Paint Calculator

Using the Paint Calculator

The calculator turns four numbers (length, width, height, opening count) into a gallon count you can bring to the paint counter. Walk the room first with a tape measure and write down the perimeter walls and an honest count of doors and windows.

1. Room dimensions - length, width, and wall height in feet.
2. Doors and windows - standard door 21 sq ft, standard window 15 sq ft. Enter the count, not the area.
3. Coverage per gallon - default 350 sq ft. The can prints the exact figure, typically 250-400 sq ft for one coat.
4. Number of coats - two is standard. Three for a dramatic light-over-dark change, one only for re-coating the exact same colour with premium paint.

What the Calculator Computes

The math is: perimeter × height gives gross wall area, subtract openings for paintable area, multiply by coats, divide by per-gallon coverage, then Math.ceil the gallon count. A 12 × 14 ft bedroom at 8-ft ceilings with one door and two windows is (52 × 8) - (21 + 30) = 365 sq ft paintable. Two coats at 350 sq ft/gallon equals 2.09 gallons, so you buy three one-gallon cans. Rounding happens at the gallon boundary, not the coat boundary - rounding per-coat over-buys.

Why Coverage Varies

A gallon labelled "350-400 sq ft" hits that number on smooth, primed drywall. On raw drywall, it soaks paint like a sponge and drops to 200-250 sq ft. Popcorn ceilings, masonry, and stucco can push under 150. Prime first with a dedicated primer for bare or patched surfaces.

When You Actually Need This

• Repainting a bedroom and you do not want leftover gallons on the shelf.
• Switching from a dark accent wall back to neutral - that transition almost always needs three coats.
• Painting a rental before move-out on a tight budget.
• Planning an entire floor at once so you can buy from the same tint lot.
• Costing a handyman job where the client supplies paint.

Pitfalls That Throw Off the Estimate

Forgetting the ceiling is the biggest miss - the calculator covers walls only. Closets get miscounted because people forget the interior walls (adds 40-60 sq ft). Dark-to-light transitions over oil-based paint need a stain-blocking primer first. Trim and baseboards consume their own paint - budget a separate quart of semi-gloss trim enamel per 15-20 linear feet. Flat and matte sheens cover in fewer coats visually than eggshell, satin, or semi-gloss.

Paint Chemistry and the Coverage Number

Coverage on the can comes from ASTM D344 (hiding power) and D2805 (contrast ratio) testing at a 4-mil wet film thickness. "One-coat coverage" claims mean 98%+ hiding at that film thickness, which is why a premium $55/gallon paint often does in one coat what a $25 contractor-grade paint needs two to do. The label's high end is the lab figure; the low end is real-world on textured walls.

A Second Worked Example

Take a 10 by 11 ft home office with a 9-ft ceiling, one door and one window, going from beige to deep navy. Perimeter is (10 + 11) times 2, or 42 ft; times the 9-ft height that is 378 sq ft gross. Subtract one door (21 sq ft) and one window (15 sq ft) for 342 sq ft of paintable wall. A dark colour over a light base needs three coats, so 342 times 3 is 1,026 sq ft, or 2.93 gallons at 350 sq ft each - round up to three. A quart of tinted primer first cuts the finish coats from three to two and saves a gallon of the pricier navy.

Buying in Gallons Versus Quarts

A gallon costs far less per square foot than four separate quarts, so for any full wall the gallon wins even with some left over. Quarts earn their place on small jobs - an accent wall under 100 sq ft, a closet, or trim and touch-up work. When the calculator lands just over a whole gallon, buy two gallons plus a quart rather than three to trim cost and leftover.

How This Compares to Big-Box Calculators

Home Depot, Lowe's, Sherwin-Williams, and Behr all have online calculators. Theirs tend to be generous - an extra gallon is revenue - and often assume 400 sq ft/gallon even on textured walls. This tool defaults to 350 because it lands closer to what mainstream latex does on typical drywall. A paint-store associate with a laser measure will beat any calculator for vaulted or dormered rooms; for a standard rectangular room with a door and a couple of windows, any calculator lands within half a gallon of correct.