Buying paint without doing the math usually ends one of two ways: a panicked mid-project trip to the store, or a garage full of half-used gallons that never get opened again. A little arithmetic before you shop avoids both.

The Paint Calculator above turns your room measurements into a shopping list. Enter your wall dimensions, subtract the doors and windows, choose your coats and paint details, and it tells you how many gallons to buy and what they will cost.

This guide explains how paint coverage works, how to measure correctly, and how to read the result so you buy exactly what you need.

What Does the Paint Calculator Do?

The calculator converts wall dimensions into gallons of paint and a total cost. It works in two groups of inputs: room measurements (wall length, wall height, number of walls, and openings to subtract) and paint details (coats, coverage per gallon, price per gallon, and waste allowance).

The headline result states how many gallons you need, rounded up to whole gallons because paint is sold by the gallon. Below that, you see the paintable area per coat, the total area including coats and waste, and the estimated total cost.

The tool assumes all walls are the same length and height. For rooms with irregular walls, measure each wall separately and add the areas, then use the total as an equivalent single-wall input.

How to Use the Paint Calculator

Enter the wall length and wall height in feet, then the number of walls. If your room is not a perfect rectangle, compute the total wall area yourself and enter it as one wall of equivalent size.

Enter the openings to subtract — the total square footage of doors, windows, and anything else you will not paint. A standard door is about 21 square feet; a typical window is 12 to 15 square feet.

Choose your coats, check the coverage per gallon (350 is the standard default), enter the price per gallon, and pick a waste allowance. Press Calculate for your gallons and cost, or Reset to start over.

How Paint Coverage Ratings Work

A coverage rating like "350 square feet per gallon" tells you how far one gallon spreads in a single coat on a smooth, primed surface. It is a laboratory number, not a promise — real rooms usually yield less.

Porous surfaces drink paint. Bare drywall, fresh plaster, and rough textures can cut effective coverage by 20 percent or more, which is why a first coat on unprimed drywall always seems to vanish.

The calculator lets you override the coverage rating. If the paint can says 400 but you are painting rough brick, entering 300 gives a far more honest estimate.

Measuring Your Walls Correctly

Measure length along the baseboard and height from floor to ceiling, in feet. Round each measurement up to the nearest half foot — small under-measurements compound across four walls.

For a standard rectangular room, all four walls share the same height, so enter the longest wall length and set the wall count to 4. For an L-shaped room, measure each wall section and sum the areas first.

Do not measure the floor area and assume it works. A 12-by-12 room with 9-foot ceilings has about 430 square feet of wall — more than triple the floor area. Walls are what you paint.

Subtracting Doors and Windows

Every door and window you skip is paint you do not need to buy. The "openings to subtract" field takes the total square footage of all of them, so add them up before entering.

A standard interior door is roughly 3 by 7 feet, or 21 square feet. A typical double-hung window is about 12 square feet, and a large picture window can reach 20 or more.

Trim and baseboards are usually painted with the same paint or a matching trim paint, so most people leave them in the area rather than subtracting. Only subtract openings you genuinely skip.

Why Coats Multiply Everything

Each coat covers the same area again, so two coats need twice the paint of one — the calculator multiplies the net wall area by the coat count before doing anything else.

Two coats are the professional standard for even color, especially when changing colors or painting over a darker shade. One coat is fine for refreshing the same color on a well-prepared wall.

A third coat is rarely needed for walls, but deep reds, bright yellows, and other hard-covering colors sometimes demand it. If you are unsure, budget for two and keep the receipt.

Worked Example: One Bedroom, Two Coats

First: a bedroom with 12-foot walls, 9-foot ceilings, 4 walls, and 40 square feet of doors and windows. Paintable area = 12 × 9 × 4 − 40 = 392 square feet.

Then: two coats at 350 square feet per gallon gives 392 × 2 = 784 square feet to cover, which is 784 ÷ 350 = 2.24 gallons.

Then: paint sells by the whole gallon, so you buy 3 gallons. At $45 per gallon, the paint costs $135.

Worked Example: Adding the Waste Allowance

First: take the same bedroom — 784 square feet of coverage needed for two coats — and add the 10 percent waste allowance.

Then: total area becomes 784 × 1.10 = 862.4 square feet, which is 862.4 ÷ 350 = 2.46 gallons.

Then: you still buy 3 gallons, but now the estimate honestly accounts for spills, roller absorption, and touch-ups instead of hoping nothing goes wrong.

Worked Example: A Tight-Budget Single Coat

First: refreshing a small office — 10-foot walls, 8-foot ceilings, 4 walls, 30 square feet of openings. Area = 10 × 8 × 4 − 30 = 290 square feet, one coat, no waste allowance.

Then: 290 ÷ 350 = 0.83 gallons, which rounds up to 1 gallon.

Then: at $38 per gallon, the whole job costs $38. The single gallon leaves a little extra for future touch-ups, which is a bonus of the round-up.

The Waste Allowance Explained

The waste allowance is a percentage added to the calculated area before rounding to whole gallons. It covers the paint that stays in the roller, spills on the drop cloth, and goes into cut-in overlap.

Ten percent is the standard recommendation for interior walls in good condition. Fifteen percent suits textured walls, first-time painters, or projects with lots of cutting in around trim.

