Paint Calculator

Where 400 square feet per gallon quietly becomes 350

The coverage rate changes the answer more than the room does, and it is the field people leave alone. This calculator defaults to 400 sq ft per gallon per coat — the generous end of the 350-400 sq ft range Sherwin-Williams and the other manufacturers publish — and that figure describes smooth, primed drywall rolled with a nap roller. Every surface that is not that covers less.

Knockdown and orange-peel texture, bare or freshly patched drywall, unsealed plaster, and any chalky or porous wall all take more paint per measured square foot, because there is more actual surface hiding inside the same rectangle you measured. Spraying uses more again: atomised paint drifts and lands where you did not aim it, so a sprayer never puts as high a share of the can on the wall as a roller does. Deep, saturated colors — strong reds and blues especially — carry more pigment and less hiding power per coat, and they show up either as a lower effective coverage rate or as an extra coat.

The move is to lower the coverage field before you buy, rather than discovering the shortfall halfway up a wall. Take the room worked through below, which needs 932 sq ft of coverage in total. At 400 sq ft per gallon that is 2.33 gallons and a 2.5-gallon purchase. At 350 it is 932 ÷ 350 = 2.66 gallons, and the purchase jumps to 3. A 12.5% cut in the assumed coverage rate moved the trip to the store by half a gallon, without a single dimension changing. The field accepts anything from 100 sq ft per gallon upward, so the rate printed on the can you actually intend to buy should always replace the default.

The second coat costs a whole gallon more, not another 1.17

Coats multiply the area before the division happens, which is why that field moves the result in steps rather than smoothly. Take the 466 sq ft of wall and ceiling from the example below. One coat is 466 ÷ 400 = 1.17 gallons, which rounds up to a 1.5-gallon purchase. Two coats is 932 ÷ 400 = 2.33 gallons, which rounds up to 2.5. The second coat added 1.17 gallons of paint but a full gallon of buying, because the half-gallon rounding lands where it lands.

Two coats is the working default here, and Benjamin Moore’s own calculator assumes two as well. One coat is realistic only for a close color match over an already-painted, unstained, non-porous wall, which is why the calculator raises a warning as soon as you set coats to 1 — it is far more often an optimistic guess than a plan.

Three coats, and the tool accepts up to four, is the honest setting for a dramatic light-to-dark or dark-to-light change, for bare or heavily patched drywall, and for a deep accent color that has to build its own opacity. At three coats the same room becomes 466 × 3 = 1,398 sq ft of coverage, or 3.5 gallons to buy — a full gallon more than the two-coat answer. Before you commit to that, price a tinted primer instead: when the third coat exists only to hide what is underneath rather than to finish the surface, a primer mixed toward the finish color usually does the hiding for less money and less labour.

One door and two windows are 50 square feet you should not buy paint for

Gross wall area is the room perimeter times the ceiling height, and it counts the door and the windows as though they were wall. The deduction is what turns that into a paintable figure. This calculator subtracts 20 sq ft for each door and 15 sq ft for each window — the majority convention across the manufacturer and retailer sources — so a room with one door and two windows loses 20 + (2 × 15) = 50 sq ft before anything is divided by a coverage rate.

Fifty square feet sounds like a rounding error until the coats and the half-gallon step get hold of it. Work the standard case: a 12 ft × 10 ft room with 9 ft ceilings, 1 door, 2 windows, 2 coats, ceiling included, at 400 sq ft/gallon. The perimeter is 2 × (12 + 10) = 44 ft, so gross wall area is 44 × 9 = 396 sq ft. Taking off 20 for the door and 30 for the two windows leaves 346 sq ft of net wall; the 120 sq ft ceiling brings it to 466 sq ft per coat, 932 sq ft over two coats, 2.33 gallons — a 2.5-gallon purchase. Skip the deduction and you divide 1,032 sq ft instead, which is 2.58 gallons and a 3-gallon purchase. The openings are worth half a gallon.

The 15 sq ft window is a convention rather than a measurement, and it is genuinely contested: This Old House uses 10 sq ft instead. Here that dispute is cheap. At 10 sq ft per window the same room needs 952 sq ft of coverage rather than 932, which is 2.38 gallons and still a 2.5-gallon purchase. It only starts to matter with unusually large openings — a picture window, a sliding door, a wall of glass — and there the fix is to measure the openings yourself and reduce the entered room dimensions to match. The same discipline covers a room that is not a rectangle: the perimeter formula assumes one, so treat an L-shaped room as two rectangles, or add the wall runs by hand and enter dimensions that reproduce the total you measured.

Primer is a second purchase at 200-300 square feet a gallon

Nothing in this estimate is primer. The gallons figure covers finish paint only, and primer is a separate product bought against a separate, worse coverage rate — roughly 200-300 sq ft per gallon, because it is applied thicker to seal the substrate rather than to look like anything.

