Civil & Structural Engineering

Paint Calculator

Calculate how many gallons of paint (and primer) a wall painting job needs, plus a total cost estimate.

Formula Paint = ⌈Net Area × n × (1 + W%) / pc⌉Reviewed Sep 8, 2026

Painting a room? This calculator turns wall dimensions into the gallons of paint you need - and, if the walls need it, primer too. Unlike this site's Drywall Calculator, where sources genuinely disagree on subtracting openings, paint has a clear, undisputed convention: doors and windows aren't painted, so their area is always subtracted from the wall area before figuring coverage. Enter your wall perimeter and height, subtract known openings, and set your coat count and coverage rate for an instant gallon count.

Calculation Bench

Wall Dimensions

01Wall Perimeter

P · Total linear length of walls being painted

02Wall Height

H · Floor to ceiling

03Opening Area

Ao · Optional - combined door/window area

Paint & Primer

04Coats

n · 2 is standard for most jobs

05Paint Coverage

pc (ft²/gal) · 350-400 ft²/gal is typical

06Primer Coverage

prc (ft²/gal) · Optional - 200-300 ft²/gal, one coat

07Waste Factor

W% · 10% is common

Pricing (Optional)

08Price per Gallon (Paint)

$p · Leave blank to skip cost estimate

09Price per Gallon (Primer)

$r · Requires Primer Coverage to compute cost

Solution

Enter wall dimensions to calculate paint needed.

Paint = ⌈Net Area × n × (1 + W%) / pc⌉

Formula Sheet

A_{net} = (P \times H) - A_o

N_{paint} = \left\lceil \dfrac{A_{net} \times n \times (1 + W\%)}{pc} \right\rceil

N_{primer} = \left\lceil \dfrac{A_{net} \times (1 + W\%)}{prc} \right\rceil

NPaint Needed
PWall Perimeter
HWall Height
AoOpening Area
nCoats
pcPaint Coverage
prcPrimer Coverage
W%Waste Factor

Variables & Units

Symbol	Variable	Description	Common Units
N	Paint Needed	Total gallons of finish paint to purchase, rounded up and including the waste allowance.
P	Wall Perimeter	Total linear length of the walls being painted.	ft, m
H	Wall Height	Floor-to-ceiling wall height.	ft, m
Ao	Opening Area	Combined door/window area subtracted from wall area.	ft², m²
n	Coats	Number of finish coats.
pc	Paint Coverage	Coverage rate of the finish paint, area per gallon.
prc	Primer Coverage	Coverage rate of the primer, area per gallon (one coat).
W%	Waste Factor	A decimal allowance for touch-ups and spillage (e.g. 0.10 = 10%).

How to Use This Calculator

01Enter the wall perimeter (the total length of walls being painted) and the wall height.
02Enter the combined area of windows and doors to subtract - typical values are about 21 sq ft per door and 15 sq ft per window if you haven't measured them directly.
03Set the number of finish coats (2 is standard for most jobs) and the paint's coverage rate (350-400 sq ft per gallon is typical for smooth, previously painted walls).
04Optionally enter a primer coverage rate (200-300 sq ft per gallon) if the walls are bare, patched, or changing to a significantly different color - primer is calculated as a single coat.
05Set a Waste Factor (10% is common, for touch-ups and spillage) and optionally price per gallon for paint and primer.
06The calculator instantly shows paint gallons, primer gallons if applicable, and total cost.

How the Formula Works

Wall area is perimeter × height, minus the combined area of doors and windows - openings simply aren't painted, so subtracting them before computing coverage is standard practice, not a judgment call the way it is for some other materials.

Paint needed scales directly with both net area and coat count: two coats need twice the paint of one, since each coat covers the full wall again. Dividing that combined area by the paint's coverage rate (gallons of coverage per square foot) gives the raw gallons needed, rounded up since paint is sold in whole gallon or quart units.

Primer is calculated separately from finish coats, using its own (typically lower) coverage rate, and - unlike the finish paint - always as a single coat, since priming more than once is rarely called for even when multiple finish coats are.

The waste factor is applied to both paint and primer, covering touch-ups, spillage, and the reality that a job rarely uses exactly the calculated amount with nothing left over.

