Paint Calculator

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Unit system:

Choose the measurement and buying units shown in the form, result rows, chart, and exports.

Measurement basis:

Choose how the wall area should be measured.

Room length:

Enter the longer or first wall dimension.

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Room width:

Enter the shorter or second wall dimension.

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Total wall length:

Use the combined wall run before multiplying by wall height.

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Wall height:

Measure the vertical wall area that receives paint.

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Opening deduction:

Use standard door/window counts or enter a measured total area.

Doors:

Count full-size door openings that will not be painted as wall surface.

doors

Windows:

Count window openings that should be subtracted from wall paint.

windows

Opening area:

Enter the combined area that should be subtracted.

{{ areaUnit }}

Wall coats:

Number of finish coats applied to the wall area.

coats

Paint coverage:

Pick a surface/paint profile or switch to Custom for the label value.

Custom coverage:

Use the coverage listed on the can or technical sheet.

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Waste allowance: {{ waste_percent }}%

Use 5-10% for smooth walls, more for texture, spraying, or strong color changes.

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Door deduction:

Used only when opening deduction is by count.

{{ areaUnit }}/door

Window deduction:

Used only when opening deduction is by count.

{{ areaUnit }}/window

Ceiling paint:

Keep off for a walls-only estimate.

Ceiling coats:

Ceiling area uses room length by width.

coats

Tin size:

Used for the metric purchase quantity and tin cost estimate.

L/tin

Metric price basis:

Affects cost only; quantities still show exact litres and rounded tins.

{{ priceLabel }}:

Use 0 to omit cost pressure from the recommendation.

$

Takeoff Item	Value	Detail	Copy
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Plan Area	Status	Action	Copy
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Coverage Profile	Coverage	Paint Need	Copy
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Enter positive wall dimensions, coverage, and coat values to build the paint estimate.

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Paint buying is a surface takeoff, not a floor-plan guess. Floor area gives a sense of room size, but paint covers walls, ceilings, doors, trim returns, patched spots, and texture. The useful number is the area that will actually receive each coat, adjusted for openings and the way the chosen product spreads on that surface.

Two rooms with the same floor plan can need different paint. Tall walls add area without changing the floor size. Large windows remove wall area. A dark-to-light color change may need another coat. Bare drywall, masonry, stucco, heavy texture, or spray application can reduce coverage enough to move the order from one container to the next. A careful estimate separates measuring, coating, coverage, allowance, and buying.

Gross wall area: The wall run multiplied by wall height before subtracting unpainted openings.
Net wall area: The wall surface left after doors, windows, and measured exclusions are removed.
Coat area: Net area multiplied by the number of coats, plus any ceiling area that receives paint.
Coverage rate: The area one gallon or litre is expected to cover for one coat under product and surface assumptions.

Paint estimate flow from measured wall area through coats and coverage to rounded paint containers.

Coverage labels are useful starting points, not guarantees. A label range usually assumes a prepared surface, ordinary application thickness, and one coat. Primer, porous surfaces, rough masonry, roller loss, cutting in, patch repairs, and touch-up reserve all pull real coverage away from a neat square-foot or square-metre figure.

The buying step adds another practical constraint because paint is sold in containers. An exact need of 3.1 L still has to be rounded to a tin size, and an exact need of 2.3 gal often becomes a 3 gal purchase unless quarts are available and suitable for the job.

How to Use This Tool:

Start with the measurement path that matches the job, then move through openings, coats, coverage, allowance, and optional buying details.

Choose Unit system. Use Metric for metres, square metres, litres, and tins, or Imperial / US for feet, square feet, gallons, and quarts.
Set Measurement basis. Use Room length and width for a rectangular room, or Total wall length for an irregular wall run, hallway, feature wall, or partial-room repaint.
Enter the wall dimensions and choose an Opening deduction. Door and window counts use standard deductions, while Measured opening area is better for glass doors, built-ins, stair openings, or unusually large windows.
Set Wall coats, then choose Paint coverage. Use a lower-coverage profile for primer, texture, sprayed work, masonry, or stucco. Use Custom label coverage when the product sheet gives a better rate.
Adjust Waste allowance. The 5% to 10% range suits smooth repainting; rough texture, spraying, repairs, heavy cut-in work, and strong color changes may need more reserve.
Open Advanced for custom door/window deductions, ceiling paint, tin size, and simple material pricing. Ceiling paint is available when room length and width define the ceiling area.
Use the validation message if results disappear. For example, Opening deduction must be smaller than gross wall area means the excluded area is larger than the measured wall surface.
Review Paint Takeoff for the area math, Coating Plan for practical buying signals, the litre or gallon sensitivity map for coverage risk, Coverage Sensitivity Data for the same comparison as rows, and JSON for a structured copy of the estimate.

