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Free Tool

Paint Calculator: How Much Paint Do You Need?

Gallons = net wall area x coats / coverage per gallon, rounded up. Enter the room, doors and windows, surface, and coats to get gallons and a 5-gallon pail breakdown. Same math as the SiteScope app.

1 Room Dimensions

2 Openings (deducted from paintable area)

Total deducted 51 sq ft

Need custom door or window sizes or per-wall openings? Customize in SiteScope →

3 Surface & Paint Details

Rough surfaces absorb more paint, reducing effective coverage per gallon.

From product label. Typical: 350–400 sq ft/gal.

10% brush/roll · 15–20% spray

Leave blank to show gallons only.

4 Primer (optional)

Results

Gross wall area
Openings deducted
Net paintable area
Effective coverage
Finish coats
Waste
Finish
Gallons needed
gallons

Rounded up to the nearest whole gallon. Order in 5-gal pails when you can. They are often cheaper per gallon than singles.

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Also need wallpaper?
Use the Wallpaper Calculator to get strip count, roll yield, pattern repeat waste, and seam overlap applied automatically.

These estimates use the same formulas as the SiteScope app. Actual quantities depend on wall conditions, installer technique, and product specifications. Always verify with your supplier before ordering.

How to Calculate How Much Paint You Need

Gallons of paint = net wall area x number of coats / coverage per gallon, then round up. Net wall area is the room perimeter times the wall height, minus doors and windows. Add a waste allowance for rolling and brushing, and adjust coverage down for textured surfaces.

Worked example: 12 x 14 ft room, 9 ft walls
  1. Wall area: 2 x (12 + 14) x 9 = 468 sq ft
  2. Minus 1 door (21) and 2 windows (30): 468 - 51 = 417 sq ft
  3. Two coats: 417 x 2 = 834 sq ft
  4. Plus 10% waste: 834 x 1.10 = 917 sq ft
  5. Divide by coverage: 917 / 350 = 2.62, round up to 3 gallons

Same room with popcorn texture: coverage drops to about 228 sq ft per gallon (350 x 0.65), so 917 / 228 = 4.03, round up to 5 gallons.

Want the full walkthrough? Read how to calculate paint for a room. Prepping new walls first? Use the drywall calculator.

Frequently Asked Questions

How much does a gallon of paint cover?
Most interior wall paints list 350 to 400 square feet per gallon for one coat on smooth, primed drywall. Rough, porous, or textured surfaces cover less, so use the label number and adjust for texture.
How many gallons of paint do I need for a 12x12 room?
For 8 ft walls: 2 x (12 + 12) x 8 = 384 sq ft, minus one door and two windows (51 sq ft) = 333 sq ft. Two coats is 666 sq ft, and at 350 sq ft per gallon that is 1.9 gallons. With a 10% waste allowance it rounds up to 3 gallons. Ceilings are extra.
Should I include the ceiling?
This calculator figures walls only. For a flat ceiling, the area is length x width. Run it separately, since ceiling paint is usually a different product.
When do I need primer?
Prime new drywall and big patches, stains, glossy surfaces, and large color changes (especially light over dark). Primer usually covers fewer square feet per gallon than finish paint, so it is calculated separately.
Why does the calculator round up to whole gallons and 5-gallon pails?
You cannot buy a partial gallon, and running short mid-wall risks a visible difference between batches. Pails are often cheaper per gallon than single gallons, so the breakdown shows how many to buy of each.
Does texture really change how much paint I need?
Yes. Texture adds surface area and soaks up more paint. The calculator reduces coverage by a factor per surface: orange peel 0.90, knockdown 0.85, popcorn 0.65, wood and metal 0.95.
Skip the calculator entirely

SiteScope does this room by room, from a 3D scan.

Scan a room with your iPhone or iPad Pro's LiDAR, open it on the Estimate page, and SiteScope calculates quantities for the room with openings already deducted. Primer, coats, surface type, and coverage applied automatically per SKU. Export to CSV or a branded quote PDF. Free includes estimating with up to 10 materials. Unlimited materials and quote PDFs are on Pro.

  • Finish and primer gallons calculated separately
  • Surface type and deep-base coats applied per SKU
  • 5-gal pail breakdown and SKU-level totals on the Estimate page
  • Your cost, charge, and margin, applied to every line
Try SiteScope Free

Estimating this for clients? See SiteScope for painting contractors.

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