Paint Coverage Calculator

Background.

This paint calculator turns four numbers — wall area, coats, coverage rate, and price — into a complete shopping list: how many gallons to buy, the metric equivalent in liters, the total material cost, and whether 1-gallon or 5-gallon cans are the better unit for the job. It exists because the single most common painting mistake is buying the wrong quantity. Run short and the second batch arrives in a different sheen or lot number that telegraphs across the wall in raking light. Overbuy and you store half-full cans that skin over and dry to the brim within eighteen months. The math behind "how much paint do I need" is simple in concept and unforgiving in execution, so this tool builds in the small corrections that professional estimators apply by reflex.

Measuring wall area is the starting point. For a rectangular room, add the four wall lengths to get the perimeter, then multiply by ceiling height. A 12-by-14-foot bedroom with 8-foot ceilings has a perimeter of 52 feet and a wall area of 416 square feet before any subtractions. Ceilings, if you are painting them, add length × width as a separate surface — a 12 × 14 ceiling is another 168 square feet. Subtract openings only if their combined area is meaningful: a single interior door (about 21 sqft) and one window (12–15 sqft) barely move the gallon count, but a wall of sliding glass or a vaulted great-room with three big windows can subtract a hundred square feet or more. The calculator exposes the subtraction field so you can be precise when it matters and ignore it when it doesn't.

The 350 square-feet-per-gallon figure used as the default is the industry consensus published by Sherwin-Williams, Behr, PPG, and Benjamin Moore on the technical data sheet of every interior latex line. It assumes a smooth, primed drywall surface, a 3/8-inch nap roller, and a normal application thickness of around 4 wet mils — about the thickness of a sheet of printer paper. Real surfaces rarely hit that ideal. Heavily textured drywall, popcorn ceilings, stucco, raw masonry, rough-sawn wood, and previously unpainted surfaces drink paint and can drop the effective spread rate to 200 sqft per gallon or worse. Color shifts compound the problem: covering a deep red, navy, or saturated yellow with a light color almost always needs three coats or a tinted primer, while painting white-over-white can sometimes finish in one. The coverage input lets you override the default whenever the manufacturer's TDS or your past experience says the surface will not deliver 350.

Coats matter as much as coverage. Two coats is the working standard for almost every interior repaint because it guarantees uniform film build, eliminates flashing, and gives manufacturers the wet film thickness their warranties assume. One coat works only when you are refreshing the same color on a smooth, clean wall with a self-priming high-hide product, and it usually still looks better with two. The waste factor — the calculator defaults to 10 percent — covers the paint absorbed into roller covers and brushes, the cut-in that overlaps the field, drips, touch-ups after furniture moves back in, and the half-gallon you keep for future patches. Professionals working from a known crew often dial waste to 5 percent; first-time DIY painters should leave it at 10 percent or push to 15 percent on complex rooms with lots of trim.

Finally, the can-count outputs help with the bulk-vs-singles decision. A 5-gallon can almost always costs less per gallon than five 1-gallon cans of the same product — the discount is typically 15–25 percent. But 5-gallon containers are heavy, harder to stir, and only worth it if you actually need most of the volume. The calculator prints both can counts so you can compare at the paint counter and pick whichever delivers the right quantity without a half-empty pail going to waste.

What is paint coverage calculator?

A paint coverage calculator converts the area you intend to paint into the volume of paint you must purchase. It takes wall area (in square feet or square meters), the number of coats you plan to apply, the manufacturer's stated coverage rate, a waste allowance for overlap and touch-ups, and any window or door area you want to exclude. Out comes the exact gallon count, its liter equivalent, the total material cost at your local price, and the number of 1-gallon or 5-gallon cans you would need to buy. The same math applies to interior latex, exterior acrylic, primer, and even single-component industrial coatings — only the coverage rate changes.

How to use this calculator.

1. 01Measure the wall area: add the perimeter of the room and multiply by ceiling height. Add ceiling area separately if you are painting it.
2. 02Enter the total wall area and pick the unit — square feet or square meters.
3. 03Set the number of coats. Use 2 for a standard repaint; use 3 when going light over dark or painting raw drywall.
4. 04Confirm the coverage rate. The default of 350 sqft per gallon matches Sherwin-Williams, Behr, and PPG for smooth interior walls. Reduce it for textured, porous, or unprimed surfaces.
5. 05Enter the price per gallon at your retailer. Premium interior latex typically lists at $35–$70.
6. 06Leave the waste factor at 10% for most DIY jobs; raise it to 15% for complex rooms with heavy cut-in.
7. 07Optionally subtract the square footage of windows and doors you will not paint.
8. 08Read the gallon count, total cost, and can breakdown. Compare 1-gallon and 5-gallon prices at the store before checking out.

The formula.

The calculator works in five steps. First it normalises the wall area to square feet — multiplying by 10.7639 if you entered square meters, since 1 sqm = 10.7639 sqft (NIST SP 811). Second it subtracts windows and doors: netArea = wallAreaSqft − subtractWindowsDoorsSqft. Third it scales by coats and the waste factor: effectiveArea = netArea × coats × (1 + wasteFactorPercent ÷ 100). Fourth it divides by the manufacturer coverage rate and rounds up to the nearest hundredth of a gallon so you never under-buy: gallonsNeeded = ceil(effectiveArea ÷ coveragePerGallon × 100) ÷ 100. Fifth it derives the downstream numbers: liters = gallons × 3.78541 (US gallon to liter, NIST SP 811), cost = gallons × pricePerGallon, cans1Gal = ceil(gallons), cans5Gal = ceil(gallons ÷ 5). The 5-gallon can count is the lever for the bulk-discount decision: if cans5Gal × bulkPrice < cans1Gal × singlePrice, buy the pail.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Sherwin-Williams — How much paint do I need? Interior paint coverage technical bulletin.
2. [2]Behr Pro — Paint Coverage Calculator and Painting Resources.
3. [3]PPG Industries — Interior Latex Technical Data Sheets (coverage and wet film thickness specifications).
4. [4]Master Painters Institute — MPI #43, Interior Latex, Institutional Low Odour / VOC (coverage and application standards).
5. [5]US Environmental Protection Agency — National Volatile Organic Compound Emission Standards for Architectural Coatings (40 CFR Part 59 Subpart D).
6. [6]NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), unit conversions for gallons and square meters.