How paint cost is worked out
Paint is sold by the gallon, but a room is priced by the wall: the wall area is twice the length plus the width, times the ceiling height, because a room has four walls and two of them are the length and two are the width. A door takes 21 square feet out of the wall and a window takes 15, because you do not paint a door slab or a pane of glass. The gallons are the paintable area times the coats, divided by the coverage — most interior paint covers about 350 square feet a gallon — rounded up to whole gallons, because the shop does not sell half a gallon. The paint cost is the gallons times the price, and the total adds $35 of supplies: rollers, brushes, tape, drop cloths and a tray.
At the defaults — a 12 × 10 ft room, 8 ft ceilings, 2 coats, paint $45 a gallon at 350 sq ft a gallon, 1 door and 1 window — the wall area is 2 × (12 + 10) × 8 = 352 sq ft, and the paintable area is 352 − 21 − 15 = 316 sq ft. The gallons are 316 × 2 ÷ 350 = 1.81, rounded up to 2 gallons. The paint costs 2 × $45 = $90.00, the supplies are $35.00, and the total is $125.00 — which is $125.00 ÷ 316 = $0.40 per paintable square foot.
| Room | Wall | Paintable | Gallons | Paint | Supplies | Total |
|---|
| 12 × 10 ft, 8 ft ceiling (default) | 352 sq ft | 316 sq ft | 2 gal | $90.00 | $35.00 | $125.00 |
| 15 × 12 ft, 9 ft ceiling | 486 sq ft | 450 sq ft | 3 gal | $135.00 | $35.00 | $170.00 |
| 12 × 10 ft, 1 coat | 352 sq ft | 316 sq ft | 1 gal | $45.00 | $35.00 | $80.00 |
| 12 × 10 ft, $60/gal paint | 352 sq ft | 316 sq ft | 2 gal | $120.00 | $35.00 | $155.00 |
| 12 × 10 ft, 3 coats | 352 sq ft | 316 sq ft | 3 gal | $135.00 | $35.00 | $170.00 |
The worked example, line by line
Setup: a 12 × 10 ft room, 8 ft ceilings, 2 coats, paint $45 a gallon at 350 sq ft a gallon, 1 door, 1 window.
Line 1 — the wall area. 2 × (12 + 10) × 8 = 352 sq ft.
Line 2 — the paintable area. 352 − 21 − 15 = 316 sq ft.
Line 3 — the gallons. 316 × 2 ÷ 350 = 1.81, rounded up to 2 gallons.
Line 4 — the paint cost. 2 × $45 = $90.00.
Line 5 — the total. $90.00 + $35.00 of supplies = $125.00, or $125.00 ÷ 316 = $0.40 per paintable square foot.
Line 6 — the budget check. A $200 budget leaves $165 for paint after the $35.00 of supplies, which buys $165 ÷ $45 = 3.67, or 3 whole gallons — and 3 gallons at 350 sq ft a gallon over 2 coats covers 3 × 350 ÷ 2 = 525 sq ft of wall.
Every branch, checked
15 × 12 ft room, 9 ft ceiling: the wall area is 2 × (15 + 12) × 9 = 486 sq ft and the paintable area is 486 − 21 − 15 = 450 sq ft, so the gallons are 450 × 2 ÷ 350 = 2.57, rounded up to 3 gallons, and the total is $170.00. A bigger room with a taller ceiling costs $45 more than the default, and the extra gallon is the reason.
1 coat: the gallons are 316 × 1 ÷ 350 = 0.90, rounded up to 1 gallon, so the paint costs $45.00 and the total is $80.00. One coat saves $45 of paint on this room — but only if the new colour matches the old one, because one coat over a dark colour will show through.
$60 a gallon paint: the gallons stay at 2, but the paint costs 2 × $60 = $120.00 and the total is $155.00. The premium paint adds $30 to this room — the price difference is multiplied by the gallons, not the wall area, so the rounding to whole gallons decides whether the upgrade is cheap or dear.
3 coats: the gallons are 316 × 3 ÷ 350 = 2.71, rounded up to 3 gallons, so the paint costs $135.00 and the total is $170.00. Three coats cost the same as the bigger room: the third coat is a whole extra gallon, and the rounding is what makes it dear.
Reverse — the $200 budget: the supplies take $35.00, leaving $165 for paint, which buys $165 ÷ $45 = 3.67, or 3 whole gallons. Three gallons at 350 sq ft a gallon over 2 coats covers 3 × 350 ÷ 2 = 525 sq ft of wall — more than the 352 sq ft of this room, so $200 covers the room with a gallon to spare.
