Paint Coverage Calculator
Paint Coverage Calculator
Walking into a hardware store to buy paint often ends in either buying too little and making a second trip, or buying too much and wasting money on cans you never open. A paint coverage calculator solves that by turning the dimensions of your room into a precise estimate of how many gallons you actually need.
Paint is sold by the gallon, but rooms are measured in feet, so the bridge between them is the coverage rate, the number of square feet one gallon covers in a single coat. Most interior paints cover about 350 to 400 square feet per gallon, but that number drops for porous surfaces, dark colors over light, and rough textures. [ppg-coverage]
The calculator multiplies your wall area by the number of coats you plan to apply, then divides by the coverage rate to get gallons. It also converts that into whole cans so you know exactly what to put in your cart. This prevents the common mistake of buying one can per wall without accounting for total area.
Knowing your paint need matters for budgeting a renovation. Paint is a meaningful line item in any room makeover, and overestimating ties up cash in unused inventory while underestimating causes frustrating mid-project store runs that may even mix slightly different batches.
The tool is useful for homeowners, renters refreshing a space, and professional painters quoting jobs. For a single accent wall or a whole house, scaling the same formula keeps estimates consistent and credible.
It also helps with sustainability. Buying only what you need reduces leftover paint that often goes to waste, and planning coats correctly means fewer applications and less material overall.
Leftover paint also creates a disposal problem, because most municipalities treat unused latex paint as household hazardous waste that cannot simply go in the trash. Estimating accurately keeps the number of half-empty cans to a minimum, which is better for your storage shelf and for the environment.
A good planning habit is to write the room name and the exact color on the lid of any leftover can, then store it upright in a temperature-stable place. The small reserve you keep from buying whole cans becomes invaluable for touch-ups after scuffs, so the extra quantity the calculator recommends is rarely wasted.
Start with the room's length and width in feet, measured wall to wall at the floor. Then enter the ceiling height, usually 8 feet in modern homes but often 9 or 10 in older or custom builds. The calculator uses these to find the total wall area around the room's perimeter.
Enter the number of coats you intend to apply. Two coats is standard for good coverage, especially when changing colors or painting fresh drywall. A single coat may suffice for a touch-up that matches the existing color closely.
Enter the paint's coverage rate in square feet per gallon. This is printed on the can; 350 is a safe default for interior latex, while primers and some finishes cover differently. If you are unsure, use 350 and adjust after checking your specific product.
Keep in mind that the first coat on bare drywall soaks in far more than the second, because the porous surface drinks the initial layer. If you are painting new walls, treat the primer as its own coat with its own lower coverage rate rather than assuming the finish paint's number applies to the bare surface.
If you plan to use a paint-and-primer combination product, the manufacturer usually quotes a single coverage number that already assumes the sealing coat is built in. In that case a two-coat job on fresh drywall may genuinely need only the quoted amount, whereas separate primer and paint would require estimating the two products independently.
Optionally add the can size in gallons so the tool can tell you how many cans to buy. Leave it blank to get gallons only. Press Calculate to see total wall area for one coat, total area including all coats, paint required in gallons, and the number of cans to purchase.
For an example, a 12 ft by 10 ft room with 8 ft ceilings, two coats, 350 sq ft per gallon coverage, and 1-gallon cans: wall area is 2 × (12 + 10) × 8 = 352 sq ft. With two coats that is 704 sq ft, needing 704 / 350 = 2.01 gallons, so buy 3 one-gallon cans.
Edge cases work cleanly. A very small closet of 4 × 4 × 8 still computes a small positive area. Setting coats to 1 returns the single-coat figure. Leaving the can size blank still gives a correct gallon total so you are never blocked from an answer.
The calculator finds the perimeter wall area, scales it by coats, and divides by the coverage rate. [benjamin-moore-coverage]
Perimeter area is used instead of floor area because paint goes on the vertical surfaces you see, not the ground. A long narrow room and a square room with the same floor space can have very different wall areas, which is why the length, width, and height all matter and why a single "per room" estimate so often misses the mark.
Define:
- L = room length (ft)
- W = room width (ft)
- H = ceiling height (ft)
- n = number of coats
- c = coverage (sq ft per gallon)
- g = gallons per can
Wall area for one coat:
Total area including all coats:
Gallons of paint required:
Cans to buy, rounding up:
When can size is omitted, N defaults to the rounded-up gallon count.
Example Calculation
Take L = 12, W = 10, H = 8, n = 2, c = 350, g = 1.
