Paint Calculator
Estimate wall paint gallons, paintable area, and material cost from room dimensions, standard openings, coats, and can coverage so you can plan an interior room repaint before buying paint.
Paint Inputs
Paint Estimate
Paint needed
2 gallons
Rounded from 1.971 gallons. Buy 2 gallons.
Based on 690 sq ft across 2 coats.
Paintable area
345 sq ft
Total wall area
396 sq ft
Total cost
$103.50
Buy as
2 gallons
Current Calculation
Wall area = 2 x (length + width) x height
2 x (12 + 10) x 9 = 396 sq ft
Paintable area = wall area - door area - window area
396 - 1 x 21 - 2 x 15 = 345 sq ft
Paint needed = (paintable area x coats) / coverage
(345 x 2) / 350 = 1.971 raw gal, 2 rounded gal
Buying Breakdown
Total cost = rounded paint cost + 15% supplies allowance
2 x $45.00 + $13.50 = $103.50
Standard interior wall estimate. Buying all finish paint in one batch helps color consistency across coats and touch-ups.
Use Scenarios
Bedroom or living-room repaint
Use the page before a weekend repaint to estimate wall-only gallons from room size, standard openings, and the number of finish coats you expect to apply.
Quick store-budget check
Enter the can coverage and per-gallon price from the brand you plan to buy so the result reflects both gallons and a rough materials-only budget.
Metric floor-plan measurements
If the room dimensions come from a metric floor plan, switch the room inputs to meters and the calculator will convert them internally before applying coverage per gallon.
Formula Explanation
1) Wall area
Wall area = 2 x (length + width) x height
For a rectangular room, the calculator adds the two long walls and two short walls, then multiplies by wall height. Meter inputs are converted internally before the coverage math is applied.
2) Opening deductions
Paintable area = wall area - doors - windows
To keep the estimate fast, the page deducts 21 square feet per standard interior door and 15 square feet per standard window unless you replace those assumptions with manual planning outside the calculator.
3) Coats and coverage
Paint needed = (paintable area x coats) / coverage
Coverage is the square-foot coverage printed on the paint can for one coat. More coats, lower-hide paint, or rougher walls all increase the gallons required.
4) Buying and cost
Rounded paint x price + 15% supplies
The raw gallon result is rounded up to the next quarter gallon so it can be expressed as gallons plus quarts. A simple 15% supplies allowance is then added for trays, rollers, tape, and cleanup items.
How to Read the Result
Paint needed
The main gallon figure is rounded up, not down, so the buying plan matches real-world gallon and quart purchases more closely than an exact decimal would.
Wall area versus paintable area
Wall area is the full wall surface before deductions. Paintable area removes the standard door and window allowances so you can see how much wall is actually left to coat.
Cost and buffer
The total cost is a materials planning number, not a labor bid. If the walls are textured, stained, or changing color sharply, add your own extra buffer before buying the final quantity.
Example Cases
Case 1: Standard bedroom refresh
Inputs
- Room size: 12 x 10 x 9 ft
- Openings: 1 door(s), 2 window(s)
- Coats: 2
- Coverage and price: 350 sq ft/gal at $45/gal
Computed Results
- Paint needed: 2 gal
- Buy as: 2 gallons
- Paintable area: 345 sq ft (32.05 m2)
- Total cost: $103.50
Interpretation
This is the classic two-coat bedroom repaint: moderate wall area, ordinary openings, and a mid-range coverage assumption.
Decision Hint
A two-gallon purchase is enough for the wall estimate here, but add a little extra if the walls are heavily patched or changing from dark to light.
Case 2: Larger living room with more openings
Inputs
- Room size: 18 x 14 x 9 ft
- Openings: 2 door(s), 4 window(s)
- Coats: 2
- Coverage and price: 400 sq ft/gal at $52/gal
Computed Results
- Paint needed: 2.5 gal
- Buy as: 2 gallons + 2 quarts
- Paintable area: 474 sq ft (44.04 m2)
- Total cost: $149.50
Interpretation
Higher coverage helps, but the project still needs more than two gallons because the room perimeter is much larger than the bedroom example.
Decision Hint
Buy the full batch together and recheck any trim, ceiling, or accent-wall paint separately before final checkout.
Case 3: Metric guest room plan
Inputs
- Room size: 4.2 x 3.6 x 2.6 m
- Openings: 1 door(s), 1 window(s)
- Coats: 2
- Coverage and price: 350 sq ft/gal at $47/gal
Computed Results
- Paint needed: 2.5 gal
- Buy as: 2 gallons + 2 quarts
- Paintable area: 400.58 sq ft (37.22 m2)
- Total cost: $135.13
Interpretation
This example shows the metric input workflow: room dimensions are entered in meters, but the wall estimate still resolves into gallons using the can coverage assumption.
Decision Hint
If the floor plan is metric, double-check that the can label and price you enter still match the gallon-based product you plan to buy.
Boundary Conditions
Sources & References
- Benjamin Moore - Paint CalculatorUsed for manufacturer-backed wall paint planning context, including room-based input workflow and the reminder that wall, opening, and coat assumptions drive the final gallon estimate.
- Sherwin-Williams - How to Use Paint CalculatorUsed for coverage guidance around roughly 350 to 400 square feet per gallon and for the boundary note that ceilings, trim, and other surfaces often need separate measurement.
- Glidden - Paint CalculatorKept as a supplementary manufacturer reference for smooth-wall coverage expectations and for the practical note that porous surfaces or major color changes can require more than one finish coat.
- Glidden - How Many Coats Of Primer Do I Need?Used for primer decision context, especially when repainting bare drywall, unfinished surfaces, or strong existing colors where the finish-coat estimate alone may not be enough.