Paint Needed Calculator: How Much Paint Do I Need?

This calculator works out how much paint you need for a room or surface. You enter the total area to be painted (in square feet or square meters), the number of coats, and the paint coverage rate (usually printed on the can), and it returns the required paint volume in liters and gallons. It also shows the total area per coat so you can double-check your measurements.

Formula

The basic formula is:

Paint needed (L) = (Total surface area × Number of coats) ÷ Coverage per liter

Where:

• Total surface area – the sum of all wall/ceiling areas you plan to paint, in square feet (or square meters).
• Number of coats – how many layers you will apply (usually 2 for a color change, 1 for a touch-up).
• Coverage per liter – the area one liter of paint covers, typically 300–400 sq ft/L (or 10–12 m²/L) for most interior paints. This number is printed on the can.

To convert liters to US gallons, divide by 3.78541. For imperial gallons, divide by 4.54609.

Worked Example

Using the default values from the calculator above:

• Total surface area = 400 sq ft
• Number of coats = 2
• Coverage per liter = 350 sq ft/L

Step 1: Multiply area by coats → 400 × 2 = 800 sq ft (total area to cover).

Step 2: Divide by coverage → 800 ÷ 350 = 2.2857 L.

Step 3: Convert to gallons → 2.2857 ÷ 3.78541 = 0.604 gal.

So the calculator returns 2.29 L (or 0.60 US gal).

When to Use This Calculator

Use this tool when you are planning any painting job – a single room, a whole house, a fence, or a piece of furniture. It is especially useful before buying paint, so you do not overbuy (waste money) or underbuy (make an extra trip).

Typical situations:

• Painting a bedroom – you measure the four walls and ceiling.
• Repainting a kitchen cabinet – you measure the front and side surfaces.
• Painting a deck – you measure the horizontal surface area.

For walls with windows and doors, subtract those areas from the total. The calculator assumes you are covering a flat, dry, primed surface. If you are painting over dark colors, bare drywall, or rough textures, you may need an extra coat.

How to Measure Your Room Accurately

Getting the area right is the most important step. Here is a simple method:

1. Measure the length and height of each wall in feet (or meters). Multiply length × height for each wall.
2. Add all wall areas together.
3. Measure the ceiling length × width, and add that if you are painting the ceiling.
4. For each door (approx. 21 sq ft / 2 m²) and window (approx. 12 sq ft / 1.1 m²), subtract that area from the total.

Example: A 12 ft × 10 ft room with an 8 ft ceiling has two walls of 12×8=96 sq ft each and two walls of 10×8=80 sq ft each. Total walls = 96+96+80+80 = 352 sq ft. Ceiling = 12×10 = 120 sq ft. If you paint walls and ceiling, total = 472 sq ft. Subtract one door (21) and one window (12) → 439 sq ft. With 2 coats and 350 sq ft/L coverage, you need 439×2/350 = 2.51 L.

Common Mistakes and How to Avoid Them

• Forgetting the second coat – Most paints require 2 coats for even color. Always multiply by the number of coats.
• Ignoring the coverage rate – Cheap paints often have lower coverage (300 sq ft/L) than premium ones (400 sq ft/L). Check the can.
• Mixing units – If you measure in meters, use m² and coverage in m²/L. Do not mix feet with liters.
• Not subtracting windows/doors – These areas do not need paint (unless you are painting the trim).
• Rounding down – Always round up to the nearest can size. It is better to have a little extra for touch-ups.

Paint Coverage: Why It Varies

Coverage per liter depends on several factors:

• Paint type – Primer, flat, eggshell, satin, semi-gloss, and high-gloss all have different thicknesses and solids content.
• Surface porosity – Bare drywall and unpainted wood absorb more paint, reducing coverage.
• Texture – Rough surfaces (stucco, brick) require more paint than smooth surfaces.
• Application method – A sprayer wastes more paint than a roller (overspray), while a brush uses more than a roller per square foot.
• Color change – Going from dark to light (or vice versa) may need an extra coat.

Always read the label for the manufacturer’s recommended coverage, and if you are unsure, buy 10% more than the calculator says.

How to Buy the Right Amount of Paint

Paint is sold in standard can sizes: 1 liter, 2.5 liters, 5 liters, and sometimes 10 liters (in metric countries) or 1 quart, 1 gallon, and 5 gallons (in the US). After the calculator gives you a number, round up to the nearest available size.

For example, if you need 2.29 L, you would buy a 2.5 L can (or a 1-gallon can in the US, which is 3.785 L). If you need 5.2 L, buy a 5 L can plus a 1 L can, or a 2.5 L + 2.5 L + 1 L.

A good rule of thumb: add 10% to the calculated amount to account for spillage, uneven application, and future touch-ups. This is especially important if you are mixing paint colors – you cannot exactly match a custom color later.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

Assuming an 8-foot ceiling, the wall area is 4 walls × (12×8) = 384 sq ft. Subtract one door (21) and one window (12) = 351 sq ft. For 2 coats, you need 351×2/350 = 2.0 L (about 0.53 gal). If you also paint the ceiling (144 sq ft), add 144×2/350 = 0.82 L, for a total of 2.82 L (about 0.75 gal).

How many square feet does a gallon of paint cover?

A US gallon is 3.785 liters. With typical coverage of 350 sq ft/L, one gallon covers about 1325 sq ft per coat. However, most manufacturers state coverage per gallon directly on the can, often 350–400 sq ft per gallon for one coat. Always check the label.

Do I need to use primer before painting?

Primer is recommended for bare drywall, bare wood, or when covering a dark color with a light one. Primer improves adhesion and reduces the number of topcoats needed. It has similar coverage to paint, so you can use the same calculator for primer, but note that primer often covers less (around 250–300 sq ft/L).

What if I am painting a textured surface like brick or stucco?

Textured surfaces have more surface area than a smooth wall, so coverage drops significantly. You may need 20–50% more paint. It is best to apply a test patch to measure actual coverage, or consult the paint manufacturer’s guidelines for masonry surfaces.

Can I use this calculator for exterior paint?

Yes, the same formula applies. For exterior walls, measure the total wall area and subtract windows and doors. However, exterior surfaces (wood siding, stucco, brick) are more porous and may require a primer and an extra coat. Also, check the coverage rate on the exterior paint can, as it may differ from interior paint.

Is it better to buy extra paint or risk running out?

Always buy a little extra – at least 10% more than the calculated amount. Running out mid-project is inconvenient, and matching a custom color later is nearly impossible. Leftover paint can be stored for touch-ups, and many recycling centers accept unused paint.

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in area | Total surface area | sq ft | 400 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Coverage per liter | sq ft/L | 350 | 10
out paint_l | Paint needed | L | (area * coats) / coverage | 2
out paint_gal | Paint needed | US gal | ((area * coats) / coverage) / 3.78541 | 2