What the defaults assume
The model assumes a rectangular room: L-shaped rooms, bay windows and built-in wardrobes change the wall area, because the walls are not four straight runs. It assumes a door takes 21 square feet and a window takes 15 — a standard interior door is 3 × 7 feet and a standard window is 3 × 5 feet — and it assumes the ceiling is not painted, the trim is not painted, and the floor is not painted. It assumes the coverage on the can is honest: 350 square feet a gallon is the standard for interior paint on a smooth wall, but a rough or porous wall drinks more paint, and a dark colour over a light one needs more coats. It assumes $35 of supplies covers the rollers, brushes, tape, drop cloths and tray for one room — a big room or a first-time painter may need more. Rounding convention: gallons are always rounded up to whole gallons, money is rounded to cents, and all figures are computed from unrounded values, rounded only for display.
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Wall area | 2 × (length + width) × height | 352 sq ft. |
| Paintable area | wall − 21 × doors − 15 × windows | 316 sq ft. |
| Gallons | ceil(paintable × coats ÷ coverage) | 2 gal. |
| Paint cost | gallons × $/gal | $90.00. |
| Supplies | $35 flat | $35.00. |
| Total cost | paint + supplies | $125.00. |
| Cost per paintable sq ft | total ÷ paintable area | $0.40. |
| 15 × 12 ft, 9 ft ceiling | same model | Wall 486 sq ft, net 450 sq ft, 3 gal, $170.00. |
| 1 coat | same model, 1 coat | 1 gal, paint $45.00, total $80.00. |
| $60/gal paint | same model, $60/gal | Paint $120.00, total $155.00. |
| 3 coats | same model, 3 coats | 3 gal, paint $135.00, total $170.00. |
| Reverse: $200 budget | ($200 − $35) ÷ $/gal, whole gallons; gallons × coverage ÷ coats | 3 gal, 525 sq ft of wall. |
Who uses this
- The homeowner painting a bedroom: a 12 × 10 ft room costs $125.00 — $90.00 of paint and $35.00 of supplies — so they know the exact number before they walk into the paint shop, and they know the $35 of supplies is not a rounding error.
- The landlord refreshing a rental: a 15 × 12 ft room with 9 ft ceilings costs $170.00 — and they learn the taller ceiling is what pushes it to 3 gallons, so they budget a gallon a room, not a room a gallon.
- The couple choosing between paints: the $60-a-gallon paint costs $155.00 against $125.00 for the $45 paint — a $30 difference on this room — so they can decide whether the better coverage is worth it, room by room.
- The renovator on a $200 budget: they learn $200 buys 3 gallons, which covers 525 sq ft of wall at 2 coats — so they know the 352 sq ft room is covered with a gallon to spare before they start.
Accuracy & limitations
The math is exact for the numbers you enter: the wall area is twice the length plus the width times the height, the paintable area subtracts 21 square feet a door and 15 a window, the gallons are the paintable area times the coats divided by the coverage and rounded up, and the total is the paint plus $35 of supplies. What the model cannot do is measure your room for you: measure wall to wall at the longest points, and for an L-shaped room split it into two rectangles and add them. It leaves out the ceiling, the trim, the primer (which a dark colour over a light one may need), and tax — ask the paint shop whether the quote includes those, and add them yourself if it does not. It assumes the coverage on the can is honest; a rough or porous wall drinks more paint, and a dark colour over a light one needs more coats. The $35 of supplies is an estimate for one room: a big room or a first-time painter may need more. For the floor of the same room, see carpet cost calculator; for how much paint the room needs by coverage alone, see paint coverage chart.
Cite this calculatorBright Side Kit. “Paint Cost Calculator: Price Per Room & Gallon 2026.” brightoolbox.com, 2026. Method: wall area = 2 × (length + width) × height; paintable area = wall − 21 × doors − 15 × windows; gallons = ceil(paintable × coats ÷ coverage); paint cost = gallons × $/gal; supplies = $35; total = paint + supplies; cost per paintable sq ft = total ÷ paintable area; $200 budget buys ($200 − $35) ÷ $/gal whole gallons, covering gallons × coverage ÷ coats sq ft of wall. Defaults: 12 × 10 ft, 8 ft ceilings, 2 coats, $45/gal, 350 sq ft/gal, 1 door, 1 window → wall 352 sq ft, net 316 sq ft, 2 gal, paint $90.00, supplies $35.00, total $125.00, $0.40 per paintable sq ft; 15 × 12 ft, 9 ft ceiling → wall 486 sq ft, net 450 sq ft, 3 gal, paint $135.00, total $170.00; 1 coat → 1 gal, paint $45.00, total $80.00; $60/gal → paint $120.00, total $155.00; 3 coats → 3 gal, paint $135.00, total $170.00; $200 budget → 3 gal, 525 sq ft of wall.Bright Side Kit. "Paint Cost Calculator: Price Per Room & Gallon 2026." https://brightoolbox.com/paint-cost-calculator/ Accessed October 24, 2026.
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