🎨 Paint Calculator

What This Calculator Does

This calculator works out how much paint a room needs. Enter the room's length, width, and wall height, tell it how many doors and windows the room has and how many coats you plan, and it returns the paintable wall area and the paint required in liters and gallons. It turns the most error-prone part of a painting project — buying the right amount — into a thirty-second calculation.

Guessing paint quantity goes wrong in both directions: run short mid-wall and you face a store run with wet edges drying; overbuy and money sits in cans in the garage. A measured calculation, minus the openings, times the real number of coats, gets it right.

How Paint Quantity Is Calculated

The math is simple and worth understanding. The wall area of a rectangular room is its perimeter — 2 × (length + width) — multiplied by the wall height. From that, subtract the openings you will not paint: a standard door is about 1.6 m² and an average window about 1.4 m², so a room with one door and two windows loses roughly 4.4 m² of paintable area. What remains is the true surface per coat.

Multiply by the number of coats — two is the standard for an even, durable finish — and divide by the paint's coverage. Typical emulsion covers roughly 10 m² per liter (about 350-400 sq ft per gallon), though the can states the real figure for your product. The result is the liters (or gallons) to buy. A worked example: a 4 × 3 m room with 2.4 m walls has 33.6 m² of wall; minus a door and window (≈3 m²) is 30.6 m²; two coats is 61.2 m²; at 10 m²/L, just over 6 liters.

Buying and Planning Wisely

Add a margin. Real-world coverage runs below the can's optimistic figure on textured, porous, or previously unpainted surfaces — fresh plaster in particular drinks paint — so buying about 10% extra is the experienced-painter default. Leftover paint from the same batch is genuinely valuable: it guarantees perfect color-matched touch-ups for years, where a can from a different batch can dry to a subtly visible difference. Slightly over is the correct side to err on.

A few planning details sharpen the estimate further. Dramatic color changes — especially light over dark — may need a third coat or a tinted primer, so budget for it when relevant. Ceilings are a separate calculation (length × width) with their own paint. Trim, doors, and window frames use different paint in small quantities, typically a liter or less for a room. And check the coverage number on your actual chosen paint rather than assuming the average, since products range meaningfully. With measured walls, honest coats, and a small margin, you will buy once and finish the job — which is the whole point.

Primer, Sheen, and Surfaces: Getting Coverage Right

The coverage number on the can assumes a smooth, sealed, similar-colored surface — and real walls frequently are not. Fresh drywall and bare plaster are porous and drink the first coat, cutting its coverage dramatically; a coat of primer, which is cheaper than paint, seals the surface so the paint above it covers as rated. Dramatic color changes are the other primer case: going light over dark can otherwise demand three or four coats, where a tinted primer plus two coats gets there cleaner and cheaper. Textured surfaces — orange peel, knockdown, brick — have more actual surface area than their measured footprint and can consume 15–25% more paint than the flat-wall math suggests. Fold these in and the calculated liters stop being optimistic.

Sheen choice affects both the look and the practical result. Flat and matte hide wall imperfections best and suit ceilings and low-traffic rooms, but mark easily; eggshell and satin balance washability with a soft look and are the usual choice for living spaces; semi-gloss and gloss stand up to scrubbing and moisture, earning their place on trim, doors, kitchens, and bathrooms — though their shine highlights every surface flaw, so preparation matters more. Buy all the paint for a room at once and have the store shake cans from the same batch, since dye lots can differ subtly. With the area from this calculator, a primer decision made honestly, and a 10% margin, the store trip happens exactly once — which is the mark of a well-planned paint job.

Quick Tips

• Wall area = perimeter × height, minus about 1.6 m² per door and 1.4 m² per window.
• Plan two coats as standard; light over dark may need three or a tinted primer.
• Typical coverage is ~10 m² per liter — but check your paint's can.
• Buy ~10% extra; same-batch leftovers make perfect touch-ups later.

Frequently Asked Questions

How much paint do I need for a room?

Calculate the wall area (perimeter x height, minus doors and windows), multiply by coats, and divide by the paint's coverage — commonly around 10 m² per liter (350-400 sq ft per gallon). The calculator does all of this from your room's measurements.

How many coats of paint do I need?

Two coats is the standard for a solid, even finish. One coat only suffices when repainting the same or a similar color; covering a dark color with a light one may need a third coat or a tinted primer.

Do I subtract doors and windows?

Yes — they can be a meaningful share of the wall area. Typical figures: about 1.6 m² (17 sq ft) per door and 1.4 m² (15 sq ft) per average window. The calculator subtracts standard sizes for the counts you enter.

Should I buy extra paint?

Buy about 10% more than the calculation. Real coverage varies with surface texture and porosity, and leftover paint from the same batch is invaluable for touch-ups later — a slightly different batch can show a visible color shift.

Practical, Instant, and Free

This calculator runs entirely in your browser, doing the area and coverage math the moment you type. It needs no account or download and works on any device. Like every tool here, it is free to use as often as you like.

Related Tools

For more practical math, the length converter switches meters and feet, the percentage calculator handles the 10% margin, and the ratio calculator scales quantities.