How this site is built
Review the calculation methodology to see which figures are exact geometry, which depend on material assumptions, and which figures have recorded sources. The source registry records every cited figure and its review status.
How the paint calculator works
The calculator finds room perimeter, multiplies by wall height, subtracts opening area, and multiplies by the number of coats. Dividing by spread rate gives US gallons before whole-can rounding.
Spread rate is a product and surface property. Smooth primed drywall, bare masonry and textured siding should not share one confident default.
Formula used for paint calculator
The calculator keeps the measured quantity separate from any purchase rounding or allowance.
- Net wall area
2 × (length + width) × height − openings - Coated area
net area × coats × allowance - Paint
coated area ÷ product spread rate
Planning choices for paint calculator
| Situation | Use | Why it matters |
|---|---|---|
| Smooth primed wall | Use product rated coverage | Closest to data-sheet test conditions |
| Porous or textured wall | Use lower label range | Absorption reduces spread |
| Large colour change | Plan primer or extra coat | Coat count changes quantity directly |
These are measurement and ordering choices, not structural specifications or code requirements.
Measure and order paint
Measure openings as combined square feet. Do not deduct every small window automatically; cutting-in and touch-up work still use paint.
Use the technical data sheet for the exact colour and base where available, then round to available container sizes.
Paint Calculator questions
Should I include the ceiling?
Use the ceiling mode when it uses the same paint and coverage rate; keep it separate when the product or coat count differs.
Are US and Imperial gallons the same?
No. This calculator labels US gallons explicitly; an Imperial gallon is larger.
Which coverage rate should I use?
Use the spread rate printed on the exact paint product. Surface texture, porosity, colour change and application method can all reduce real coverage.
Variables and units
For the default “Room walls” mode. Other modes label their own fields in the calculator.
length- Room length: ft, in, yd, m, cm.
width- Room width: ft, in, yd, m, cm.
height- Wall height: ft, in, yd, m, cm.
openings- Doors and windows area: ft²; optional.
coats- Number of coats: unitless.
coverage- Coverage per gallon: ft²/US gal.
waste- Touch-up allowance: %; optional.
price- Price per gallon: unitless; optional.
Worked example: a 14 ft by 12 ft room
A 14 ft by 12 ft room has 8 ft walls, 40 ft² of openings, two coats and 350 ft² per gallon coverage.
| Field | Value |
|---|---|
| Room length | 14 ft |
| Room width | 12 ft |
| Wall height | 8 ft |
| Doors and windows area | 40 ft² |
| Number of coats | 2 |
| Coverage per gallon | 350 ft²/US gal |
| Touch-up allowance | 5 % |
| Result | Value |
|---|---|
| Whole gallons to buy | 3 US gal |
| Paint before rounding | 2.26 US gal |
| Net paintable area | 376 ft² |
These figures are generated by the same calculation the calculator above uses, so they update automatically if the method changes.
Where these numbers come from
| Assumption | Value used |
|---|---|
| Paint spread rate on smooth, primed drywall.Spread rate is a property of the specific product and is printed on the tin. It falls sharply on porous or textured surfaces; bare drywall, masonry and stucco can take half as much area per gallon. Enter the figure from your own product data sheet. Applicability: Launch-market starting value only. A future market must supply reviewed regional or product-specific provenance before overriding it. You can replace this with your own figure. | 350 ft²/US gal |
Sources
- Manufacturer technical data sheets (spread rate): Individual paint manufacturers
- Master Painters Institute architectural painting specification standards: Master Painters Institute
Content release:
Published by Material Totals. Review the methodology and source assumptions.
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