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Trowelly

Paint Calculator

Enter your room size, doors, windows and number of coats to see how many gallons of paint to buy.

Units
ft
ft
ft

Each door removes about 21 ft² (3×7 ft).

Each window removes about 15 ft² (3×5 ft).

ft² per gal

Check the can; most interior paint covers 350–400 ft² per gallon.

Add prices for a cost estimate (optional)
per gallon

Results

Gallons to buy
2 gal
Exact paint needed
1.72 gal
Exact paint needed (metric)
6.5 L
Wall area
301 ft²
  • New drywall, dark-to-light color changes and textured walls often need a primer coat or an extra coat.

Where to buy

Quantities from your result, with links to search each store.

Your ZIP code stays on this device. We use it to show stores and suppliers in your area.

You may also need

Links search each store’s website. Check prices and stock before you go.

How to use the paint calculator

  1. Enter the Room length, Room width and Wall height.
  2. Enter the number of Doors and Windows to subtract.
  3. Under Include the ceiling?, choose walls only or walls and ceiling.
  4. Set the number of Coats and the Coverage per gallon from the paint can (350 ft² by default).
  5. Optionally enter a price per gallon to estimate cost.

How it works

The calculator finds the area of all four walls, then subtracts about 21 ft² for each door (3 × 7 ft) and 15 ft² for each window (3 × 5 ft). If you include the ceiling, its area of length × width is added.

Total area is multiplied by the number of coats and divided by the coverage per gallon. The default is 350 ft² per gallon, the low end of the 350–400 ft² most interior paints list. In metric mode, coverage is entered in m² per liter.

Wall area = 2 × (length + width) × height − doors × 21 ft² − windows × 15 ft²

Paint area = wall area (+ length × width if painting the ceiling)

Gallons = paint area × coats ÷ coverage per gallon, rounded up

Worked example

With the default inputs, a 12 × 10 ft room with 8 ft walls has 352 ft² of wall. Subtracting one door (21 ft²) and two windows (30 ft²) leaves 301 ft².

Two coats at 350 ft² per gallon need 301 × 2 ÷ 350 = 1.72 gallons (6.5 liters), so buy 2 gallons.

Tips before you buy

  • Use the coverage figure printed on your paint can. Textured and porous surfaces cover less.
  • New drywall, dark-to-light color changes and patched walls often need primer or an extra coat.
  • Buy all the paint you need at once, or mix cans together, to avoid slight color differences.
  • Trim, doors and cabinets are usually painted with a different sheen, so estimate them separately.

Frequently asked questions

How much area does a gallon of paint cover?

Most interior paints list 350–400 ft² per gallon for one coat on smooth walls. Check the can, and expect less on textured or unprimed surfaces.

How much paint do I need for a 12×10 room?

With 8 ft walls, one door and two windows, the walls are about 301 ft². Two coats at 350 ft² per gallon need 1.72 gallons, so buy 2 gallons.

Do I need two coats of paint?

Two coats usually give the most even color. Going from dark to light, or painting new drywall, often needs a primer first.

How much paint do I need for a ceiling?

A ceiling’s area is the room length × width. Choose walls and ceiling to include it, or calculate it separately if you are using a different paint.

Should I buy quarts or gallons?

If your exact need is well under the gallons to buy, the calculator suggests replacing the last gallon with quarts; 4 quarts equal 1 gallon. Gallons are usually cheaper per ounce.

Results are estimates based on the measurements you enter and typical material properties. Products vary, so check quantities with your supplier and follow product labels and your local building codes. Read the full disclaimer. Last reviewed October 9, 2026.