Paint Calculator

{{ summaryHeading }} {{ summaryPrimary }} {{ summaryLine }} Area {{ displayArea(values.adjusted_coat_area_m2) }} Coats {{ formatInteger(values.wall_coats) }} Spare {{ formatNumber(values.spare_percent, 1) }}%

Measurement basis:

Choose how the gross wall surface should be measured.

Room length:

First room dimension used to calculate the perimeter.

Room width:

Second room dimension used to calculate the perimeter.

Combined wall run:

Total horizontal wall length before multiplying by height.

Wall height:

Vertical wall surface receiving paint.

Opening deduction:

Subtract surfaces that will not receive wall paint.

Doors:

Count full-size door openings excluded from wall paint.

doors

Windows:

Count window openings excluded from wall paint.

windows

Opening area:

Combined unpainted opening area.

Wall coats:

Finish coats applied to the net wall area.

coats

Product coverage:

One-coat coverage for the selected paint and surface.

Waste allowance:

Ten percent is a practical planning default; verify difficult surfaces and application methods.

Container size:

Purchase package used for whole-container rounding.

Include ceiling:

Uses room length × width; unavailable for a wall-run-only measurement.

Ceiling coats:

Applied only while the ceiling option is enabled.

coats

Price per container:

Paint-only subtotal; excludes tax, labor, primer, tools, and supplies.

Measure	Value	Planning use	Copy
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Buying guidance

Coverage assumption

{{ displayCoverage(values.coverage_m2_per_l) }} is the one-coat rate used for every painted surface.

Confirm the product label, surface porosity, application method, primer plan, and color change before ordering.

Container rounding

{{ displayVolume(values.exact_liters) }} exact volume rounds to {{ formatInteger(values.purchase_containers) }} containers, or {{ displayVolume(values.purchased_liters) }} purchased.

The calculated spare is a paper allowance; opened containers, tinting, and return policies can limit how excess paint is reused.

Order boundary

This estimate covers wall paint and the selected ceiling coats only.

It excludes separate primer, trim and door paint, tax, labor, masking, tools, delivery, cleanup, and disposal.

Buying paint is a surface-area problem before it is a container-count problem. Wall length and height establish the gross area, openings remove unpainted sections, and every additional coat repeats the net area. Coverage and waste then turn that coated area into liquid volume.

A room's floor area is not its wall area. For a rectangular room, the wall run is the perimeter of the floor plan, and multiplying that run by wall height gives the gross wall surface. Irregular rooms are usually easier to measure as one combined horizontal wall run. Ceilings are separate because they use floor-plan area and may need a different number of coats.

Coverage rate: The one-coat area a litre or US gallon is expected to cover on the selected surface.
Waste allowance: Extra volume for roller loading, cutting in, texture, repairs, touch-ups, and measurement uncertainty.
Exact volume: The calculated liquid requirement before package size is considered.
Purchased volume: The whole-container quantity bought after exact volume is rounded up.

Coverage printed on a product sheet is a planning rate, not a guarantee. Porous or textured surfaces, major color changes, application method, primer use, and product choice can change the real yield. Measure carefully and use the selected paint's stated coverage before ordering.

How to Use This Tool:

Choose the measurement method that matches the room shape, then keep the units attached to each value.

Select Room dimensions for a rectangular room or Combined wall run when wall segments have already been added.
Enter wall height and subtract openings by standard door and window counts or by a measured total area. Use measured openings for unusual sizes.
Set wall coats and the paint's one-coat Product coverage. Add a waste allowance that reflects the surface and application method.
Open Advanced to choose the container size, include a rectangular ceiling, or enter an optional price per container.
Use Paint takeoff to check net wall area, exact volume, whole containers, and spare volume before buying.

Interpreting Results:

Exact paint volume is the formula result; Purchase containers is the buying result. The difference appears as spare paint, so a large spare percentage may simply mean the chosen container size is coarse relative to the project.

The waste scenarios show how reserve assumptions affect exact volume and whole-container count. A higher allowance may leave the container count unchanged when both values still fall inside the same package boundary. Confirm the product coverage and scope before treating that flat step as evidence that no extra paint is needed.

Technical Details:

All lengths are converted to metres, areas to square metres, coverage to square metres per litre, and container volume to litres. Opening deductions are removed once from gross wall area before coat multiplication. The ceiling is included only when room dimensions are used.

Formula Core:

The calculation moves from wall run to net area, coated area, adjusted area, exact litres, and finally whole containers.

\begin{array}{rcl} P & = & 2 \times (L + W) or the measured wall run \\ A_{net} & = & (P \times H) - A_{openings} \\ A_{coated} & = & (A_{net} \times C_{walls}) + (A_{ceiling} \times C_{ceiling}) \\ V_{exact} & = & \frac{A_{coated} \times (1 + w)}{Coverage} \\ Containers & = & ⌈ \frac{V_{exact}}{V_{container}} ⌉ \end{array}

w is the waste percentage expressed as a decimal. Count-based opening deductions use 1.9 m² per door and 1.4 m² per window as planning defaults. Measured opening area replaces those defaults. Openings must remain smaller than gross wall area, wall and ceiling coats must be whole numbers from 1 to 6, and the container count always rounds upward.

Paint calculation units and conversions
Quantity	Canonical unit	Conversion used
Length	metre	1 ft = 0.3048 m
Area	square metre	1 m² = 10.7639104167 ft²
Volume	litre	1 US gal = 3.785411784 L
Coverage	m²/L	ft²/gal is converted before division

Limitations and Accuracy Notes:

The estimate covers wall paint and an optional rectangular ceiling only. Primer, trim, doors, cabinetry, labor, masking, tools, tax, delivery, cleanup, and disposal are excluded. Count-based openings are approximate, and opened or custom-tinted containers may not be returnable even when the calculation shows substantial spare paint.

Worked Examples:

Rectangular room with counted openings

A 4 m by 3 m room with 2.7 m walls has a 14 m perimeter and 37.8 m² of gross wall area. Subtracting one standard door and two standard windows leaves 33.1 m². Two coats plus 10% waste create 72.82 m² of adjusted coat area. At 8.6 m²/L, the exact need is about 8.47 L, which rounds up to three 4 L containers and 12 L purchased.

References:

SI units: volume, National Institute of Standards and Technology.
How to use a paint calculator, Sherwin-Williams.