Measure the surfaces
Enter room length, width, and wall height for each room, or enter a known paintable surface area directly.
Paint Calculator: Calculate How Much Paint You Need
Quick Answer
Measure the length, width, and wall height of each room, then use the room perimeter × wall height to calculate wall area and length × width to calculate ceiling area. Subtract door and window openings from the wall area, add selected surfaces such as trim or doors, multiply the paintable surface by the number of coats, and divide the total coat area by the paint's coverage rate to determine the paint needed. The calculator can convert feet, inches, metres, centimetres, and yards, estimate gallons or litres, calculate containers to buy, account for surface texture and colour changes, include primer, and estimate material and labour costs.
A Paint Calculator helps you estimate the quantity of interior or exterior paint required before starting a painting project. The calculation uses measurable surfaces rather than guesswork, so the result can account for wall dimensions, ceiling area, doors, windows, trim, paint coverage, coats of paint, primer, and the selected paint container size.
To calculate paint for a room, measure the length (L), width (W), and wall height (H). For walls, calculate the room perimeter and multiply the perimeter by wall height. For a rectangular ceiling, multiply length by width. Enter multiple rooms separately when a project contains different room sizes, because each room can have its own dimensions and quantity.
Measure doors and windows when they are present because their opening areas can be deducted from the wall surface. Enter the number of doors and windows together with their width and height. Select ceilings, skirting and trim, or doors and frames when those surfaces need paint so the calculator includes the selected paintable areas in the final estimate.
Paint coverage is normally stated on the product label as an area covered per gallon or litre. Enter the manufacturer's spread rate, select the appropriate coverage unit, choose the number of coats, and account for surface texture or a significant colour change. The calculator then estimates the total paint volume and converts the result into the selected container size, such as gallons, litres, quarts, or larger paint pails.
For a more complete paint quantity estimate, you can include primer or sealer and enter optional paint price, primer price, preparation and repair cost, labour cost, and sundries. The result provides estimated paint volume, paintable area, area over all coats, effective coverage, containers to buy, primer requirements, and an optional project cost.
Walls · Ceilings · Coats
Paint Calculator
ft / in / m / cm / yd
Doors & windows
Gallons or litres
Estimated cost in $
Room size
Walls use the perimeter × Height (H); ceiling uses L × W
L
LengthFront wall to back wall
W
WidthLeft wall to right wall
H
HeightFloor up to ceiling
Input method
Surfaces to paint
Door and window areas are taken off the wall area automatically.
Paint & coverage
Spread rate comes off the tin label
Primer / sealer
Optional
Include primer
Paint to buy
0 cans
5 gal pails
Cost estimate ($)
Leave blank for a quantity-only result
How is your paint priced?
Paint sells by the can, gallon or litre; painters quote per ft² or m² of surface.
How the Paint Calculator Works
The Paint Calculator converts room measurements into paintable surface area, accounts for openings and selected surfaces, then applies the number of coats and paint coverage rate to estimate the required paint volume. The result can be shown in gallons and litres and converted into the selected paint container size.
Adjust the paint area
Select walls, ceilings, trim, doors, and frames, then deduct door and window openings from the wall area.
Calculate paint quantity
Apply coats, coverage rate, surface texture, colour change, primer requirements, and container size to calculate the final estimate.
Paint Calculator Formula
The calculation uses surface dimensions, opening deductions, coating requirements, and effective paint coverage to estimate the amount of paint required.
Core Paint Calculation
Use the formulas below to understand how the calculator converts measurements into paint quantity.
Wall Area
Wall Area = 2 × (Length + Width) × Height
For a rectangular room, calculate the perimeter from the length and width, then multiply the perimeter by wall height.
Ceiling Area
Ceiling Area = Length × Width
A rectangular ceiling uses the room length multiplied by the room width to determine the ceiling surface.
Opening Area
Opening Area = (Door Count × Door Area) + (Window Count × Window Area)
Door and window openings are calculated from their quantity, width, and height before being deducted from wall area.
