Work out how much paint a room needs from its dimensions, doors, windows, and coat count, then solve for the cheapest combination of tins that actually covers it.
Change the inputs that matter to your decision. Optional assumptions stay collapsed until you need them.
Measure the room once. The ceiling is the floor plan, counted a single time.
Doors and windows come off the wall area, never off the ceiling.
Enter all three and the purchase plan solves for the cheapest basket that covers the job.
397 ft² of paintable surface at 2 coats, covering 350 ft² per gal.
With a price against each container size, the purchase plan solves for the cheapest combination that covers the volume rather than rounding up to whole large tins.
| Walls | 448 ft²Perimeter × wall height |
|---|---|
| Doors (1) | −21 ft²Deducted |
| Windows (2) | −30 ft²Deducted |
| Paint needed | 2.38 gal397 ft² × 2 coats ÷ 350 ft² per gal |
| Buy: 1 gallon can | 2 ×2 gal |
| Buy: 1 quart can | 2 ×0.5 gal |
| Touch-up reserve | 0.11 gal5% held back, already inside the total above |
Coverage rates are the mid-range figures manufacturers publish for a smooth, previously painted surface. Bare plaster, new drywall, and heavy texture can take half again as much, so check the tin and adjust the rate above.
Practical notes about the inputs, assumptions, and result.
As the floor plan — room length × room width — counted once for the room. It is a common bug to derive a ceiling from a single wall length, which squares that one dimension and then repeats it for every wall; a 20 × 10 ft room would report 400 ft² four times over instead of one 200 ft² ceiling.
Because it solves for cost rather than rounding up. With a 2.74 gallon requirement, two gallons plus three quarts can beat three gallons or eleven quarts depending on the prices you enter. Whatever it picks, the total volume always covers the requirement and the price shown is the sum of exactly those lines.
For one or two openings in a large room the difference is small and the extra is useful. For a wall that is mostly glazing it matters a great deal, so the deduction is there and editable. Openings only ever come off the wall area — a window cannot reduce the ceiling.
The one on your tin. The defaults here are 350 ft² per gallon for a standard interior paint, 400 for a premium, and 300 for primer, which are mid-range published figures for a smooth previously painted surface. New drywall, bare plaster, and textured surfaces can drop that by a third or more.
Usually another coat rather than a different rate. Going from dark to light, or applying a strong colour over white, commonly needs three coats or a tinted primer. Set the coat count to what the job actually requires rather than adjusting the coverage rate to compensate.
The touch-up reserve defaults to 5% and is part of the total shown, not an extra line you have to remember. Paint from a different batch rarely matches exactly, so keeping a sealed quart from the original run is worth more than the small saving from buying tight.
Other tools in construction & diy.
Size a slab, footing, round column, wall, or flight of stairs with thickness in its own unit, then compare bagging the pour against a ready-mix delivery.
Estimate tiles, boxes, and setting materials for a room from the real row-by-row layout, with grout taken from the joint geometry and no grout quoted for a floating floor.
Turn a finished concrete volume into cement, sand, gravel, and water for a chosen strength, working back through an explicit dry-volume factor rather than splitting the wet volume.