Paint Calculator

A paint project may begin with color, but the purchase quantity comes from wall area. The Paint Calculator measures a rectangular room’s wall perimeter, subtracts doors and windows, applies the number of coats, and divides the coated area by the paint’s coverage rate. An optional waste allowance helps account for touch-ups and application loss.

What the Paint Calculator Estimates

The calculator returns the estimated paint needed in gallons and liters, together with the measurements used to produce that result:

• gross wall area before subtracting openings
• combined door and window area
• paintable wall area for one coat
• total coated area for all coats
• paint volume after the waste allowance
• alternative estimates for 1-gallon cans and 5-gallon pails

This is a wall-paint estimate. Ceilings, trim, baseboards, doors, cabinets, and accent areas are not automatically included.

Paint Calculation Formula

For a rectangular room, begin with the wall perimeter:

Room perimeter = 2 × (Length + Width)

Multiply the perimeter by the wall height:

Gross wall area = Room perimeter × Wall height

Subtract the openings:

Paintable area = Gross wall area − Door area − Window area

Then account for coats, waste, and product coverage:

Paint needed = Paintable area × Coats × (1 + Waste% / 100) / Coverage

The coverage unit must match the area system. Use ft²/gal with measurements calculated in square feet or m²/L with measurements calculated in square meters.

How to Use the Calculator

1. Measure the room length, width, and wall height, then select feet or meters.
2. Enter the number of coats recommended for the paint and project.
3. Enter the number of doors and their average area.
4. Enter the number of windows and their average area.
5. Use the coverage rate printed on the paint label or technical data sheet.
6. Add a reasonable waste allowance for application loss and future touch-ups.
7. Click Calculate and check the calculation details before buying.

If doors or windows have different sizes, calculate their areas separately and use an accurate average, or divide the project into smaller wall sections.

Worked Example: Paint for a Room

Suppose a room is 12 ft long, 10 ft wide, and 8 ft high. It has one 20 ft² door and two 15 ft² windows. The walls need two coats, the paint covers 350 ft²/gal, and the waste allowance is 10%.

Calculate the room perimeter and gross wall area:

Perimeter = 2 × (12 + 10) = 44 ft

Gross wall area = 44 × 8 = 352 ft²

Subtract the openings:

Paintable area = 352 − 20 − (2 × 15) = 302 ft²

Apply two coats and the waste allowance:

Total adjusted area = 302 × 2 × 1.10 = 664.4 ft²

Divide by the coverage rate:

Paint needed = 664.4 / 350 = 1.90 gallons

The project needs approximately 1.9 gallons, or about 7.2 liters. If buying only 1-gallon cans, round up to two cans.

Choosing an Accurate Coverage Rate

A gallon of interior paint commonly covers about 350 to 400 square feet, but a generic value is only a planning estimate. Product coverage can differ by paint line, sheen, application method, surface texture, porosity, and color change. Use the coverage printed on the exact product whenever possible.

Fresh drywall, masonry, textured walls, repairs, and bare wood may absorb more coating. Primer can improve uniformity, but primer should be estimated separately because its coverage and purpose may differ from the finish paint.

Coats, Openings, and Waste

The number of coats multiplies the paintable area. Two coats do not mean doubling the room dimensions; they mean covering the same net wall area twice. Strong color changes, low-hide colors, or porous surfaces may require additional coats even when a product is marketed for one-coat coverage under specific conditions.

Subtract only openings that will not receive the wall paint. If a door will be painted with the same product, do not deduct it from the project—or estimate it separately using its actual surface area. A small waste percentage can cover roller and brush retention, transfer loss, and touch-ups, but it cannot correct inaccurate room measurements.

Measurement Tips

• Measure wall height at more than one point in rooms with sloped or uneven ceilings.
• For irregular rooms, calculate each wall as width × height and add the results.
• Measure unusually large doors, windows, or built-in openings individually.
• Calculate ceilings separately as length × width.
• Keep some labeled paint for future touch-ups, especially with custom-tinted colors.

Assumptions and Limitations

• The room is treated as rectangular with a consistent wall height.
• Door and window deductions use the entered counts and average areas.
• The calculator assumes one coverage rate for every included wall.
• It does not estimate primer, ceilings, trim, doors, cabinets, exterior siding, labor, price, drying time, or application equipment.
• Real coverage can be lower on rough, porous, damaged, or previously unpainted surfaces.
• Container estimates are purchase-planning alternatives; available sizes vary by product and location.

Common Mistakes to Avoid

• Using floor area instead of total wall area
• Forgetting to multiply by the number of coats
• Mixing feet with square meters or meters with ft²/gal coverage
• Using a generic coverage rate when the product label provides a different value
• Subtracting doors or windows that will also be painted
• Adding the suggested 1-gallon cans and 5-gallon pails together

Paint Calculator FAQs

How do I calculate how much paint I need?

Calculate the total wall area, subtract unpainted doors and windows, multiply by the number of coats and waste factor, then divide by the paint’s stated coverage.

How much area does one gallon of paint cover?

A gallon commonly covers about 350 to 400 square feet, but the exact rate depends on the product and surface. Always check the label or data sheet.

Should I subtract doors and windows?

Subtract openings only when they will not receive the same wall paint. Large openings make a meaningful difference; very small deductions may be absorbed by the waste allowance.

Does two coats require twice as much paint?

In the calculator’s area model, yes: two coats double the coated area. Actual use can vary because the first and second coats may spread differently.

Does the estimate include the ceiling and trim?

No. The calculator estimates room walls only. Calculate ceilings, trim, doors, and cabinets separately using the appropriate product coverage.

Should I round paint quantities up?

Yes. Round up to available container sizes so the project is not interrupted. Keep package combinations practical and avoid treating alternative can-size results as cumulative.

Related Calculators

Continue a room-material estimate with the Drywall Calculator, Flooring Calculator, Tile Calculator, or Concrete Calculator.

References

• Sherwin-Williams: Paint Calculator FAQs and Coverage Guidance
• Sherwin-Williams: How Much Paint to Buy
• Sherwin-Williams: Painting Frequently Asked Questions