Project notes
Understand this estimate
Room paint quantity starts with four rectangular walls, subtracts unpainted openings once, and optionally adds the ceiling. Primer uses its own product rate.
How to use the calculator
Measure the room and openings
Use inside room dimensions and total the doors or windows that will not be painted.
Inputs Room length · Room width · Wall height · Doors and windows to deduct
Choose painted surfaces
Select walls only or walls plus a rectangular ceiling.
Inputs Painted surfaces
Enter separate paint and primer data
Use each product’s label coverage and instructed coats. Primer fields are optional and are used only when a primer rate is provided.
Inputs Paint coverage · Paint coats · Primer coverage · Primer coats · Extra allowance
Formula and variables
Gross wall area = 2 × (length + width) × height. Net wall area = gross wall area − openings. Add length × width when ceiling is selected. Estimate each product as area ÷ its coverage × its coats × (1 + allowance ÷ 100).
- 2(L + W)HGross wall area
- Four rectangular room walls.ft²
- OOpening area
- Area subtracted once from the walls.ft²
- L × WCeiling area
- Added only in walls-and-ceiling mode.ft²
What the result means
Result interpretation
Paintable area reflects the selected scope. Paint and primer gallon estimates use their own coverage and coat counts. Irregular rooms and surfaces require measured sections beyond this rectangular model.
Limits and assumptions
The room formula assumes four rectangular walls and a rectangular ceiling. Measure irregular spaces as wall sections and account for openings once. Primer uses its own rate; product coverage is not a universal constant.
Worked examples
Paint walls and estimate separate primer
Assumptions
- Room is rectangular and all four walls are painted.
- 42 ft² of openings are excluded.
- Paint and primer rates are 400 and 200 ft²/gal.
Calculation
- 2 × (12 + 10) × 8 − 42 = 310 ft².
- 310 ÷ 400 × 2 = 1.55 gal paint; 310 ÷ 200 = 1.55 gal primer.
| Paintable area | 310 ft² |
|---|---|
| Paint estimate | 1.55 US gal |
| Primer estimate | 1.55 US gal |
Include the ceiling and primer
Assumptions
- The rectangular ceiling is included.
- The same 42 ft² opening deduction applies to walls only.
Calculation
- 310 ft² net wall area + 12 × 10 = 430 ft².
- 430 ÷ 400 × 2 = 2.15 gal paint; 430 ÷ 200 = 2.15 gal primer.
| Paintable area | 430 ft² |
|---|---|
| Paint estimate | 2.15 US gal |
| Primer estimate | 2.15 US gal |
Common questions
- Are openings also subtracted from the ceiling?
- No. The entered opening deduction applies to gross wall area only.
- Can I use one coverage number for paint and primer?
- Enter each product’s own rate and coat count; they may differ.