Paint

Turning Wall Area Into Gallons to Buy

Paint needed is a room’s wall area, minus doors and windows, divided by how much area one gallon covers per coat. Enter a room’s length, width, and wall height, plus how many doors and windows it has and how many coats you’re planning, and this calculator estimates how much paint to buy, along with a rough cost range and a few things worth considering before you start.

Doors and windows don’t get painted (at least not with wall paint), so their area is subtracted from the total before estimating how much paint you need — otherwise the estimate would run high for any room with more than the average opening or two.

The Formula

Wall area, then paint volume:

$$\vG{\text{Gross Wall Area}} = \vF{\text{Perimeter}} \times \vC{\text{Wall Height}} = 2 \times (\vA{\text{Length}} + \vB{\text{Width}}) \times \vC{\text{Height}}$$ $$\vH{\text{Net Wall Area}} = \vG{\text{Gross Wall Area}} - (\vD{\text{Doors}} \times {\sim}20 \text{ sq ft}) - (\vE{\text{Windows}} \times {\sim}15 \text{ sq ft})$$ $$\text{Paint Needed} = \frac{\vH{\text{Net Wall Area}} \times \text{Number of Coats}}{\text{Coverage per Gallon}}$$

The coverage figure — about 350 square feet per gallon per coat — is the paint industry’s widely-cited estimate for a smooth interior wall; textured walls or very porous surfaces may need more.

Worked Example

A 13 ft × 11.5 ft room (about 4 m × 3.5 m) with 7.9 ft walls, 1 door, 1 window, and 2 coats:

1. Perimeter: $2 \times (\vA{13} + \vB{11.5}) = \vF{49} \text{ ft}$.
2. Gross wall area: $\vF{49} \times \vC{7.9} \approx \vG{387} \text{ sq ft}$.
3. Net wall area, after subtracting the door and window: $\vG{387} - 20 - 15 = \vH{352} \text{ sq ft}$.
4. Paint needed: $\vH{352} \times 2 \div 350 \approx 2 \text{ gallons}$ — rounded up to whole gallons for what to actually buy, that’s 3 gallons (paint is sold in whole cans, and running short mid-coat is worse than a little extra left over).

Key Factors to Consider

• A dramatic color change (light to dark, or the reverse) often needs a dedicated primer coat first, on top of the paint itself. Priming a wall before a big color shift or over a stained/ patched surface generally produces more even coverage with fewer paint coats than trying to cover the difference with paint alone — this calculator’s coat count refers to actual paint coats, not a separate primer layer.
• Paint finish (sheen) affects both durability and how forgiving the surface is of imperfections. Flat/matte finishes hide wall imperfections well but wipe clean less easily; satin and semi-gloss are more washable and durable but show more surface flaws — the right finish depends on the room (a high-traffic hallway or kids’ room often favors more washable finishes than a rarely-touched ceiling).
• Textured, very porous, or previously unpainted (like new drywall) surfaces typically need more paint than the standard coverage estimate assumes. The 350 sq ft/gallon figure used here is calibrated for a smooth, previously-painted wall — a rough or highly absorbent surface can need noticeably more paint per coat to reach full coverage.
• Buying a little extra paint for future touch-ups is usually worth the small added cost. Paint colors can be difficult to precisely rematch later (especially if the exact product or batch is discontinued), so keeping a small amount of leftover paint on hand for future scuffs or repairs is a common practical recommendation beyond just covering the initial job.

Common Mistakes

• Forgetting to subtract the ceiling, or forgetting the ceiling needs its own paint entirely. This calculator estimates wall paint only — a ceiling is a separate surface, usually a different paint (flat ceiling paint resists the “flashing” that glossier wall paint can show overhead), and needs its own coverage calculation from its floor-area footprint.
• Rounding down to the nearest gallon instead of up. Running out mid-coat partway across a wall is a much bigger problem than having a partial gallon left over — always round a paint estimate up to the next whole can, never down.
• Assuming one coat is enough for a big color change. Going from a dark wall to a light one (or the reverse) almost always needs either two full topcoats or a tinted primer first — a single coat over a strongly contrasting color typically dries patchy or see-through.
• Skipping the doors-and-windows subtraction on an oddly-shaped or opening-heavy room. The standard ~20 sq ft door and ~15 sq ft window estimates used here work well for typical rooms, but a room with unusually large openings (a big picture window, French doors) will overestimate paint needs unless those larger openings are accounted for separately.

Useful to Know

• Painting drywall that’s brand new or freshly patched? Drywall Joint Compound Calculator estimates the joint compound needed to get walls smooth before paint ever goes on.
• Doing the whole room, not just the walls? Flooring Calculator and Baseboard / Trim Calculator estimate the other two surfaces most paint jobs touch.
• Considering wallpaper instead of paint for an accent wall? Wallpaper Calculator estimates rolls needed the same way this calculator estimates gallons.