Trim Area
Trim Area = Perimeter × Trim Band Height
When skirting or trim is selected, the calculator uses the room perimeter and the entered trim band height.
Paintable Area
Paintable Area = Wall Area − Openings + Ceiling Area + Selected Trim / Door Area
Only the surfaces selected in the calculator are included, while door and window openings are deducted from wall area.
Coat Area
Total Coat Area = Paintable Area × Number of Coats
Two coats on the same surface require twice the calculated single-coat surface area.
Effective Coverage
Effective Coverage = Base Coverage × (1 − Texture Loss − Colour Loss)
Surface texture and colour change reduce the effective spread rate used for the paint quantity estimate.
Paint Volume
Paint Needed = Total Coat Area ÷ Effective Coverage
The final surface area over all coats is divided by the adjusted paint coverage rate to determine required paint volume.
Final Working Formula
Paint Needed = [(Selected Paintable Area × Coats) ÷ Effective Coverage]
Paint Calculator Example
Here is a complete example using room dimensions, openings, paint coverage, coats, surface conditions, primer, and a 5-gallon paint container.
Example: One Rectangular Room
The following measurements demonstrate how a typical wall and ceiling paint calculation is built.
Room Length
20 ft
Room Width
15 ft
Wall Height
8 ft
Room Quantity
1 room
Doors
1 × 3 × 6.75 ft
Windows
2 × 3 × 4 ft
Paint Surfaces
Walls + Ceiling
Number of Coats
2 coats
Base Coverage
375 ft²/gal
Surface Texture
Smooth
Colour Change
Similar shade
Paint Container
5 gal pail
Room perimeter
2 × (20 + 15) = 70 ft
Gross wall area
70 × 8 = 560 ft²
Ceiling area
20 × 15 = 300 ft²
Door opening
1 × 3 × 6.75 = 20.25 ft²
Window openings
2 × 3 × 4 = 24 ft²
Total openings deducted
44.25 ft²
Net wall area
560 − 44.25 = 515.75 ft²
Total paintable area
515.75 + 300 = 815.75 ft²
Area over 2 coats
815.75 × 2 = 1,631.50 ft²
Effective coverage
375 ft²/gal
Paint required
1,631.50 ÷ 375 = 4.35 gal
Estimated Paint Requirement
About 4.35 gallons of paint
A 5-gallon pail provides enough volume for this example based on the entered coverage rate and two-coat requirement. Actual coverage can vary with application method, surface condition, paint formulation, substrate porosity, and manufacturer specifications.
Helpful Information Before Using the Paint Calculator
Accurate measurements and the correct product coverage rate produce a more useful paint quantity estimate.
Measure Every Room
Measure length, width, and wall height for each room when calculating from room dimensions. Enter each room separately when room sizes differ, because a single average measurement can distort the total paint estimate.
Use the Paint Label Coverage
Use the manufacturer's stated spread rate whenever possible. Coverage can be listed as square feet per gallon (ft²/gal), square metres per litre (m²/L), or another product-specific measurement.
Count Doors and Windows
Enter the number, width, and height of doors and windows when openings are present. Examples include one 3 × 6.75 ft door or two 3 × 4 ft windows. The calculator deducts the selected opening area from wall surface.
Choose the Correct Surfaces
Select walls, ceiling, skirting and trim, or doors and frames according to the surfaces that need painting. Selecting additional surfaces increases the paintable area and therefore increases the required paint quantity.
Account for Number of Coats
Enter the actual number of coats planned for the project. One coat, two coats, and three coats produce different total coating areas because the surface area is multiplied by the number of coats.
Consider Surface Texture
Smooth plaster or drywall generally requires less adjustment than textured surfaces. Light texture, heavy texture, render, and masonry can reduce effective coverage because more paint may be required to cover an uneven or porous surface.
Consider Colour Changes
A similar shade generally needs less coverage adjustment than a strong colour change. Light paint over a dark colour or a deep colour can require additional material or coats depending on the existing surface and product.
