for touch-ups later. Always save leftover paint -- you will need it for nail holes, scuffs, and repairs within the first year.
A general rule of thumb: one gallon of quality interior paint covers approximately 350-400 square feet on smooth, primed surfaces with a single coat. Textured walls, unprimed drywall, and porous surfaces reduce coverage by 15-25%. Always plan for two coats unless you are painting the same color over the existing paint with a high-quality paint-and-primer product.
Let us calculate paint for a master bedroom: 14 x 16 feet, 8-foot ceilings, 2 doors, 3 windows.
Step 1: Calculate total wall perimeter.Perimeter = 2 x (14 + 16) = 60 linear feet
Step 2: Calculate total wall area.Wall area = 60 x 8 = 480 square feet
Step 3: Subtract openings.
2 doors x 21 sq ft = 42 sq ft
3 windows x 15 sq ft = 45 sq ftPaintable area = 480 - 42 - 45 = 393 square feet
Step 4: Multiply by number of coats.393 x 2 coats = 786 square feet total coverage needed
Step 5: Divide by coverage rate.786 / 350 = 2.25 gallons
Round up: buy 3 gallons (you will use the extra for touch-ups).
Flat vs. eggshell vs. semi-gloss -- when to use each:
Flat/matte: Best for ceilings and low-traffic areas. Hides wall imperfections well but is harder to clean. Not recommended for kitchens, bathrooms, or kids' rooms.
Eggshell: The most popular choice for bedrooms, living rooms, and dining rooms. Slight sheen makes walls easy to wipe clean while still hiding minor imperfections.
Satin: More durable than eggshell. Great for hallways, family rooms, and kids' bedrooms where walls get touched frequently.
Semi-gloss: Highly washable and moisture-resistant. Use for kitchens, bathrooms, trim, doors, and cabinets. Shows wall imperfections more than flatter sheens.
Do you really need primer? Yes, in these situations: new drywall, covering dark colors with light ones, painting over stains, or switching between oil-based and latex paint. For repainting a similar color over existing paint in good condition, a quality paint-and-primer-in-one product usually works fine.
Quality matters. Premium paints ($40-$60/gallon) provide better coverage, often requiring fewer coats, and last significantly longer than budget paints ($20-$30/gallon). The cost difference for a typical room is only $30-$60, but the result lasts years longer.
Interior latex paint: $30-$55 per gallon for quality brands (Benjamin Moore, Sherwin-Williams, Behr). Economy brands run $20-$30 per gallon.
Exterior paint: $35-$70 per gallon. Exterior paint must withstand UV, moisture, and temperature extremes.
Primer: $20-$40 per gallon. Tintable primer is recommended when making drastic color changes.
Supplies: Budget $30-$60 for rollers, brushes, tape, drop cloths, and trays per room.
Professional painting: Expect $2-$6 per square foot of wall area for interior painting, including labor and materials. A typical bedroom costs $300-$700 professionally painted.
Prices are approximate US averages for 2026 and vary by region and brand.
| Room | Typical Size | Wall Area | Gallons (2 coats) | Est. Cost |
|---|---|---|---|---|
| Small bedroom | 10 x 12 ft | ~300 sq ft | 2 gallons | $60-$110 |
| Master bedroom | 14 x 16 ft | ~400 sq ft | 3 gallons | $90-$165 |
| Living room | 15 x 20 ft | ~500 sq ft | 3-4 gallons | $90-$220 |
| Bathroom | 8 x 10 ft | ~220 sq ft | 1-2 gallons | $30-$110 |
| Whole house (3 bed) | ~1,500 sq ft home | ~3,000 sq ft | 10-15 gallons | $300-$825 |
Always buy all your paint at once from the same batch. Color can vary between batches. If you need multiple gallons, mix them together in a 5-gallon bucket for perfect color consistency.
Use the "W" technique when rolling: paint a W shape on the wall, then fill it in without lifting the roller. This distributes paint evenly and avoids roller marks.
Do not skip cutting in. Use a quality angled brush to "cut in" 2-3 inches around trim, ceiling edges, and corners before rolling.
Remove painter's tape while the paint is still slightly tacky -- not fully dry. Pull at a 45-degree angle to avoid pulling off paint.
Most stores accept returns on unopened, untinted paint within 30-90 days. Buy an extra gallon as insurance and return it if unused.
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