Paint Calculator by Square Footage
Start with paintable wall area, not room floor area. Use the area calculator below, then work out gallons from your product's coverage, coat count and available container sizes.
Last updated:This is an area-only tool for paint planning. Enter wall length as Length and wall height as Width on the Rectangle tab. Add non-overlapping rectangular walls with Irregular. It does not deduct openings or calculate gallons automatically. Use unadjusted Total Area before any optional flooring-style waste factor, subtract excluded surfaces manually, then use the formulas below. Do not enter paint coverage in the box field and treat its box count as a paint order.
Tip: If you only have the diameter, divide by 2 to get the radius.
L-shape = two rectangles sharing a corner.
Add one row per rectangle and we'll sum them.
Measure the Surfaces Receiving Each Paint
- For equal-height rectangular walls, calculate perimeter × height. Otherwise measure and calculate each wall separately.
- Subtract the measured openings, trim or other surfaces excluded from this coating; do not use a universal door or window size.
- Calculate ceilings and trim separately. Group areas only when product, color, sheen and coat requirements are compatible.
- Use
US gallons = net sq ft × coats ÷ coverage per gallon per coat. Keep full precision until choosing containers.
For a 12 ft × 15 ft room with four 8-ft-high walls: 2 × (12 + 15) × 8 = 432 sq ft. If one excluded door is 3 ft × 7 ft and two excluded windows are each 3 ft × 5 ft, the net wall area is 432 − 21 − 30 = 381 sq ft. Ceiling and trim paint are not included.
The wall measurement guide handles unequal heights and openings that overlap only part of a finish section. The Sherwin-Williams Painter Skills Program, pages 31–36, explains wall measurements and separate surface calculations. Its sample coverage is not a guarantee for another paint or surface.
Coverage Sensitivity: The Same Wall Can Need Different Quantities
For 381 sq ft and two coats, these are hypothetical coverage inputs, not product-category ratings. Use your actual label and surface/application conditions.
| Assumed coverage per US gallon per coat | Required volume, rounded for display | If buying only one-gallon cans |
|---|---|---|
| 300 sq ft | 762 ÷ 300 = 2.54 gal | 3 cans |
| 350 sq ft | 762 ÷ 350 ≈ 2.177 gal | 3 cans |
| 400 sq ft | 762 ÷ 400 = 1.905 gal | 2 cans |
At the 350 example rate, two gallons are not enough. Two gallons plus one US quart provide 2.25 gallons, leaving 2.25 − 762 ÷ 350 ≈ 0.073 gallon after the theoretical application—not an unused full quart for touch-ups. Three one-gallon cans provide more reserve but are not necessarily the cheapest available combination.
Two Colors: Round Each Product's Containers Separately
Split the same 381 sq ft into 200 sq ft of color A and 181 sq ft of color B. Assume each needs two coats at 350 sq ft per US gallon per coat, there is no usable existing stock, and the colors cannot substitute for one another.
| Product group | Theoretical gallons | Whole-gallon-only purchase |
|---|---|---|
| Color A | 200 × 2 ÷ 350 = 400 ÷ 350 ≈ 1.143 | 2 gallons |
| Color B | 181 × 2 ÷ 350 = 362 ÷ 350 ≈ 1.034 | 2 gallons |
The order is 4 gallons if each product is available only in one-gallon cans. Pooling both colors gives ceil(762 ÷ 350) = 3, but three one-gallon cans cannot supply more than one gallon of both colors. Excess color A cannot cover a shortage of color B.
If both products are sold in one-gallon and one-quart cans, an alternative is one gallon plus one quart of each color: 1.25 gallons per color, 2.5 gallons total. This satisfies the assumed application volumes before an extra reserve. Compare actual container prices; a smaller-volume combination is not automatically cheaper. Check reserve and product compatibility separately for each color.
When using leftover paint, subtract only the usable quantity of the same suitable product, color and sheen from its own group before rounding purchases. Do not pool primer, ceiling paint, trim paint or incompatible stock merely because all are measured in gallons.
Repeat the order with matching paint already on hand
Now suppose you have 0.25 gallon of usable matching color A and none of color B. Keep all other example inputs unchanged.
| Product | Need after usable inventory | New purchase using gallons and quarts |
|---|---|---|
| Color A | 400 ÷ 350 − 0.25 ≈ 0.893 gal | 1 gallon |
| Color B | 362 ÷ 350 − 0 ≈ 1.034 gal | 1 gallon + 1 quart = 1.25 gallons |
Buy 2.25 gallons in the stated color allocation. Total paint available is 2.25 + 0.25 = 2.5 gallons; expected combined remainder is 2.5 − 762 ÷ 350 ≈ 0.323 gallon, split between colors. If only one-gallon cans were available, the new order would instead be one can of A plus two of B: 3 gallons. Pooling the net need and rounding it to two gallons would leave color B short. Check any desired touch-up reserve per color rather than relying on the combined remainder.
