Paint Calculator | How Much Paint Do I Need? (By Square Feet, Room Size)

Paint Calculator (Estimate Gallons by Square Feet or Room Size)

Use this paint calculator to figure out how much paint you need for interior or exterior projects. Enter your room dimensions or total square footage, and the calculator estimates gallons based on real-world coverage rates, number of coats, doors, windows, and waste. This is one of the construction calculators on OnSiteCalculator.

Paint Coverage Per Gallon (Interior vs Exterior)

One gallon of interior latex paint covers 350–400 square feet on smooth, primed drywall. One gallon of exterior paint covers 250–350 square feet because surfaces like siding, stucco, and brick are rougher and more absorbent.

These are the coverage rates to use when estimating by hand or checking the calculator’s output:

Key factors that reduce coverage: Porous or textured surfaces, dark colors, unpainted substrates, and windy conditions (exterior). If your surface is rough or unpainted, use the lower end of the range. For a deeper breakdown of rates by finish, surface texture, and trim type, see the paint coverage per gallon guide.

Paint Calculator by Square Feet

The fastest way to estimate paint is by total square footage. Measure each wall (height times width), subtract openings, and enter the total into the calculator. Here is the process:

1. Measure each wall. Multiply height by width. A standard 8-foot ceiling on a 12-foot wall = 96 square feet.
2. Subtract openings. Deduct 20 sq ft per standard door and 15 sq ft per standard window. For sliding or French doors, use 40 sq ft.
3. Add the ceiling if you are painting it. Ceiling area = room length times room width.
4. Enter the total into the calculator, select interior or exterior, and choose the number of coats.

Example: A room with four walls totaling 480 sq ft, minus 2 doors (40 sq ft) and 2 windows (30 sq ft) = 410 sq ft of paintable wall. At 375 sq ft/gal coverage and 2 coats, you need about 2.2 gallons. Buy 3 gallons to cover waste and touch-ups.

For precise wall measurements, the fraction to decimal chart converts tape measure fractions to decimal feet. The tape measure guide walks through reading every mark on your tape if you are not comfortable with fractions.

How Much Paint for Common Room Sizes

These estimates assume 8-foot ceilings, standard interior latex at 375 sq ft/gal, and two coats. Door and window deductions are included.

How to estimate any room: Multiply the perimeter (total wall length) by the ceiling height, subtract openings, multiply by the number of coats, and divide by coverage per gallon. For more room types — including kitchens, laundry rooms, garages, and accent walls — see the room-by-room paint guide.

Formula: (Perimeter × Height − Openings) × Coats ÷ Coverage Rate = Gallons

How to Calculate Paint Quantity (Step by Step)

Follow these steps to calculate paint for any project without the calculator:

1. Measure all paintable surfaces. For each wall, multiply width by height. For the ceiling, multiply room length by width. For trim, measure the linear feet and multiply by the trim width (usually 3–6 inches).

2. Subtract non-paintable areas. Deduct 20 sq ft per standard door, 15 sq ft per window, and the full area of any built-ins, cabinets, or tile that will not be painted.

3. Apply the coverage rate. Divide total paintable square footage by the coverage rate for your paint type:

   • Interior on smooth drywall: 375 sq ft/gal (midpoint)
   • Exterior on siding: 325 sq ft/gal (midpoint)
   • Exterior on stucco/brick: 275 sq ft/gal (midpoint)

4. Multiply by coats. Two coats is standard for most projects. New drywall or dramatic color changes may need three.

5. Add waste. Add 5–15% depending on surface texture and complexity. Simple rectangular rooms need less waste; rooms with many cuts, corners, or obstacles need more.

6. Calculate primer separately if needed. New drywall, patched areas, and dark-to-light changes require a primer coat. Primer covers 350–400 sq ft/gal and needs one coat.

7. Round up to the nearest whole gallon. Paint stores do not sell partial gallons (except quarts for small jobs). If your estimate is 2.3 gallons, buy 3. The extra covers touch-ups and future repairs.

Worked example — 10x12 bedroom:

• Perimeter: 10 + 12 + 10 + 12 = 44 feet
• Wall area: 44 × 8 = 352 sq ft
• Subtract 1 door (20 sq ft) + 1 window (15 sq ft) = 317 sq ft
• Two coats: 317 × 2 = 634 sq ft total coverage needed
• At 375 sq ft/gal: 634 ÷ 375 = 1.69 gallons
• Round up: buy 2 gallons

Primer Guide: When You Need It and What Type

Primer typically covers 350–400 sq ft per gallon and requires only one coat. Factor primer into your paint project budget and timeline, as it adds drying time between coats. Use the drywall calculator to estimate sheets and finishing materials for new drywall before priming and painting.

Common Paint Calculation Mistakes

Not accounting for primer on new drywall. New drywall is porous and absorbs the first coat of paint unevenly. Always apply a dedicated PVA primer before painting bare drywall. Paint-and-primer combos are not a substitute for this step.

Forgetting the ceiling. If you plan to paint the ceiling, add its area (length × width) to your total. Ceilings add significant square footage — a 12x14 room adds 168 sq ft.

Skipping the door and window deduction. Failing to subtract openings overestimates your paint needs. Standard doors are roughly 20 sq ft and windows about 15 sq ft.

Using the wrong coverage rate. Interior coverage (350–400 sq ft/gal) is higher than exterior (250–350 sq ft/gal). Applying interior rates to exterior projects leads to buying too little paint.

Buying all 1-gallon cans. For projects requiring 4+ gallons, a 5-gallon bucket is cheaper per gallon and ensures color consistency across the entire job. Mix all your paint in one bucket (“boxing”) before starting for uniform color.

Related Tools

• Drywall Calculator — Estimate sheets, joint compound, tape, and screws for new drywall before priming and painting.
• Flooring Calculator — Calculate square footage and boxes of flooring for rooms you are also painting.
• Fraction to Decimal Chart — Convert tape measure fractions to decimal inches for precise wall measurements.
• Lumber Dimensions Chart — Look up actual board widths for baseboards, casings, and trim when estimating paint coverage on woodwork.
• How to Read a Tape Measure — Step-by-step guide to reading fraction marks on your tape for accurate wall measurements.
• Paint Coverage Per Gallon Guide — Full coverage rates chart for interior, exterior, and trim by paint type, surface, and finish.
• How Much Paint for a Room — Gallon estimates for 10x12 bedrooms, bathrooms, kitchens, garages, and more with step-by-step formulas.