Wall area:
Total with coats:
Gallons:
Cans:
Paint Needed by Room Size (8 ft ceilings, 2 coats, 350 sq ft/gal)
| Room (L × W ft) | Wall Area (sq ft) | Gallons Needed |
|---|---|---|
| 10 × 10 | 320 | 1.83 |
| 12 × 10 | 352 | 2.01 |
| 14 × 12 | 416 | 2.38 |
| 16 × 14 | 480 | 2.74 |
| 20 × 16 | 576 | 3.29 |
Effect of Coverage Rate (12 × 10 room, 2 coats, 8 ft ceilings)
| Coverage (sq ft/gal) | Gallons Needed |
|---|---|
| 250 | 2.82 |
| 300 | 2.35 |
| 350 | 2.01 |
| 400 | 1.76 |
Lower coverage rates, typical of primers and deep colors, raise the gallons needed by more than half compared with a high-coverage finish.
Always buy a little extra for touch-ups. Keep at least a quart of the same batch so future repairs match, since different production lots can vary slightly in shade.
Account for doors and windows only if they are large. The wall-area formula ignores them, but a room with a big picture window may need slightly less, while a wall of solid surface needs the full amount.
Prime bare or dramatic-color changes. Primer has its own coverage rate, often lower than paint, so calculate it as a separate coat when switching from dark to light or painting new drywall.
Use the can's stated coverage, not a generic guess. Manufacturers print the real number for that product, and trusting it improves accuracy over a default.
Round up to whole cans. Paint is rarely sold in fractions at retail, and having a spare can avoids a mid-job run that might mix a mismatched batch.
Stir and mix all cans together when using multiple. Boxing the paint in a bucket evens out slight batch differences so the whole room looks uniform.
Consider the finish when judging how much you need. Flat and matte paints tend to cover imperfections in one or two coats, while high-gloss and deep saturated colors often demand an extra coat to look even, so build that extra coat into your estimate for those finishes.
- Walls Only: The formula covers perimeter walls and excludes the ceiling and any interior partition walls, which add area if you are painting them too.
- No Opening Deduction: Doors and windows are not subtracted, so estimates run slightly high for very open rooms, which is a safe bias but not exact.
- Flat Rate Assumption: Coverage is held constant across all walls. Textured or porous surfaces can cut real coverage well below the can's claim.
- Single Coverage Value: One coverage rate is used for every coat. Primer coats and finish coats often differ, so treat mixed jobs as approximate.
- No Waste Factor: The estimate is mathematical and does not add a spillage or roller-loss margin, though rounding up to whole cans usually covers it.
- Ceiling Not Included: If you are painting the ceiling the same color, add the floor area (length times width) as a separate surface, since the wall formula deliberately ignores it to avoid double counting.
- Trim Not Counted: Baseboards, crown molding, and window frames are excluded. These are usually painted with a separate trim product, so estimate them with their own smaller quantity rather than folding them into the wall total.
- Why does the calculator not subtract doors and windows?
- It uses a simple perimeter wall area for a safe overestimate. Most rooms have enough trim and touch-up that the small opening deduction is minor, and rounding up to whole cans covers it.
- How many coats do I really need?
- Two coats is standard for even color and good hide, especially over a different color or bare drywall. One coat may work only for a close-match refresh.
- What coverage rate should I use?
- Check the paint can. Interior latex is commonly 350 to 400 sq ft per gallon; primer and deep-base colors often cover closer to 250.
- Should I include the ceiling?
- Not in this wall calculator. Add ceiling area separately as length times width, then divide by coverage for that surface.
- Why buy whole cans instead of exact gallons?
- Retail paint is sold by the can, and keeping a matching spare helps future touch-ups. The calculator rounds up so you leave with a workable quantity.
- Does texture change how much paint I need?
- Yes. Textured and porous walls soak up more paint, lowering real coverage below the can's flat-surface claim, so budget a bit extra for those.
- How do I paint a room with an interior wall?
- Measure that wall's two sides and add them to the perimeter area before applying the formula, since both faces get painted.
- Can I use this for exterior painting?
- The math is the same, but exterior coverage rates and surface porosity differ. Use the exterior product's coverage number for an accurate estimate.
- [1]PPG Paints. (n.d.). Paint calculator and coverage guide.
- [2]Benjamin Moore. (n.d.). Paint calculator and coverage estimator.
- [3]Behr Paint. (n.d.). Paint coverage calculator.
- [4]U.S. Environmental Protection Agency (EPA). (n.d.). Healthy painting and paint waste guidance.
- [5]HGTV. (n.d.). How to estimate paint needed for a room.
Last updated: July 20, 2026
UnByte — Independent Software Engineering
Every calculator references authoritative sources — Editorial policy