Plan for Primer or Sealer
Primer can be useful for new drywall, porous substrates, repaired areas, stains, significant colour changes, and surfaces requiring a suitable base coat. Enter primer coats and primer coverage separately when primer is part of the project.
Select the Container Size Carefully
Paint may be sold in different container sizes, including quarts, gallons, litres, 4-litre containers, 5-gallon pails, 10-litre containers, and 20-litre containers. Select the size that matches the product you plan to purchase.
Treat the Result as an Estimate
A paint quantity calculator provides an estimate rather than a guaranteed purchase quantity. Application method, roller or brush type, spray application, surface porosity, substrate condition, wastage, touch-ups, and manufacturer coverage can affect actual paint consumption.
Keep Units Consistent
The calculator supports feet (ft), inches (in), metres (m), centimetres (cm), and yards (yd) for dimensions, plus square feet (ft²), square metres (m²), and square yards (yd²) for known surface area. Use the unit that matches your measurements.
Add Cost Inputs for a Budget Estimate
Enter paint price, primer price, preparation and repair cost, labour rate, and sundries when a project budget is required. Cost estimates depend on the selected pricing method and the values entered for the specific project.
Related Calculators
Explore other home improvement calculators for planning materials, measurements, and project quantities.
Frequently Asked Questions About the Paint Calculator
Find answers to common questions about paint coverage, room measurements, coats, openings, primer, containers, and paint quantity estimates.
How does the Paint Calculator determine how much paint I need?
The Paint Calculator calculates the selected paintable surface area, multiplies that area by the number of coats, and divides the resulting coat area by the effective paint coverage rate. The calculation can include walls, ceilings, trim, doors, and frames while deducting selected door and window openings from wall area.
What measurements do I need for the Paint Calculator?
For a room-size calculation, enter length, width, and wall height. You can additionally enter door and window quantities, widths, and heights when openings need to be deducted. The calculator can handle multiple rooms, so different room dimensions can be entered separately.
What is the formula for calculating paint for walls?
The wall area formula is 2 × (Length + Width) × Height for a rectangular room. Door and window opening areas can then be deducted from the gross wall area before the remaining wall surface is combined with other selected paintable surfaces.
Does the Paint Calculator include the ceiling?
Yes. Select the ceiling surface to include it in the calculation. For a rectangular room, ceiling area is calculated as Length × Width.
Does the calculator subtract doors and windows from the wall area?
Yes. Enter the number, width, and height of doors and windows, and their calculated opening areas are deducted from the selected wall area. Examples include one 3 × 6.75 ft door or two 3 × 4 ft windows.
How many coats of paint should I enter?
Enter the number of coats planned for the surface, such as 1, 2, or 3 coats. Two coats double the total coating area compared with one coat, while three coats triple the coating area before coverage is applied.
What does paint coverage or spread rate mean?
Paint coverage, also called the spread rate, describes the surface area a given quantity of paint can cover. Common measurements include ft² per gallon (ft²/gal) and m² per litre (m²/L). Use the coverage figure provided on the paint manufacturer's label whenever possible.
Does surface texture affect how much paint I need?
Yes. Textured, rendered, or masonry surfaces can require more paint than smooth plaster or board because effective coverage can decrease. The calculator includes surface texture adjustments to account for this difference.
Does a colour change affect the paint estimate?
Yes. The calculator provides adjustments for a similar shade, light paint over a dark colour, and strong or deep colour changes. A substantial colour transition can require additional coverage or coats depending on the existing colour and paint product.
Can the Paint Calculator calculate primer as well as paint?
Yes. Enable the primer option, select the number of primer coats, enter the primer spread rate, and the calculator estimates primer volume and the number of selected containers required.
Can I calculate paint in litres instead of gallons?
Yes. The calculator displays the estimated paint volume in both gallons and litres, while the coverage input can use either ft² per gallon or m² per litre.
Can the Paint Calculator estimate the project cost?
Yes. The optional cost section can calculate paint, primer, preparation and repair, labour, and sundries when prices or rates are entered. The result provides an estimated total and an estimated cost per square foot.