Interpreting Results:

The summary leads with the rounded purchase quantity because that is the amount you can normally buy. In metric mode, the purchase is whole tins based on the selected tin size. In imperial mode, the purchase is whole gallons, with a quart-equivalent value shown in the takeoff for small-job planning.

Exact paint volume is the mathematical need after openings, coats, ceiling scope, and waste allowance. Metric purchase and Imperial purchase round that exact volume to containers. A small spare amount is normal and usually helpful for roller loading, cutting in, patching, and future touch-ups.

Paint Takeoff is the audit trail. Check gross wall area, opening deduction, net wall area, adjusted coat area, exact paint volume, and estimated paint cost before buying.
Coating Plan flags practical risks. A tight allowance, very high spare quantity, low-coverage surface, or included ceiling can change the purchasing decision even when the summary number looks simple.
Litre Sensitivity Map or Gallon Sensitivity Map shows how far the estimate moves under other coverage profiles. If nearby coverage assumptions cross a tin or gallon boundary, verify the product label and surface condition.
Exact-looking decimals do not make the estimate exact in the room. Recheck the coverage rate, opening deduction, and waste allowance whenever the rounded purchase leaves little spare material.

Technical Details:

A paint estimate begins by turning the measured wall run into area. For a rectangular room, the wall run is twice the length plus twice the width. For a non-rectangular run, the measured total wall length replaces the room perimeter. Openings are removed before coats are applied, so a door or window is not counted once for every finish coat.

Coverage is a one-coat spread rate. Lower coverage means more liquid paint for the same adjusted coat area. The calculation uses square feet and US gallons for the core equations, then shows litres and metric area when metric units are selected. Rounded buying quantities are calculated after the exact volume, not before it.

Formula Core:

The wall area, opening deduction, coats, ceiling scope, waste allowance, and coverage rate combine into one exact paint volume.

\begin{array}{lll} P & = & 2 \times (L + W) or measured wall run \\ A_{gross} & = & P \times H \\ A_{net} & = & \max (0, A_{gross} - A_{openings}) \\ A_{coat} & = & A_{net} \times C_{walls} + A_{ceiling} \times C_{ceiling} \\ A_{adjusted} & = & A_{coat} \times (1 + W / 100) \\ G_{exact} & = & A_{adjusted} / R \\ L_{exact} & = & G_{exact} \times 3.785411784 \end{array}

Paint estimate variables and units
Symbol	Meaning	Unit or source
`P`	Wall run used for area	Room perimeter or entered total wall length
`H`	Painted wall height	Floor to ceiling or trim stop
`Aopenings`	Doors, windows, or measured excluded area	Counted deductions or entered area
`Cwalls`	Wall finish coats	Whole coats, minimum 1
`W`	Waste allowance	Percent added after coat multiplication
`R`	Coverage rate	Square feet per gallon for one coat

Purchase rounding happens after the exact gallons and litres are known.

\begin{array}{lll} G_{buy} & = & ceil (G_{exact}) \\ T_{buy} & = & ceil (L_{exact} / S_{tin}) \\ L_{buy} & = & T_{buy} \times S_{tin} \end{array}

Coverage assumptions used by the calculator
Coverage assumption	Rate	Best use
Standard interior	350 sq ft/gal	Smooth painted drywall when no product label is available.
Premium interior	400 sq ft/gal	Higher label coverage on prepared interior walls.
Primer/new drywall	275 sq ft/gal	Porous first coats and surfaces that absorb more paint.
Textured or sprayed	250 sq ft/gal	Rough wall texture, spraying, and heavier application loss.
Masonry/stucco	150 sq ft/gal	Very porous or rough masonry surfaces that often need primer.

Paint planning boundary rules
Signal	Boundary	Meaning
Opening validation	Opening deduction must be less than gross wall area	Excluded openings cannot exceed the surface being painted.
Waste allowance	Below 5%	Little reserve for texture, cut-in work, touch-ups, or measuring error.
Waste allowance	5% to 20%	Practical reserve for many interior jobs.
Waste allowance	Above 20%	Conservative allowance for rough, sprayed, repaired, or uncertain work.
Purchase rounding	Spare under 8%	Tight spare material after rounding up.
Purchase rounding	Spare 45% or higher	High leftover risk, often seen on small jobs or large container sizes.

