Measurement & project guide
Buy each paint system and color separately
Measure walls, openings and ceilings, then apply coats and the exact product's coverage. Primer and finish paint are separate products; leftover paint from one color cannot satisfy another color's order.
- A Room perimeter
- B Wall height
- C Separate ceiling
Before you calculate
- Surface areas
- Measure each room or individual surface. Deduct only areas that will not be painted, using the same dimensions as the wall estimate.
- Coats and coverage
- Use the planned coat count and coverage from the exact product. Texture, porosity and application method can reduce practical coverage.
- Containers and colors
- Select real can sizes. Round each color, ceiling product and primer separately rather than pooling all liquid volume.
Read the result before using it
- Finish volume
- Painted area × finish coats ÷ coverage, including the explicit allowance.
- Whole cans
- A purchase count for each selected color and container size. This is why pooled gallons can understate the shopping list.
- Primer quantity
- A separate calculation with its own coats and coverage; enabling it does not remove the finish coats.
Check before ordering or handing over
- Check preparation, repairs, dryness and the specified primer on new drywall.
- Record sheen, tint, batch and container size for each product.
- Follow label instructions for ventilation, drying time and recoating.
Detailed results and planning tools
View calculations, report and comparisons
Calculation breakdown
These values reflect the editable inputs and assumptions above.
| Quantity | Value |
|---|
Calculation report
Includes formulas, entered assumptions, limitations and sources.
Compare scenarios
Keep up to three alternatives to compare measured values and assumptions.
Measure the surfaces you will actually paint#
Use room mode for four rectangular walls, or surface mode for a single wall, door or other rectangular surface. Deduct openings only when their surfaces will not receive the same product. Select ceilings explicitly; they have a separate paint purchase.
Calculate coats and coverage explicitly#
Room wall area = 2 × (length + width) × height − unpainted openings. Multiply net area by the selected coat count and application allowance, then divide by coverage per coat. Coverage is entered in ft² per US gallon or m² per liter.
The starting 350 ft²/US gal, two coats and 10% allowance are editable examples. Read the chosen product’s instructions and account for your surface and application method. A coverage override can be entered for each finish product; blank overrides inherit the main finish coverage.
Worked example: a room and separate ceiling#
A 12 × 10 ft room with 8 ft walls has 352 ft² of wall area. Excluding a 21 ft² door leaves 331 ft². At two coats, 350 ft²/US gal and 10% allowance, wall finish paint is 331 × 2 × 1.10 ÷ 350 = 2.081 US gal, rounded to three 1-gallon containers.
The 120 ft² ceiling at the same coverage and coat count needs 0.754 US gal, rounded separately to one 1-gallon container. These products cannot share a container merely because their combined liquid quantity fits another package arrangement.
Keep primer and colors separate#
Three wall or surface groups allow separate colors or products. Areas in one group must use the same coverage, coat count and container size. Primer has its own coverage and coats and applies to all included surfaces when enabled. Run separate scenarios when different surfaces require different primers or finish coat counts.
Enter a price for every used product to obtain a complete purchase cost. Price is per whole container; delivery/fixed cost and entered tax are then applied. Currency changes the label, not the exchange rate.
Sources and surface preparation#
Benjamin Moore’s calculator, BEHR’s product specifications and USG’s new-wallboard guidance provide manufacturer context. Follow the coating system’s preparation and primer instructions. For panel quantity before painting, use the Drywall Calculator.
New drywall: primer is a separate purchase#
New board, joint compound and repaired areas can absorb coatings differently. Follow the selected coating system's preparation and priming instructions rather than assuming an extra finish coat replaces primer. Let the surface reach the required condition before estimating from the label coverage.
For an illustrative 451 ft² of combined surfaces, one primer coat at 300 ft² per US gallon and 10% extra needs 451 × 1.10 ÷ 300 = 1.654 gallons, or two whole 1 gallon containers. Two finish coats at 350 ft² per gallon need 2.835 gallons, purchased separately. Different finish colors and ceiling paint still round separately; surplus in one product is not a replacement for another.
Measure and price each paint group, record the actual product and sheen, and confirm the manufacturer's recoat conditions. The estimate covers the entered surfaces and coats; repairs, sanding, masking and labor are separate scope items.
Frequently asked questions
How much paint does a rectangular room need?
Measure wall area as twice the sum of length and width, multiplied by height, then subtract unpainted openings. Multiply by coats, apply the chosen allowance, and divide by the selected product coverage.
Are ceilings and primer included?
Only when selected. Ceiling paint is a separate product with the finish coat count. Primer is separately calculated across all included painted surfaces.
Why should colors be calculated separately?
Each product or color needs its own whole-container purchase. Combining two small color requirements into one total can understate what you must buy.
What coverage should I enter?
Use the selected product label and surface conditions. The starting 350 ft² per US gallon is an editable estimating example, not guaranteed coverage.
Does paint and primer in one eliminate separate primer?
Not automatically. Follow the product instructions for the substrate and condition. Enable a separate primer estimate when your coating system requires it.