Short answer
Painting the walls of a 20×24 garage with 8-foot ceilings, 2 doors, 2 windows, and 2 coats requires 3.6 gallons of wall paint. Add 1.4 gallons for the ceiling (1 coat) and 2 quarts for trim and door frames. Buy 4 gallons of wall paint and 2 gallons of ceiling paint to cover rounding and roller waste. That's the direct answer to how many gallons to paint a 20x24 garage interior.
How this calculator works
The calculator builds up paintable wall area from your room dimensions, subtracts door and window openings, multiplies by the number of coats, then divides by a realistic coverage rate.
Step 1 — Calculate gross wall area
A 20×24 room has a perimeter of 88 linear feet (20 + 24 + 20 + 24). Multiply that by the 8-foot ceiling height and you get 704 square feet of gross wall surface.
Step 2 — Subtract openings
Every standard door opening is roughly 21 square feet (3 ft × 7 ft). Every average window is about 15 square feet (3 ft × 5 ft). Two doors and two windows subtract 72 square feet:
- 2 doors × 21 sq ft = 42 sq ft
- 2 windows × 15 sq ft = 30 sq ft
- Total deduction: 72 sq ft
Net paintable wall area: 704 − 72 = 632 square feet.
Step 3 — Multiply by coats
Two coats means covering that 632 square feet twice: 632 × 2 = 1,264 square feet of total paint coverage needed.
Step 4 — Divide by coverage rate
Most interior paints spread at about 350 square feet per gallon on a smooth, previously painted or primed surface. Dividing 1,264 by 350 gives 3.61 gallons.
The calculator rounds nothing internally — it hands you the exact decimal so you can decide whether to buy 4 one-gallon cans or 15 quart cans (also shown as a secondary output). Quarts cost more per unit but let you use exactly what you need if you have leftover paint from a previous job.
Ceiling output
Ceiling area is simply length × width: 20 × 24 = 480 sq ft. At 350 sq ft per gallon that's 1.37 gallons for one coat. Ceiling paint is usually applied in a single coat with a thick-nap roller, so this figure stands on its own.
Trim output
Trim and door paint is estimated at 0.25 quarts per door or window opening, plus 1 quart base. With 2 doors and 2 windows: (4 × 0.25) + 1 = 2 quarts. This covers door slabs, frames, window casings, and a run of baseboard. It's a rough estimate — if your garage has heavy crown moulding or you're painting the garage door itself, add another quart.
Coverage rate caveat
The 350 sq ft/gallon figure assumes a surface that's already sealed and in reasonable condition. Bare concrete block drops to 250–300 sq ft/gallon. Raw drywall (no primer) can be just as thirsty. If your garage walls haven't been painted before, run the calculator at 1.5× the gallon output and plan on a dedicated masonry or drywall primer as a separate coat not counted here.
What this calculator doesn't include
- Paint for the garage floor (that's a separate epoxy or porch-and-floor coating calculation)
- Primer coats
- The garage door itself (exterior paint product, different coverage)
- Touch-up paint you may want to set aside after the job
The outputs are a purchasing baseline. Round up to the next whole gallon when buying — returned paint cans are a hardware-store hassle, not a guaranteed option.
Recommended materials
For a 20×24 garage, a durable washable finish matters more than in a living room — expect the walls to take tool brushes, grease splatter, and seasonal humidity swings. A high-hide interior paint with a satin or semi-gloss sheen handles that well. Use a 9-inch roller with a 3/8-inch or 1/2-inch nap to get into any texture without wasting paint in excess fabric. Tape off the floor line, door frames, and any electrical panel covers before you start — painter's tape pulled while the paint is still slightly tacky gives the cleanest lines.
- Behr Premium Plus interior paint (1 gallon) — good coverage and washability, widely available for color-matching
- Purdy 9-inch roller cover (3-pack) — a quality nap that holds enough paint to cover large wall spans without constant reloading
- Frog Tape painter's masking tape (1.41-inch x 60yd) — worth using at the floor line and around the service door frame where clean edges are visible
FAQ
How many gallons do I need to paint a 20x24 garage interior?
With 8-foot ceilings, 2 doors, 2 windows, and 2 coats, you need about 3.6 gallons of wall paint. Buy 4 gallons to have a small buffer for touch-ups and roller waste.
Does that 3.6 gallons include the ceiling?
No. The 3.6-gallon figure covers the four walls only. The 480-square-foot ceiling adds roughly 1.4 gallons for a single coat, so budget separately for ceiling paint.
Why subtract area for doors and windows?
Doors and windows aren't painted with wall paint, so including them would overestimate your needs. The calculator deducts 21 square feet per door and 15 square feet per window from the total wall area.
Should I paint a garage ceiling the same color as the walls?
That's a personal choice, but many garages use a flat white ceiling paint regardless of wall color. Ceiling paint is typically thicker and formulated to reduce dripping overhead.
What coverage rate does this calculator use?
The formula uses 350 square feet per gallon, which is a realistic spread rate for most mid-grade interior paints on a primed or previously painted surface. New drywall or bare concrete will absorb more, dropping coverage to 250–300 sq ft per gallon.
Is 2 coats standard for a garage?
Yes, if you're changing colors or painting over a stained or bare surface. One coat can work when repainting the same color with a high-hide paint, but two coats give more durable coverage in a high-traffic space like a garage.
How much trim and door paint do I need?
For 2 doors and 2 windows, the calculator estimates 2 quarts of trim paint. That covers door frames, window casings, and baseboard in a standard garage.
Can I use exterior paint inside a garage?
Technically yes, but VOC levels are higher and the smell lingers longer indoors. A durable interior paint or a dedicated garage floor and wall coating is a better fit for an enclosed space.
What finish sheen works best for garage walls?
Satin or semi-gloss holds up to scuffs, moisture, and cleaning better than flat or eggshell. Semi-gloss is especially practical near workbenches and around the garage door opening.
Do I need a primer coat?
If the walls are bare drywall, new concrete block, or you're making a dramatic color change, prime first. Primer is not included in the 3.6-gallon wall paint total and should be calculated separately at roughly the same coverage rate.
How do I account for textured or porous concrete block walls?
Drop the coverage estimate to around 250 square feet per gallon for the first coat on unsealed block, and plan on a masonry primer. You may end up closer to 5–6 gallons total for a 20x24 garage with block walls.
What if I only want to do 1 coat?
Halve the wall paint figure: roughly 1.8 gallons for a single coat on a 20x24 garage with 2 doors and 2 windows. One coat is only reliable if the existing paint is the same color and in good condition.