Paint Calculator

Estimate wall paint litres, cans and material cost from room length, width, height, openings, coats, coverage rate and waste allowance, with a printable job note.

Paint required39.2 m2 net wall area x 2 coats = 78.4 m2 coated area; 7.84 L before 10% allowance; 3 cans at 4 L.

Room lengthmRoom widthmWall heightmDoors/windows to subtractm²Number of coatsPaint coveragem²/L per coat

Live math canvas

Your numbers, formula and explanation together

Paint Calculator: 8.62 L paint. 39.2 m2 net wall area x 2 coats = 78.4 m2 coated area; 7.84 L before 10% allowance; 3 cans at 4 L.

Formula applied

The exact method behind this answer

CalculationTime keeps the method visible so the number can be checked instead of blindly trusted.

Wall area = 2 × (room length + room width) × wall height. Net paint area = max(0, wall area − openings). Coated area = net paint area × coats. Paint litres = coated area ÷ coverage per litre. Order litres = paint litres × (1 + waste percent ÷ 100). Cans = ceiling(order litres ÷ can size).

Apply the formulaWall area = 2 × (room length + room width) × wall height. Net paint area = max(0, wall area − openings). Coated area = net paint area × coats. Paint litres = coated area ÷ coverage per litre. Order litres = paint litres × (1 + waste percent ÷ 100). Cans = ceiling(order litres ÷ can size).8.62 L paint39.2 m2 net wall area x 2 coats = 78.4 m2 coated area; 7.84 L before 10% allowance; 3 cans at 4 L.

Your live breakdown

Current inputs in the calculation

These values come from the controls above and update when the calculator changes.

Room length: 5 m
Room width: 4 m
Wall height: 2.4 m
Doors/windows to subtract: 4 m²
Number of coats: 2
Paint coverage: 10 m²/L per coat
Waste/touch-up allowance: 10 %
Paint can size: 4 L

Resulting answer

8.62 L paint

39.2 m2 net wall area x 2 coats = 78.4 m2 coated area; 7.84 L before 10% allowance; 3 cans at 4 L.

Answer: 8.62 L paint
Live support: 39.2 m2 net wall area x 2 coats = 78.4 m2 coated area; 7.84 L before 10% allowance; 3 cans at 4 L.

Assumptions used

What this answer assumes

Best for homeowner shopping lists, painter quote notes, rental make-ready records, classroom area examples and job files where wall area, coverage rate and can rounding need to be visible.

Room dimensions are entered in metres and paint coverage is entered as square metres per litre per coat.
The calculator estimates vertical wall paint for a rectangular room or wall run; ceilings, trim, doors, cabinets and exterior surfaces should be calculated separately.
Openings are subtracted before coats and waste are applied. Small cutouts may not reduce real purchase quantity because edging and touch-ups still consume paint.
Coverage depends on paint type, colour change, surface texture, porosity, application method and manufacturer data. The entered coverage rate should come from the paint label or data sheet when possible.
The result is an estimating aid for shopping, quotes and records. It does not choose primer, surface preparation, coating system, ventilation or safety requirements.

Master’s Tip

How to use the result well

Master’s Tip: do not hide the coverage rate. Paint labels often give a best-case range, and porous plaster, strong colour changes, textured walls or spray application can move the real litres. Print the rate and waste allowance beside the result before buying.

Printable record

What belongs in the saved calculation

Save the inputs, result, formula, assumptions, page URL and date together so the calculation can be reviewed later.

Room length: 5 m
Room width: 4 m
Wall height: 2.4 m
Doors/windows to subtract: 4 m²

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Embed this calculator

Copy a clean iframe version with the required CalculationTime attribution link built in.

Formula

Wall area = 2 × (room length + room width) × wall height. Net paint area = max(0, wall area − openings). Coated area = net paint area × coats. Paint litres = coated area ÷ coverage per litre. Order litres = paint litres × (1 + waste percent ÷ 100). Cans = ceiling(order litres ÷ can size).

Worked example

For a 5 m by 4 m room with 2.4 m walls, gross wall area = 2 × (5 + 4) × 2.4 = 43.2 m². Subtract 4 m² of openings to get 39.2 m². Two coats cover 78.4 m². At 10 m²/L, base paint is 7.84 L. With 10% allowance, order about 8.62 L, or 3 cans if cans are 4 L each.

Professional note

Regional and unit assumptions

Standard or basis: transparent rectangular wall-area arithmetic using metric dimensions, user-entered paint coverage in m²/L per coat, upward can rounding and a separate waste/touch-up allowance. Manufacturer instructions and local safety guidance override generic estimates.

Assumptions and limitations

Room dimensions are entered in metres and paint coverage is entered as square metres per litre per coat.
The calculator estimates vertical wall paint for a rectangular room or wall run; ceilings, trim, doors, cabinets and exterior surfaces should be calculated separately.
Openings are subtracted before coats and waste are applied. Small cutouts may not reduce real purchase quantity because edging and touch-ups still consume paint.
Coverage depends on paint type, colour change, surface texture, porosity, application method and manufacturer data. The entered coverage rate should come from the paint label or data sheet when possible.
The result is an estimating aid for shopping, quotes and records. It does not choose primer, surface preparation, coating system, ventilation or safety requirements.

Methodology & Accuracy

How this calculator is checked

CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.

Formula used

Standard or basis

Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.

Master's Tip

Questions

How do I calculate how much paint I need?

Calculate wall area, subtract large openings, multiply by the number of coats, divide by the paint coverage rate, then add a waste or touch-up allowance.

How many litres of paint do I need for a room?

It depends on room size, wall height, openings, coats and coverage. A 5 m by 4 m room with 2.4 m walls, 4 m² of openings, two coats and 10 m²/L coverage needs about 8.62 L with a 10% allowance.

Should I subtract doors and windows?

Subtract large doors, windows or skipped sections when they materially reduce painted area. Small openings may not reduce the number of cans because cutting-in and touch-ups still use paint.

What coverage rate should I use?

Use the coverage rate from the paint tin or manufacturer data sheet when possible. If you do not know it, 10 m² per litre per coat is a common planning value, but real coverage varies by product and surface.

What should I print for a paint quote or shopping note?

Print room dimensions, openings, coats, coverage rate, waste allowance, litres, rounded cans, optional price, formula and notes about surface condition or colour change.

Calculation note

Paint estimating turns room geometry into liquid quantity, but the real-world variable is coverage. A useful paint record keeps surface area, coats, manufacturer coverage, waste and can rounding separate instead of hiding them inside one unexplained number.

Area comes before litres

Paint is usually specified by coverage: how many square metres one litre covers for one coat. That means the room must first be reduced to a net wall area before litres can be estimated.

Coats multiply the work area

Two coats do not change the room size, but they double the coated area. Primer, undercoat or stain blocking should be planned separately when the product system requires it.

Coverage is a product claim, not a universal constant

Smooth sealed walls can use less paint than porous plaster, masonry or textured surfaces. The calculator therefore makes coverage editable and prints it beside the answer.

Rounded cans are part of the job note

Paint is purchased in tin sizes, not exact decimal litres. Rounding litres up to cans keeps the shopping list practical while preserving the calculated litres for checking.

Sources and further readingDulux UK — paint calculator and coverage guidance Sherwin-Williams — paint calculator and estimating guidance NIST: SI Units — area and length context