Paint Calculator

How much paint do I need?

Start with the net paintable surface area, multiply that area by the number of coats, then divide by the coverage rate for the paint you plan to use. ProjectFigures performs that math in one canonical metric model and converts the result for display.

The built-in coverage value is an editable planning assumption. The product label or technical data for your actual paint and surface should take priority when it provides a specific coverage rate.

Measuring a rectangular room

For a rectangular room, wall area begins with the perimeter:2 × (length + width). Multiply the perimeter by wall height to get gross wall area, then subtract doors, windows or other openings.

Add multiple room sections when dimensions differ. If several rooms are genuinely identical, use the identical-room quantity instead of re-entering the same measurements.

Custom walls and known net area

Custom Walls mode is useful when a room is irregular or you only want to paint selected walls. Enter width × height for each rectangular wall and subtract any opening area that belongs to that wall.

Known Net Area mode is for a surface area you already calculated elsewhere. Enter the net area before coats; doors and windows should already have been deducted.

Doors and windows

Room mode offers two mutually exclusive deduction methods. Quick counts use editable planning assumptions of 20 ft² per door and 15 ft² per window. Those values are convenient estimates, not universal door or window sizes.

For more precise work, enter one combined excluded area per room. ProjectFigures never allows excluded openings to exceed gross wall area and never silently clamps a negative wall area to zero.

When should I include the ceiling?

The room ceiling uses length × width. Include it only when that ceiling is being estimated with the same finish-paint coverage rate and coat count as the walls in this calculation. When the ceiling uses a different product, coverage or coat count, calculate it separately.

How many coats should I enter?

ProjectFigures starts at two coats as a planning default, but the field is intentionally editable. Product, colour change, opacity, surface porosity, texture, preparation and application method can all change the real number of coats required.

Paint coverage and why it varies

Coverage is area per unit of paint: ft² per US gallon or m² per litre. The calculator starts at 375 ft² per US gallon per coat, exactly normalised to 9.203395030166686 m²/L internally. This is a ProjectFigures planning default, not a manufacturer guarantee.

Rough, porous or highly textured surfaces can use more paint. Large colour changes or different application methods can also affect practical coverage. Use the rate published for your actual product and surface whenever possible.

Extra paint and touch-ups

No extra percentage is added by default. If you want an allowance for touch-ups or practical project variation, enter it explicitly. The base calculated volume remains visible so the allowance is never hidden inside the result.

Calculated volume vs purchase planning

Mathematical paint requirement and store packaging are different questions. The primary result is the calculated paint volume. If you enter one real container size, ProjectFigures separately shows how many whole containers that size would require and the resulting purchase volume.

Package sizes differ by manufacturer, product and market, so V1 does not assume a universal size list and does not attempt multi-size package optimisation.

US gallons and litres

United States mode uses US gallons and US liquid quarts. Metric mode uses litres. The calculator does not use an Imperial gallon, including for UK users.

Paint vs primer

Primer is intentionally outside this V1 estimate. Primer and finish paint can have different coverage and coat requirements, so treating them as interchangeable could produce a misleading quantity.

Worked US example

A 12 ft × 10 ft room with 8 ft walls has 352 ft² of gross wall area. Subtract one 20 ft² door and two 15 ft² windows to get 302 ft². At two coats and 375 ft² per US gallon, the calculated requirement is about 1.61 US gallons before any optional extra allowance.

Worked metric example

If your net paintable area is 20 m², you choose two coats, and your paint is rated at 10 m²/L, the coated area is 40 m² and the mathematical requirement is 4 L before any optional extra allowance.

Common paint-estimating mistakes

• Using floor area as wall area without calculating wall perimeter and height.
• Subtracting openings twice or combining two opening methods.
• Assuming one coat for every colour and surface.
• Using a generic coverage rate when the chosen product lists another rate.
• Rounding the calculated volume too early instead of rounding only a purchase count.
• Including primer as though it has the same coverage as finish paint.

Frequently asked questions

Related tool

Estimating a contractor job as well as the material quantity? Continue to theContractor Job Pricing Calculator and use your own supplier paint cost in the wider pricing workflow.