Worked Example 01

12x15 ft room, 8 ft walls, standard 2 coats

Known

Wall Perimeter (P): 54 ft
Wall Height (H): 8 ft
Opening Area (Ao): 40 ft²
Coats (n): 2
Paint Coverage (pc): 350 ft²/gal
Waste Factor (W%): 10%

Formula

Paint = ⌈Net Area × n × (1 + W%) / pc⌉

Substitution

Wall = 54×8 = 432 ft², Net = 432−40 = 392 ft², Paint = ⌈392×2×1.10/350⌉

Result

3 gallons

A 12 by 15 foot room with 8-foot walls, 40 sq ft of doors and windows, and two coats needs about 2.46 gallons before rounding - 3 gallons to actually purchase, since paint is sold by the whole gallon.

Worked Example 02

Same room, with primer

Known

Wall Perimeter (P): 54 ft
Wall Height (H): 8 ft
Opening Area (Ao): 40 ft²
Coats (n): 2
Paint Coverage (pc): 350 ft²/gal
Primer Coverage (prc): 300 ft²/gal
Waste Factor (W%): 10%

Formula

Primer = ⌈Net Area × (1 + W%) / prc⌉

Substitution

Net = 392 ft², Primer = ⌈392×1.10/300⌉

Result

3 gallons paint · 2 gallons primer

The same room, primed before painting, needs 2 gallons of primer (one coat) on top of the 3 gallons of finish paint - primer uses its own, typically lower coverage rate and is never multiplied by the finish coat count.

Applications

01Estimating paint (and primer) gallons for a room repaint or new-construction painting job
02Comparing paint needs across different coat counts or coverage rates
03Getting a quick cost estimate before requesting contractor quotes or shopping for paint

Assumptions

01Walls are smooth, previously painted drywall or a comparable surface - textured, raw drywall, brick, or masonry absorb significantly more paint than the coverage rates used here.
02Primer, when included, is applied as a single coat - additional primer coats are uncommon even when multiple finish coats are used.
03This calculator covers wall paint only, not ceiling paint - ceilings are usually a different product/sheen, so run the calculator again with the ceiling's own dimensions if it also needs painting.

Where This Model Stops

01Does not model different coverage rates for different colors or paint types (e.g. deep-base colors typically need more coats or reduced coverage per gallon).
02Does not calculate trim, baseboard, or ceiling paint - wall paint only.
03Rounds up to the nearest whole gallon; check with your supplier if quarts are available for a closer match on small jobs.
04Coverage varies with roller nap, surface texture, porosity, repairs, and color contrast. If the surface is rough masonry, bare drywall, or a dramatic color change, use a lower coverage rate or add primer.

References

[1]
Paint Calculator - Estimate Paint and Primer
Inch Calculator
Reference for the wall-area-minus-openings formula, standard door/window area estimates (21/15 sq ft), and typical paint (350-400 sq ft/gal) and primer (200-300 sq ft/gal) coverage rates.

Frequently Asked Questions

How many coats of paint do I actually need?

Two coats is standard for most repaints and covers most color changes evenly. A single coat can work for a very close color match on an already well-painted wall, while a significant color change (especially going lighter over a darker existing color) sometimes needs a third coat or a tinted primer instead.

Do I need primer?

Primer is recommended for bare or patched drywall, a significant color change (especially going from dark to light), or a surface with stains or uneven porosity. Repainting a wall that's already the same or a similar color, in good condition, usually doesn't need it.

Why is primer calculated as one coat while paint can be two or more?

Primer's job is to seal the surface and give the finish paint something consistent to bond to - a single coat does that. Finish coats are about achieving full, even opacity and color, which is why they're the ones that typically need a second application.

Should I subtract door and window area, or is that overkill for a normal room?

For a typical room, it makes a real difference - a single door and a couple of windows can easily total 40-60 sq ft, several percent of an average room's wall area. It's a straightforward subtraction and, unlike some other materials, there's no genuine debate about whether to do it: openings simply aren't painted.

How is this different from the Drywall Calculator?

The Paint Calculator estimates liquid coating from net wall area, coats, coverage, and waste. The Drywall Calculator estimates sheet counts and may treat openings differently because offcuts and layout can still matter for board purchases.

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