Interior Painting & Drywall Coverage Calculator: Estimate Paint, Primer & Wall Area

Key formulas used for interior paint coverage

The calculator relies on a few simple geometry and coverage formulas. Understanding them can help you sanity-check results and adjust inputs for your specific project.

1. Wall area

For a rectangular room, the total wall area (ignoring doors and windows) is:

Wall area (sq ft) = 2 × (Length + Width) × Height

This works by taking the room perimeter (two long walls plus two short walls) and multiplying by the ceiling height.

Where:

• L = room length (ft)
• W = room width (ft)
• H = ceiling height (ft)

2. Ceiling area (if painted)

If you choose to paint the ceiling, the area is simply the footprint of the room:

Ceiling area (sq ft) = Length × Width

3. Total painted area for multiple coats

Every coat of paint covers the same physical area. To account for multiple coats:

Total painted area (sq ft) = (Wall area + Ceiling area) × Number of coats

4. Base coverage per gallon

Coverage per gallon depends on paint quality, brand, and surface. The calculator uses typical interior ranges:

• Budget (flat): about 250–300 sq ft/gal
• Standard (eggshell/satin): about 325–375 sq ft/gal
• Premium (high quality): about 375–450 sq ft/gal

Primer usually has similar or slightly higher coverage (often around 300–400 sq ft/gal) but may require separate coats depending on the substrate.

5. Adjusted coverage for textured or rough surfaces

Rough or heavily patched drywall absorbs more paint. The Surface Adjustment (%) field lets you reduce the effective coverage rate. A positive percentage means you expect coverage to drop compared to a smooth wall.

The adjusted coverage rate is:

Adjusted coverage (sq ft/gal) = Base coverage × (1 − Texture % ÷ 100)

6. Paint gallons needed

Once the calculator has the total painted area and adjusted coverage rate, it estimates how many gallons you should buy:

Gallons of paint = Total painted area ÷ Adjusted coverage

Results are typically rounded up slightly so you do not run out mid-project.

Interpreting your paint and drywall coverage results

When you run the calculator, you will usually see a few distinct outputs:

• Total wall area in square feet
• Ceiling area (if painting the ceiling)
• Total painted area across all coats
• Recommended gallons of paint, based on your paint type, quality, and surface adjustment
• Conceptual drywall coverage, i.e., how much area could be covered by common drywall sheet sizes

Use the wall and ceiling area numbers as a cross-check against any measurements your contractor or paint store provides. The gallons estimate helps you decide whether to buy quart touch-up cans, single gallons, or multiple 5-gallon buckets.

If you are planning new construction or major remodeling, the total wall area figure also approximates the minimum drywall surface required for that room. While the calculator does not output a precise sheet count, you can divide the total wall area by the area of a typical sheet:

• 4×8 ft sheet = 32 sq ft
• 4×10 ft sheet = 40 sq ft
• 4×12 ft sheet = 48 sq ft

Because drywall layouts must account for stud spacing, seams, and offcuts, a contractor will always add a waste factor beyond this simple division. Treat these drywall figures as conceptual guidance only.

Worked example: 12×14 room paint coverage with 8 ft ceilings

To see how the calculator works, imagine a standard bedroom with the following details:

• Room length: 12 ft
• Room width: 14 ft
• Ceiling height: 8 ft
• Paint ceiling: Yes
• Paint type: Interior latex
• Number of coats: 2 (standard)
• Paint quality: Standard
• Surface adjustment: 0% (smooth, previously painted walls)

Step 1: Calculate wall area

Perimeter = 2 × (12 + 14) = 2 × 26 = 52 ft.

Wall area = Perimeter × Height = 52 × 8 = 416 sq ft.

Step 2: Calculate ceiling area

Ceiling area = Length × Width = 12 × 14 = 168 sq ft.

Step 3: Total painted area for 2 coats

Single-coat area (walls + ceiling) = 416 + 168 = 584 sq ft.

Total painted area = 584 × 2 coats = 1,168 sq ft.

Step 4: Choose a coverage rate

For a standard interior latex at a mid-range quality, assume about 350 sq ft/gal on smooth walls. With no surface adjustment, the adjusted coverage remains 350 sq ft/gal.

Step 5: Estimate gallons of paint

Gallons needed = 1,168 ÷ 350 ≈ 3.34 gallons.

