Paint Calculator
Gallons = wall area (minus doors/windows) × coats ÷ coverage, rounded up to the nearest half gallon.
A paint calculator turns the dimensions of a room, the number of coats you plan to apply, and a standard coverage rate into the number of gallons you need to buy, so a painting project gets the right amount of material without a rushed extra trip to the store. This page explains how the paint calculator works, walks through a worked example for a bedroom, and covers the mistakes that lead to buying too little or noticeably too much paint.
What the Paint Calculator Finds
Paint coverage is usually quoted as square feet per gallon, typically 350 to 400 square feet for one coat on a primed, smooth wall, and a paint calculator turns your room's dimensions into that same square footage so the two numbers line up. Enter the wall height and the room's perimeter, or the total square footage directly if you already have it, along with how many coats you plan, and the paint calculator returns the number of gallons needed, rounded up to a purchasable quantity. Because paint is sold in fixed can sizes, usually quarts, gallons, and five-gallon buckets, the calculator's rounding matters just as much as the raw math.
How to Use the Paint Calculator
Measure the height of the walls and the total perimeter of the room, or measure each wall separately if the ceiling height varies, and enter those numbers into the paint calculator. Add the combined area of windows and doors if you want a more precise estimate, since this tool can subtract that area from the total before calculating gallons. Select the number of coats, typically two for a full repaint or one for a close-color touch-up, and it returns the gallon count along with the equivalent square footage covered. Always round the result up to the nearest full gallon or quart, since running short mid-wall creates a visible touch-up line once the paint dries.
Paint Formula and Worked Example
The formula is total area = (wall height × room perimeter) − (window and door area), then gallons = (total area × number of coats) ÷ coverage rate per gallon. For a bedroom with 8-foot ceilings and a 50-foot perimeter, with 40 square feet of windows and doors: area = (8 × 50) − 40 = 360 square feet. For two coats at 375 square feet per gallon coverage: gallons = (360 × 2) ÷ 375 ≈ 1.92, which the paint calculator rounds up to 2 gallons. That single worked example shows why a paint calculator is useful even for a small room: the raw math is simple, but rounding to a purchasable can size is easy to get wrong without checking it directly.
Coverage by Room Size
| Room Size | Wall Area (approx.) | Gallons (2 coats) |
|---|---|---|
| Small bedroom (10x10) | ~320 sq ft | 2 gal |
| Standard bedroom (12x14) | ~400 sq ft | 2-3 gal |
| Living room (16x20) | ~560 sq ft | 3 gal |
| Large open room (20x25) | ~700 sq ft | 4 gal |
A paint calculator applies these same coverage ratios to your exact room dimensions, so use this table mainly as a quick check before opening the tool for a precise number.
Primer, Ceilings, and Trim
Bare drywall or a major color change, especially going from a dark wall to a light one, usually needs a separate coat of primer before the finish coats, and a paint calculator run separately for primer at a similar coverage rate keeps that estimate accurate. Ceilings are typically calculated as their own area, length times width, since ceiling paint coverage and finish differ from wall paint. Trim and doors use a much smaller quantity per piece, so a paint calculator estimate for trim is usually given in quarts rather than gallons unless a home has an unusually large amount of woodwork.
Mistakes to Avoid
The most common paint calculator mistake is forgetting to multiply by the number of coats, which understates gallons needed by roughly half for a standard two-coat job. Not subtracting large window or door areas in a room with several oversized openings can lead to buying more paint than necessary, though this is a smaller error than under-buying. Assuming every paint brand and sheen covers exactly 400 square feet per gallon is another common gap, since some formulas, especially deep or bright colors, cover closer to 300 square feet per gallon and need an adjusted coverage rate in the paint calculator. Finally, buying paint in several small trips instead of one larger purchase risks a slight color variation between cans from different production batches.
Buying Paint Efficiently
Buying a five-gallon bucket instead of individual gallon cans is usually cheaper per gallon once a paint calculator shows a project needs more than 3 to 4 gallons of one color. Many stores will mix leftover base paint into a slightly different can size if you bring an exact gallon figure from a paint calculator, reducing waste from over-ordering. Keeping a small labeled leftover can of each color used in a home makes future touch-ups easier to color-match, since paint formulas can shift slightly between production runs years apart.
Related Calculators
Paint is often the final step on a renovation or new-build project. If the room is part of a larger addition, check the concrete yard calculator for any new footings or slabs, and the stair calculator or stair stringer calculator for connecting staircases. For exterior painting tied to a roofing project, the roof pitch calculator and rafter calculator help confirm the structure underneath. Running the room through the paint calculator before shopping keeps the finishing stage of any project on budget.
FAQ
How does a paint calculator estimate gallons?
A paint calculator multiplies wall height by perimeter to get square footage, subtracts window and door area, then divides by a coverage rate of roughly 350 to 400 square feet per gallon, multiplied by the number of coats planned.
How many coats of paint does a wall need?
Most walls need two coats for even color and full coverage, especially when changing to a noticeably different color. A paint calculator set to one coat is usually only accurate for a matching touch-up over existing paint in good condition.
Should I subtract windows and doors from the paint calculator?
Yes, for a room with several large windows or doors, subtracting their combined area can meaningfully reduce the gallons needed. For a typical room with one door and one or two average windows, many painters skip the subtraction since it falls within the calculator's rounding buffer.
Does paint sheen affect coverage?
Coverage rate varies slightly by sheen and by whether the paint is a standard formula or a paint-and-primer-in-one product. A paint calculator using the standard 350-400 square feet per gallon range is accurate for most flat, eggshell, and satin finishes.