This free paint calculator estimates the amount of paint required for a rectangular room. It works in Imperial and Metric units, subtracts standard doors and windows, supports multiple coats, can include the ceiling and lets you replace the default coverage with the figure printed on your chosen paint.
Measure the walls, subtract openings, multiply the paintable area by the number of coats and divide by the paint coverage. HomeCalc does that calculation automatically and can also add an optional allowance and cost estimate.
FREE PAINT ESTIMATOR
Paint Calculator
Estimate wall and ceiling paint from your room dimensions, openings, number of coats and the coverage printed on your paint can.
Estimated paint needed
Recommended purchase: 2 gallons
This is an estimate. Actual coverage varies with paint, color change, surface porosity, texture and application method. Always check the coverage stated by the manufacturer.
How to use the paint calculator
- Choose Imperial for feet and gallons or Metric for meters and liters.
- Enter the room length, width and wall height.
- Add the number of standard doors and windows.
- Use the extra non-paintable-area field for unusually large openings or built-ins.
- Select how many coats you plan to apply.
- Check the paint can or technical sheet and enter its actual coverage.
- Enable the ceiling only if you want it included in the same estimate.
- Adjust the extra allowance. Set it to 0% if you want the formula-only requirement.
- Optionally enter a price per gallon or liter for a rough material-cost estimate.
How the paint calculation works
For a rectangular room, HomeCalc first calculates the total wall surface from the room perimeter and wall height.
2 × (room length + room width) × wall height
Next, the calculator subtracts the estimated area occupied by doors, windows and any additional opening you enter. If you include the ceiling, it adds length × width. The remaining area is multiplied by the number of coats and by your optional extra allowance.
(paintable area × coats × allowance factor) ÷ paint coverage
Sherwin-Williams gives the same basic planning method: measure the wall area, subtract large windows and doors, then use the coverage shown on the selected paint label. Its current consumer guidance specifically recommends checking the product label because coverage depends on the paint you choose. See Sherwin-Williams guidance.
How many square feet does a gallon of paint cover?
There is no single coverage number that is correct for every paint. Sherwin-Williams training material says an average gallon covers roughly 350–400 sq ft, while BEHR states that one gallon of several of its interior products covers approximately 250–400 sq ft with one coat. HomeCalc therefore uses 350 sq ft per gallon as an editable starting point rather than treating it as a universal rule.
| Coverage assumption | 1 gallon, 1 coat | 5 gallons, 1 coat |
|---|---|---|
| 350 sq ft/gal | 350 sq ft | 1,750 sq ft |
| 400 sq ft/gal | 400 sq ft | 2,000 sq ft |
Smooth, sealed surfaces can behave differently from porous or textured ones, and a strong color change may require additional coats or primer. Use the manufacturer's number whenever it is available.
Should you subtract doors and windows?
Yes, if they will not be painted. For convenience, HomeCalc deducts 20 sq ft per standard door and 15 sq ft per standard window. Those figures come from a Benjamin Moore planning guide that uses the same deductions in its worked room example.
They are only shortcuts. A sliding glass door, picture window or unusually large opening can be much bigger. For those, measure the actual opening and use the calculator's extra non-paintable-area field.
How many coats of paint should you calculate?
Benjamin Moore's current paint calculator explicitly describes its result as a rough estimate and assumes two coats. Two coats are therefore a useful planning starting point, but the correct number depends on the product, substrate, existing finish, new color and whether primer is needed. See Benjamin Moore's calculator.
HomeCalc lets you select one to four coats so the estimate reflects your project instead of forcing a fixed assumption.
How to calculate paint for a ceiling
For a flat rectangular ceiling, the area is simply room length × room width. Enable Include the ceiling to add that area to the calculation.
If your ceiling uses a different product with a different coverage rate, it is more accurate to calculate it separately using that paint's own coverage figure rather than combining wall and ceiling paint into a single purchase estimate.
Paint calculator examples for common room sizes
The examples below assume an 8 ft wall height, one 20 sq ft door, two 15 sq ft windows, two coats and 350 sq ft of coverage per gallon. They show the formula-only requirement before any extra allowance.
| Room size | Paintable wall area | Area for 2 coats | Theoretical paint needed |
|---|---|---|---|
| 10 × 10 ft | 270 sq ft | 540 sq ft | 1.54 gal |
| 10 × 12 ft | 302 sq ft | 604 sq ft | 1.73 gal |
| 12 × 12 ft | 334 sq ft | 668 sq ft | 1.91 gal |
| 12 × 14 ft | 366 sq ft | 732 sq ft | 2.09 gal |
| 14 × 16 ft | 430 sq ft | 860 sq ft | 2.46 gal |
For example, a 12 × 10 ft room with 8 ft walls has 352 sq ft of gross wall area. Deducting one standard door and two standard windows leaves 302 sq ft. Two coats require coverage of 604 sq ft, which is approximately 1.73 gallons at 350 sq ft per gallon. With HomeCalc's default 10% extra allowance enabled, the estimate becomes approximately 1.90 gallons.
Why HomeCalc lets you add an extra allowance
The allowance is optional and is not presented as an industry standard. It is simply a planning buffer for touch-ups, small measuring differences, texture and application loss. You can select 0%, 5%, 10%, 15% or 20%. If you want to compare the tool directly with a manufacturer's stated coverage, choose 0%.
Before buying paint, check these details
- Product coverage: replace the default with the figure on the paint can or technical sheet.
- Surface: new drywall, masonry, texture and porous surfaces can change real-world coverage.
- Primer: primer is not included in the HomeCalc paint total and should be estimated separately.
- Color change: moving between very different colors can affect the number of coats required.
- Ceiling paint: calculate separately when its coverage differs from the wall paint.
- Can sizes: buy a practical combination of the sizes actually sold for your selected product.
- Manufacturer instructions: they take priority over any generic calculator assumption.
Paint calculator FAQ
How much paint do I need for a room?
How many square feet does one gallon of paint cover?
How much paint do I need for a 12 × 12 room?
Does the calculator include two coats?
Does the paint calculator include the ceiling?
Does HomeCalc calculate primer?
Can I use meters and liters instead of feet and gallons?
Is the cost estimate exact?
Sources and methodology
HomeCalc uses simple geometry and editable product assumptions. The default figures and explanatory guidance on this page were checked against paint-manufacturer resources rather than treated as universal specifications.
- Sherwin-Williams — How Much Paint to Buy : measure wall area, subtract large openings and check the selected paint label for coverage.
- Sherwin-Williams Painter Skills Training : describes average coverage of roughly 350–400 sq ft per gallon.
- BEHR — Determine How Much Interior Paint to Buy : lists approximately 250–400 sq ft per gallon with one coat for cited interior products.
- Benjamin Moore Paint Calculator : current calculator describes its result as an estimate and assumes two coats.