Primer Needs Its Own Calculation
Whether primer is needed, and how many coats to apply, depends on the specified coating system and surface preparation. For an example requiring one primer coat at an assumed 250 sq ft per gallon, 381 ÷ 250 = 1.524 gallons. Two finish coats at 350 require 762 ÷ 350 ≈ 2.177 gallons. There is no fixed primer-to-finish gallon ratio.
Even for equal coat counts, reducing coverage from 350 to 250 means 28.57% less coverage per gallon but 350 ÷ 250 − 1 = 40% more volume—not 25% more. These numbers illustrate reciprocal rates, not a primer recommendation.
Room Examples with Explicit Opening Assumptions
All examples have four 8-ft-high walls. Each door is assumed 3 ft × 7 ft and each window 3 ft × 5 ft; no other deductions are modeled. Ceiling and trim are excluded. Volumes assume two coats at 350 sq ft per US gallon per coat.
| Room (ft) | Openings | Net walls (sq ft) | Required gallons |
|---|---|---|---|
| 10 × 10 | 1 door, 1 window | 320 − 21 − 15 = 284 | 568 ÷ 350 ≈ 1.623 |
| 12 × 12 | 1 door, 1 window | 384 − 21 − 15 = 348 | 696 ÷ 350 ≈ 1.989 |
| 12 × 15 | 1 door, 2 windows | 432 − 21 − 30 = 381 | 762 ÷ 350 ≈ 2.177 |
Before Buying
- Confirm the product's intended substrate, preparation, coverage, coat count and drying requirements.
- Keep different products and colors separate. Follow product guidance on mixing compatible cans; do not mix unrelated coatings.
- Include a chosen reserve without counting container leftovers twice. Follow storage instructions rather than assuming a fixed shelf life.
- Compare quotes on the same measured surfaces and scope: preparation, primer, finish coats, trim, labor and taxes may be separate.
Frequently Asked Questions
How many gallons of paint for a 10x10 room?
With four 8-ft-high walls, gross wall area is 320 sq ft. Subtract an assumed 3 × 7 ft door and 3 × 5 ft window to get 284 sq ft. At an assumed 350 sq ft per US gallon per coat, two coats need 568 ÷ 350 ≈ 1.623 gallons before any extra reserve. Two one-gallon cans cover that arithmetic estimate.
How many gallons of paint for a 12x15 bedroom?
For four 8-ft-high walls, one 21-sq-ft door and two 15-sq-ft windows, net wall area is 381 sq ft. Two coats at an assumed 350 sq ft per US gallon need 762 ÷ 350 ≈ 2.177 gallons. Two gallons are insufficient. For one product/color, three one-gallon cans or two gallons plus one quart provide enough volume for this example before a separate reserve.
How much paint do I need per square foot?
Divide the number of coats by the product's coverage in square feet per gallon per coat. Multiply that rate by the net surface area. Coverage and coat requirements depend on the selected product and surface; 350 is an example input, not a universal rating.
Do I need to subtract trim?
Subtract measured trim surfaces excluded from the wall finish, regardless of whether the colors match. Measure trim separately when it receives another product or application. Avoid subtracting the same area twice through both opening and trim deductions.
How much extra paint should I keep for touch-ups?
Choose a reserve for your project and account for usable paint left after container rounding. Check the manufacturer's storage and shelf-life instructions; no fixed percentage or storage lifetime suits every paint. Label the product, color, sheen and room.
How many square feet does a 5 gallon bucket cover?
At an assumed 350 sq ft per US gallon per coat, five gallons cover 1,750 sq ft for one coat or 875 sq ft for two coats, before other losses. These are paintable surface areas, not house floor areas.
Is primer calculated the same way as finish paint?
Use the same area × coats ÷ coverage formula, but enter the primer's own coverage and required coats. Primer and finish paint are separate products. Their quantities are not related by a universal percentage or gallon ratio.
Can I combine ceiling and wall paint quantities?
Only if they use the same suitable product, color and sheen, and their coating requirements are accounted for. Otherwise calculate and round each product's containers separately. The same rule applies to accent walls and trim.
Does the calculator automatically return gallons?
No. The embedded tool calculates area and optional area-based allowances, prices and box counts. It has no coat, paint-coverage or gallon inputs. Use its unadjusted Total Area, subtract excluded surfaces manually, then apply the paint formulas on this page.