How to use (3 steps)
- Pick Metric or US units, coats, coverage, and waste allowance.
- Use Room mode for length, width, height, doors/windows, and ceiling; use Area mode to type a known surface.
- Check the live results, set can size if needed, then copy the URL.
A sample room is prefilled so you see results instantly.
Settings
Typical interior wall paints cover around 8–12 m² per L.
Calculation mode
Door and window areas are subtracted automatically.
Area, paint, and cans refresh as you edit.
Results
Area, paint, and cans refresh as you edit.
If your openings or walls differ a lot from this simple model, increase the extra allowance or treat tricky areas as a separate run.
Interpretation (and a quick example)
How the estimate is computed
- Per coat area is your paintable surface for one coat (from room geometry or your known area).
- All coats total = per coat area × coats.
- Theoretical paint = all coats total ÷ coverage.
- With extra allowance adds your waste % (edges, roller texture, spillage, touch-ups).
- Cans rounds up to your chosen can size.
Mini example (area mode)
If your total area for all coats is 800 ft² and your paint covers 350 ft²/gal, the theoretical amount is about 2.29 gal. With 10% extra, that’s about 2.52 gal, so you’d buy 3 one-gallon cans.
When to increase the extra allowance
- Rough or porous surfaces (new drywall, textured walls, masonry).
- Spraying (overspray), lots of cutting-in/trim work, or frequent touch-ups.
- Changing from a dark to a light colour (often needs more coats).
Need more control (primer, substrate absorption, custom openings)? Try the Paint Coverage Calculator.
Related calculators
How to use this calculator effectively
This guide helps you use Interior paint calculator (simple) in a repeatable way: define a baseline, change one variable at a time, and interpret outputs with explicit assumptions before you share or act on results.
How it works
The page applies deterministic logic to your inputs and shows rounded output for readability. Treat it as a comparison workflow: run one baseline case, adjust a single parameter, and measure both absolute and percentage deltas. If a result seems off, verify units, time basis, and sign conventions before drawing conclusions. This approach keeps your analysis reproducible across teammates and sessions.
When to use
Use this page when you need a fast estimate, a classroom check, or a practical what-if comparison. It works best for planning and prioritization steps where you need direction and magnitude quickly before investing in deeper modeling, manual spreadsheets, or formal external review.
Common mistakes to avoid
- Changing multiple parameters at once, which hides the true cause of output movement.
- Mixing units (percent vs decimal, monthly vs yearly, gross vs net) across scenarios.
- Comparing with another tool without aligning defaults, constants, and rounding rules.
- Using rounded display values as exact downstream inputs without re-checking precision.
Interpretation and worked example
Run a baseline scenario and keep that result visible. Next, modify one assumption to reflect your realistic alternative and compare direction plus size of change. If the direction matches your domain expectation and the size is plausible, your setup is usually coherent. If not, check hidden defaults, boundary conditions, and interpretation notes before deciding which scenario to adopt.
See also
FAQ
What if my doors or windows are a different size?
This tool subtracts a standard door and window size. If yours differ a lot, adjust the counts or switch to the area mode with your measured surface.
How should I plan for multiple colors or finishes?
This calculator estimates a single paint. Split the area by color/finish, run the calculation for each, and round up cans separately.
What should I do first on this page?
Start with the minimum required inputs or the first action shown near the primary button. Keep optional settings at defaults for a baseline run, then change one setting at a time so you can explain what caused each output change.
Why does this page differ from another tool?
Different pages often use different defaults, units, rounding rules, or assumptions. Align those settings before comparing outputs. If differences remain, compare each intermediate step rather than only the final number.
How reliable are the displayed values?
Values are computed in the browser and rounded for display. They are good for planning and educational checks, but for regulated or high-stakes decisions you should validate assumptions with official guidance or professional review.