Choose "none" only when you are repainting the same color on smooth walls and you already own leftover paint for touch-ups. For everyone else, the allowance is cheap insurance.

Common Paint Estimating Mistakes

The most common mistake is measuring the floor instead of the walls, which underestimates the job by a factor of three or more. Always measure wall length times wall height.

Another frequent error is forgetting the second coat in the mental math — people price one coat, buy one coat's worth of paint, and discover the shortfall on a Saturday afternoon. The calculator's coat multiplier exists to prevent exactly this.

Finally, many shoppers ignore the round-up. You cannot buy 2.24 gallons, so 2 gallons will leave you short. The calculator rounds up for you; trust the rounded number, not the decimal.

Paint Finishes and How They Affect Coverage

Flat and matte paints hide wall imperfections and usually hit their stated coverage on the first coat. Eggshell and satin, the most popular interior finishes, spread similarly but may need a fuller second coat for even sheen.

Semi-gloss and gloss paints are thinner and often cover slightly less per gallon, but they are chosen for trim and kitchens where washability matters more than coverage.

Primer is a separate purchase with its own coverage rating, typically 200 to 300 square feet per gallon. If your project needs primer, run the calculator a second time with the primer's coverage and price.

How to Interpret Your Result Correctly

The headline gallon count is the purchase number — buy that many gallons. The decimal underneath (gallons before rounding) tells you how tight the estimate is: 2.9 gallons means almost no margin, while 2.1 means a comfortable cushion.

The paintable area per coat lets you sanity-check your measurements. If it looks wildly off from what you expected, recheck the wall dimensions before trusting the gallon count.

The total cost covers paint only. Brushes, rollers, tape, drop cloths, and primer are separate — budget another 15 to 25 percent of the paint cost for supplies on a first project.

Where Paint Calculations Are Useful

The obvious use is a single room refresh: measure once, buy once, and skip the second trip. Landlords repainting between tenants use it to standardize costs across units.

Contractors use the same math scaled up — whole houses, offices, and apartment buildings are just bigger versions of the same area-times-coats formula, and the waste allowance becomes critical at that scale.

It is also handy in reverse. If you already own 2 gallons of a color, enter your room size and see whether one coat is feasible or whether you need to buy more before opening the first can.

Frequently Asked Questions

1. What is the Paint Calculator?

It estimates how many gallons of paint a project needs and what they will cost. You enter wall dimensions, subtract doors and windows, choose coats and paint details, and it returns whole gallons rounded up plus the total price.

2. How many square feet does a gallon of paint cover?

Most interior paints claim 350 to 400 square feet per gallon per coat on smooth surfaces. Rough, porous, or unprimed surfaces yield less — the calculator lets you enter your own coverage figure.

3. Why does the calculator round up to whole gallons?

Because paint is sold by the whole gallon. A calculation of 2.24 gallons means you must buy 3 gallons — 2 gallons would leave you short mid-project.

4. Should I include a waste allowance?

Yes, in most cases. Ten percent covers roller absorption, spills, and touch-ups. Skipping it is only sensible when repainting the same color on smooth walls with leftover paint on hand.

5. How do I measure openings to subtract?

Multiply each door or window's height by its width in feet and add them together. A standard door is about 21 square feet; a typical window is 12 to 15 square feet.

6. Do I need to double the paint for two coats?

Yes. Each coat covers the same wall area again, so two coats need twice the paint. The calculator multiplies the net area by the coat count automatically.

7. What if my walls are different lengths?

Measure each wall's area (length times height) and add them up, then enter the total as a single equivalent wall. Subtract the openings from the grand total as usual.

8. Does ceiling paint use the same calculation?

The math is identical — area times coats divided by coverage — but ceilings usually need a flat ceiling paint with its own coverage rating. Run the calculator separately with the ceiling's area and the ceiling paint's details.

9. How accurate is the cost estimate?

It is accurate for the paint itself given your inputs: gallons times price per gallon. It does not include brushes, rollers, tape, primer, or drop cloths, so budget extra for supplies.

10. What coverage should I enter for textured walls?

Reduce the can's stated coverage by 15 to 25 percent for heavy texture, stucco, or brick. Entering 280 to 300 when the can says 350 is a reasonable starting point.

11. Can I use this for exterior painting?

Yes. Measure the exterior wall areas, subtract windows and doors, and enter the exterior paint's coverage and price. Choose the 15 percent waste allowance — exteriors are rougher and weather interrupts work.

12. Do I count the trim and baseboards?

Usually not separately. If you paint trim the same color, leave its area in the total. If trim gets its own paint, subtract a rough allowance and run the calculator again for the trim paint.

13. How much extra paint should I keep for touch-ups?

Keep at least a quart of the exact paint, stored sealed and labeled with the room and date. Touch-ups with a different batch can show a visible sheen difference.

14. Why did my last project need more paint than calculated?

The usual causes are painting over bare drywall without primer, a darker color underneath showing through, or a coverage rating that assumed smoother walls than yours. Adjust the coverage input downward next time.

15. Is one coat ever enough?

Yes, when refreshing the same color on a well-prepared wall with quality paint. Changing colors, covering dark shades, or painting bare surfaces almost always needs two coats for an even finish.