🎨 Paint Calculator

Calculate exactly how much paint you need for a room. Enter room dimensions, subtract doors and windows, choose number of coats, and get gallons or litres needed along with cost estimate.

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Doors

Windows

Coats

Coverage (sq ft/gal)

Can Size (gallons)

Price per can ($) opt.

What is a Paint Calculator?

A paint calculator estimates the quantity of paint needed to cover a given surface area, accounting for the number of coats, the coverage rate of the paint, and deductions for windows and doors. Buying the right amount of paint is more important than it might seem: too little forces a mid-project trip to the store and risks a colour match failure if the original batch is discontinued; too much wastes money and creates disposal challenges, as leftover paint cannot simply be poured down the drain due to environmental regulations.

Paint coverage rate — the area one litre or gallon covers at the recommended spreading rate — varies by paint type, brand, and surface porosity. Standard interior latex paints typically cover 350–400 square feet per gallon (roughly 8.5–9.5 m² per litre) for smooth surfaces in good condition. Rough, porous, or previously unpainted surfaces absorb more paint and may reduce coverage to 250–300 sq ft per gallon. Primers are typically applied at lower coverage rates, and deep or vivid colours may require an extra coat compared to lighter neutrals.

Professional painters typically add a 10–15% overage allowance to calculated quantities to account for touch-ups, waste, and slight variations in application technique and surface condition. For large projects, it is also worth considering the finish — matte and eggshell finishes generally cover better than satin or semi-gloss, which may need an additional coat on previously painted surfaces of a different sheen level. This calculator helps homeowners and contractors determine the right number of gallons (or litres) to purchase before starting any interior or exterior painting project.

How the Paint Calculator Works

Formula, assumptions, and calculation steps for this engineering tool.

Methodology

Engineering calculators apply standard unit conversions and formula relationships after normalizing measurements to compatible units.

Calculation Steps

Enter dimensions, loads, rates, or electrical values.
Convert the inputs into the formula unit system.
Apply the engineering equation or conversion factor.
Return the result with units and supporting context.

Assumptions and Limits

Material behavior is assumed ideal unless fields specify otherwise.
Code checks, safety factors, and site conditions may require professional review.
Use a qualified engineer for design-critical decisions.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most interior latex paints cover 350–400 square feet per gallon on smooth, primed surfaces. On rough or porous surfaces (bare drywall, textured walls, older surfaces), coverage can drop to 250–300 sq ft/gallon. Always check your specific paint label for its listed coverage rate.

How many coats of paint do I need?

Two coats are standard for most interior painting projects, especially when changing colors or painting over a dark shade. One coat may suffice if you are painting the same color. Three coats are sometimes needed for highly saturated colors (reds, yellows) or when painting over a very dark color with a light one.

Should I include the ceiling in my paint calculation?

This calculator covers walls only. For ceilings, calculate the floor area (length × width) and add it to your paint needs. Ceiling paint is usually a separate product — white flat finish — and is sold separately.

Should I prime before painting?

Priming is recommended for: bare drywall, major color changes (dark to light), stained or repaired surfaces, and new plaster. Primer is typically cheaper than paint and improves adhesion and coverage, often allowing you to use one less coat of finish paint.

Real-World Applications

🏠

Interior Room Repainting

Calculate gallons for repainting a bedroom — measuring wall area, deducting windows and doors, selecting two coats for a colour change, and adding 10% overage for touch-ups and future repairs.

🏡

Exterior House Painting

Estimate paint for the full exterior — total wall area minus windows and doors, adjusted for the lower coverage rate of exterior surfaces (rough siding, brick, stucco absorbs more paint than smooth interior walls).

🎨

Interior Designer Colour Sampling

Calculate the minimum quantity of each colour required for a multi-room project — preventing mid-project shortage, which risks colour batch mismatch when a new tin is opened from a different production run.

🏗️

Construction Contractor Bidding

General contractors use paint quantity estimates to price painting subcontracts accurately — material cost is a significant component of a painting quote, and systematic calculation prevents underestimation.

🏢

Commercial Office Refurbishment

Facilities managers calculate paint requirements for large open-plan offices and corridors — where multiple colours, feature walls, and ceiling finishes must each be separately quantified and ordered.

🧱

Masonry and Concrete Coating

Bare masonry, concrete block, and unpainted brick have significantly lower coverage rates than pre-painted surfaces — this distinction is critical for first-coat application on new construction where absorption is highest.

Common Mistakes

Not deducting windows and doors from wall area

A standard door is approximately 20 sq ft; a standard window is approximately 15 sq ft. Failing to deduct these from total wall area inflates the paint quantity estimate — potentially requiring an unnecessary extra tin, especially in room with many windows.

Using the coverage rate from the label for all surfaces

Label coverage rates (typically 350–400 sq ft/gallon) assume smooth, previously painted surfaces in good condition. Textured walls, bare drywall, raw wood, and masonry all absorb more paint — reducing effective coverage to 250–300 sq ft/gallon.

Not adding a buffer for touch-ups and future repairs

Without a buffer, buying exactly the calculated quantity risks running short — and reformulated or batch-matched paint rarely matches the original colour perfectly months later. Professional practice is to add 10–15% and store the remainder for touch-ups.

Underestimating coats required for dark or vivid colours

Deep, saturated colours (navy, forest green, burgundy) typically require 3 coats over a white or light base to achieve full, even colour. Planning for only 2 coats results in a short purchase that becomes apparent only after the first coat is applied.

Forgetting to measure ceiling area for ceiling paint

Ceiling paint is often a separate product and should be calculated separately. A 12×15 ft room has 180 sq ft of ceiling area — at 400 sq ft/gallon coverage, one gallon is typically sufficient for a ceiling, but must be budgeted separately from the wall paint.

Paint Coverage by Surface Type

Surface Type	Coverage (sq ft/gal)	Typical Coats
Smooth interior wall (repaint)	350–400	1–2 coats
Bare/new drywall	250–300	1 primer + 2 coats
Smooth exterior (wood/vinyl)	300–350	2 coats
Rough exterior siding	200–250	2–3 coats
Masonry / concrete block	150–200	1 filler + 2 coats
Interior ceiling	350–400	1–2 coats

References

Sherwin-Williams. Paint Coverage Calculator & Buying Guide. sherwin-williams.com, 2024.
The Spruce. How to Calculate How Much Paint You Need. thespruce.com, 2024.
US EPA. Proper Disposal of Latex and Oil-Based Paints. epa.gov, 2024.
Painting and Decorating Contractors of America. Production Rate Manual. PDCA, 2019.
Bob Vila. Estimating Paint: A Room-by-Room Guide. bobvila.com, 2024.