You need it for new or bare drywall, for a dramatic color change, for stain-blocking over water marks, smoke, or crayon, and for adhesion over glossy or oil-based surfaces. You do not need it for a repaint in a similar color over sound, previously painted, unstained walls — which is the job this calculator is really sized for, and the reason primer is not folded into the default.

When you do need it, price it as one coat over the same area this tool has already worked out for you, at primer’s own rate rather than the wall paint’s. The example room’s 466 sq ft comes to 466 ÷ 300 ≈ 1.55 gallons at the optimistic end of that range and 466 ÷ 200 = 2.33 gallons at the pessimistic end: a second purchase of roughly 1.5 to 2.5 gallons that the wall figure above never hinted at. A tinted primer, mixed toward the finish color, frequently replaces a third finish coat outright — the one case where buying more primer means buying less paint.

The ceiling shares your coverage rate but not your can

Ticking the ceiling box adds length × width to the area at the same coverage rate as the walls. That is a convenience for getting to one number, not a claim that the two are the same product. Ceiling paint is usually a flat or matte formulation built to hide roller lap marks across a wide, evenly-lit plane, and in a kitchen or bathroom to resist moisture. Wall paint in eggshell or satin is a different can at a different price. Whenever the sheens differ, the two totals belong on separate lines before you reach the counter.

The tool hands you the pieces to split them: the breakdown beneath the result reports net wall area and ceiling area separately, so each can be divided by its own coverage rate and bought as its own product. In the example room the ceiling is 12 × 10 = 120 sq ft, roughly a quarter of the 466 sq ft total. Walls only is 692 sq ft across two coats — 1.73 gallons, so 2 gallons to buy. Adding the ceiling takes the same room to 2.5 gallons. That half gallon is the entire ceiling decision, and it is worth making deliberately rather than leaving a checkbox ticked.

Textured ceilings are where the shared coverage rate genuinely breaks down. Popcorn, knockdown, and heavy stipple all hold far more surface than their flat footprint suggests, and the same 400 sq ft per gallon that is fair for a smooth wall is optimistic there. Lower the coverage field exactly as you would for a textured wall, or leave the ceiling out of the calculation and estimate it separately at its own rate.

2.33 gallons is not two gallons and a quart

The result rounds up to the nearest half gallon because that is how paint is sold and mixed at a counter, and the rounding is deliberately generous rather than tight. Running out three-quarters of the way up a wall means a fresh batch, and a re-mixed can of the same color code is not guaranteed to be an invisible match. That seam costs far more than the leftover paint you were trying not to buy, so when you are between sizes, lean toward the larger can.

Read the rounded gallons-to-buy figure as the purchase and the unrounded gallons-needed figure as the arithmetic behind it. When the two sit far apart, the gap is your margin for a spill, a second touch-up round, or a wall that turned out thirstier than the coverage rate promised. When they nearly coincide, you have no margin at all, and that is the moment to round up a further half gallon on purpose.

Quarts look like the way to shave the difference, and they only sometimes are. A quart is a quarter of a gallon, so two gallons plus one quart is 2.25 gallons — less than the 2.33 the example room actually needs, and the wrong answer despite looking like the clever one. Two gallons plus two quarts reaches the 2.5 the calculator asked for. Whether that beats simply buying three gallons is a pricing question rather than a coverage one: per square foot, gallons are almost always cheaper than quarts, so the saving only survives if the leftover fraction is large and the quart is priced near a quarter of the gallon. Keep quarts for trim, a single accent wall, or a genuine top-up.

Trim and baseboards are bought by the quart, not by the wall

The doors and windows you just subtracted still have to be painted, and this calculator does not estimate them. Trim, baseboards, casings, and door faces are nearly always a different product — semi-gloss or a satin enamel, chosen for washability and for the crisp edge it holds against the wall color — and they are estimated by linear feet of run rather than by wall area. Folding them into the wall total would mix two incompatible units and two different cans.

Budget them separately, usually a quart or two for an ordinary room, and buy them alongside the wall paint so the tint base and sheen stay consistent across the whole job. The same goes for a feature wall in a second color: calculate its area on its own rather than averaging it into the room, since the point of an accent color is that it is not spread over the other three walls.

What remains is a planning estimate with real limitations, and they are worth naming plainly. It cannot know your brand’s actual spread rate, the porosity of the wall you are about to roll, how much paint a sprayer will lose to the air, or the coverage figure printed on the can you finally pick up — and that number on the label beats every default on this page. Use the gallons figure to size the trip and the budget; use the can to settle the argument.

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Sources

• Paint Calculator (coverage: 350-400 sq ft per gallon) — Sherwin-Williams
• Paint Calculator (assumes 2 coats of paint) — Benjamin Moore
• How Much Interior Paint to Buy — Behr