For a 12 ft by 14 ft room with 8 ft walls, one 20 sq ft door, two 15 sq ft windows, two wall coats, 10% allowance, and 350 sq ft/gal coverage, gross wall area is 416 sq ft. The openings remove 50 sq ft, net wall area is 366 sq ft, adjusted coat area is about 805 sq ft, and exact paint volume is about 2.3 gal. Whole-gallon buying rounds that to 3 gal.

Limitations and Accuracy Notes:

Paint estimates depend on measurements and product assumptions. Product labels, surface porosity, primer, repair patches, roller nap, spraying, temperature, film thickness, color contrast, and the painter's application method can all change real coverage.

Opening counts are quick assumptions. Measure large glass, cabinets, built-ins, stair voids, and partial-height sections when they materially change the surface area.
Primer is not automatically included unless you model it as a separate coat estimate or choose a lower-coverage assumption.
Material cost excludes tax, labor, masking supplies, tools, delivery, cleanup, disposal, and any separate primer purchase.
The estimate is based on numbers entered in the page. No room photo, drawing, or paint label is read automatically.

Worked Examples:

Small rectangular room: A 4 m by 3 m room with 2.7 m walls, one door, two windows, two wall coats, 10% allowance, standard interior coverage, and 4 L tins produces about 37.8 sq m gross wall area and 33.1 sq m net wall area. Paint Takeoff shows roughly 72.8 sq m adjusted coat area and about 8.48 L exact paint volume, so Metric purchase rounds to 3 tins, or 12 L.

Feature wall: A wall run entered as 5 m long by 2.7 m high with no openings, two wall coats, and 10% allowance has 13.5 sq m gross wall area. Exact paint volume is about 3.46 L under the standard interior coverage assumption, and one 4 L tin leaves a modest spare amount.

Ceiling added late: The 12 ft by 14 ft room in the formula example rounds to 3 gal for walls only. Adding one ceiling coat increases adjusted coat area to about 990 sq ft and exact paint volume to about 2.83 gal. Imperial purchase still shows 3 gallons, but Coating Plan treats the spare as tight because the rounded overage is close to 6%.

Opening error: A 4 m wall run with 2.4 m height has only 9.6 sq m gross wall area. Entering 12 sq m of measured openings triggers the opening validation message because the excluded area is larger than the wall surface. Reducing the measured opening area below 9.6 sq m restores the takeoff rows.

FAQ:

Should I subtract every door and window?

Subtract openings that materially change the painted area. Use door and window counts for a quick room estimate, and use measured opening area for oversized windows, glass doors, built-ins, and unusual wall gaps.

Why is the purchase amount higher than the exact paint volume?

The exact volume is the calculated liquid need. The purchase amount rounds up to whole tins or whole gallons, so spare paint is expected unless the job happens to land near a container boundary.

Can I include primer?

Run primer as its own estimate using the primer coverage rate, or choose a lower-coverage profile when the first coat is likely to behave like primer on bare or porous material.

Why did the result panel disappear?

The result panel appears only after valid dimensions, coats, coverage, and opening deductions are entered. Check the red validation message first, especially for zero wall dimensions or an opening deduction larger than gross wall area.

Does the material cost include labor?

No. The cost row uses only the paint price entered in Advanced. It does not include primer unless estimated separately, and it excludes labor, tax, tools, supplies, delivery, masking, cleanup, and disposal.

Glossary:

Gross wall area: Wall run multiplied by wall height before openings are removed.
Net wall area: Gross wall area after doors, windows, and measured exclusions are subtracted.
Adjusted coat area: Coat area after the waste allowance is added.
Coverage rate: The one-coat spread rate used to convert adjusted coat area into liquid paint volume.
Waste allowance: Extra paint reserved for roller loading, cut-in work, texture, repairs, touch-ups, and measurement uncertainty.
Purchase rounding: The final step that rounds exact litres or gallons up to whole tins or gallons.

References:

Paint Calculator FAQs, Sherwin-Williams.
Interior & Exterior Paint Calculator, BEHR.
Regal Select Waterborne Interior Paint Specifications, Benjamin Moore.