Understand the estimate
Formula and assumptions
Net paint area = wall perimeter × height + selected ceilings − doors − windows. Paint volume = net area × coats ÷ stated coverage, then the waste allowance is applied before rounding to whole containers.
Net paint area = wall perimeter × height + selected ceilings − doors − windows. Paint volume = net area × coats ÷ stated coverage, then the waste allowance is applied before rounding to whole containers.
Assumptions to confirm
- Every room is rectangular.
- All walls in a room use the same height.
- Coverage is entered per gallon or litre and should match the product label.
- Doors and windows are treated as unpainted openings.
Label every measurement and keep exclusions visible. Add the shapes in the calculator instead of hiding them inside one total.
Worked example
Input
A 12 × 10 × 8 ft room, one 21 sq ft door, two 15 sq ft windows, two coats, 400 sq ft/gal coverage and 10% waste.
Planning result
Net walls are 301 sq ft. Two coats with waste require 1.66 gal, so the shopping quantity is 2 one-gallon cans.
Common package sizes
Package conventions vary by brand and region. Enter the usable coverage or yield shown on the actual product instead of relying on the name or weight alone.
- 1 qt / 0.95 L sample or trim cans
- 1 US gal / 3.78 L standard cans
- 5 US gal / 18.9 L pails
Frequently asked questions
Should I include the ceiling?
Only enable the ceiling for rooms where it will receive the selected paint. Its area is length × width.
Why does the calculator round up?
Paint is bought in discrete containers. The exact volume is shown, but the shopping quantity rounds up to avoid a shortfall.
Does primer use the same coverage?
Not always. Enter the primer coverage printed on its label; porous or patched surfaces may use more.
Related measurement guides
How to measure a room for paint
Measure wall perimeter, height, ceilings, doors, and windows for a paint estimate you can audit.
How much paint does a gallon cover?
Understand label coverage, surface porosity, colour changes, and why exact paint volume differs from cans to buy.
Do I need one coat or two?
Choose a realistic coat count based on colour change, surface condition, sheen, and product instructions.
How to subtract doors and windows
Decide which openings are worth subtracting and keep trim calculations separate.
Interior painting shopping checklist
Build a practical paint-day list covering preparation, protection, application, cleanup, and safety.