In practice, you would round up and buy 4 gallons. If you expect touch-ups later, you might choose a single 5-gallon bucket instead, especially if the color will be used in multiple rooms.

Step 6: Conceptual drywall coverage

Total wall area: 416 sq ft.

If using 4×8 drywalls sheets (32 sq ft): 416 ÷ 32 ≈ 13 sheets, before waste.

If using 4×12 sheets (48 sq ft): 416 ÷ 48 ≈ 9 sheets, before waste.

A contractor might plan for 10–15% extra to account for cuts, windows, and layout. That means around 10–15 sheets in real-world planning, depending on sheet length and framing.

Comparison: paint types, coats, and coverage assumptions

Different paint types and project choices change how much product you will use. The table below summarizes typical coverage expectations and when you might choose each option.

Assumptions & limitations of this coverage calculator

Every interior space is a bit different. For clarity and to set expectations, keep these assumptions and limitations in mind when using the calculator:

• Rectangular rooms only: Calculations assume a simple rectangular floor plan. L-shaped or irregular rooms should be split into rectangles and calculated separately.
• Doors and windows are not subtracted by default: The wall area estimate treats walls as continuous. In many rooms, the extra paint required for cutting in and touch-ups offsets the area “saved” by openings, so this simplification is usually acceptable.
• Standard, flat ceilings: Ceiling area is length × width. Vaulted, tray, or sloped ceilings will have more surface area than the calculator assumes.
• Typical coverage ranges: Coverage rates are based on common manufacturer guidelines for interior products applied correctly. Your actual coverage can vary with brand, roller nap, application method, color, and temperature/humidity.
• Surface condition matters: New, unprimed drywall, heavy texture, or patched areas can soak up more paint. Use the surface adjustment field (for example, 10–25%) if you expect significant additional absorption.
• Drywall sheet counts are approximate: The tool focuses on area, not precise material takeoff. Real-world drywall planning must consider stud spacing, sheet orientation, seam placement, and waste.
• Estimates only, not specifications: The outputs are intended for planning and budgeting. Always review individual product labels and, for large or complex jobs, confirm quantities with a paint supplier or contractor.

Tips for using the calculator effectively

• Measure in feet: Use a tape measure to get length and width to the nearest inch, then convert to decimal feet (e.g., 12 ft 6 in ≈ 12.5 ft).
• Decide whether to paint the ceiling: If you select "Yes" for painting the ceiling, the calculator adds that area to the total and adjusts gallons accordingly.
• Choose coats based on your project: One coat is often fine for very light refreshes. For most color changes, plan for two coats. Dark colors, reds, and bright yellows may need a third coat or a tinted primer.
• Use surface adjustment thoughtfully: Leave this at 0% for smooth, previously painted drywall. Increase to around 10–15% for light texture, and 20–30% for very rough or porous surfaces.
• Add a small safety margin: Even with a good estimate, buying a little extra paint (for example, 5–10%) provides insurance for touch-ups and future repairs.

Common questions about interior paint and drywall coverage

How many gallons of paint do I need per room?

For an average bedroom (about 12×14 ft with 8 ft ceilings), painting walls only with two coats of standard latex usually takes 2.5–3.5 gallons. Adding the ceiling, using darker colors, or painting over bare drywall can push that closer to 4–5 gallons. The calculator tailors this estimate to your exact room size and settings.

Do I need primer on new drywall?

Yes. New drywall and joint compound are very porous, and a dedicated primer (or a high-quality paint-and-primer-in-one) helps seal the surface, reduce flashing, and improve topcoat coverage. In many cases you will apply one coat of primer plus two coats of paint. Use the calculator to estimate paint quantities, then treat primer separately using a similar coverage rate.

Should I buy extra paint for touch-ups?

It is usually wise to keep at least half a gallon of leftover paint for touch-ups in each major color, especially in high-traffic areas. If the calculator suggests, for example, 2.7 gallons, buying 3 gallons is often better than trying to stretch or under-coat the surface.

Why does coverage per gallon vary so much?

Coverage varies with pigment content, solids, finish, application method, and how heavily you load the roller. Premium paints can cover more area per coat and hide better, but they also cost more. The calculator uses mid-range assumptions that work well for planning; always check the label on your chosen product for the manufacturer’s stated coverage.