How to Calculate Interior Paint for a House
The quickest formula for estimating interior wall paint is: (floor area × 2.5) ÷ 400 = gallons per coat for walls. The 2.5 multiplier accounts for the fact that total wall area in a typical home is about 2.5 times the floor plan square footage when all rooms and ceiling heights are considered.
Double the result for two coats. Add 15% of the wall total for ceilings. Add 10% of the wall total for trim. This gives you a complete interior paint estimate.
For example: a 2000 sq ft house — (2000 × 2.5) ÷ 400 = 12.5 gallons per coat × 2 coats = 25 gallons of wall paint. Add 3.75 gallons for ceilings and 2.5 gallons for trim — total approximately 31 gallons for the complete interior.
Interior House Paint — Reference Table by Home Size
| Home Size | Walls (2 coats) | Ceilings (2 coats) | Total Interior |
|---|---|---|---|
| 800 sq ft | 5–6 gal | 2 gal | ~9 gal |
| 1000 sq ft | 6–8 gal | 2.5 gal | ~12 gal |
| 1500 sq ft | 9–12 gal | 4 gal | ~17 gal |
| 2000 sq ft | 12–15 gal | 5 gal | ~22 gal |
| 2500 sq ft | 15–19 gal | 6 gal | ~28 gal |
Tips for Estimating and Buying Interior House Paint
Use the room-by-room method for the most accurate result — measure each room individually rather than using the whole-house formula.
Buy 5-gallon buckets for your main colour — cheaper per gallon and batch-consistent.
Use one colour for multiple rooms where possible — this allows you to use leftovers between rooms and reduces total cans purchased.
Always add 10% buffer to your calculated total for waste, touch-ups, and coverage variation.
Store all leftover paint labelled by room — a sealed can of latex paint stays usable for up to 5 years.
Related Paint Calculators
- Whole House Paint Calculator →
- How Much Paint for a 2000 Sq Ft House? →
- How Much Paint for a 1500 Sq Ft House? →
- How Much Paint for a 1000 Sq Ft House? →
- Paint Cost Calculator →
- Paint Coverage Calculator →
- Free Paint Calculator →
Frequently Asked Questions
How do I calculate paint for the interior of my house?
Use the formula: (floor area × 2.5) ÷ 400 = gallons per coat for walls. Double for two coats, add 15% for ceilings, and 10% for trim.
How much does it cost to paint a house interior?
Paint alone runs $400 to $1,200 depending on home size. Professional labour adds $1,500 to $6,000 for a complete interior job.
Should I use the same colour throughout the house?
Using one colour throughout saves money by allowing bulk buying and cross-room use of leftovers. A single accent wall per room adds variety without requiring additional colours.
How much paint for ceilings in a whole house?
Budget 1 gallon of flat white ceiling paint per 400 sq ft of floor area for two coats. A 1500 sq ft house needs 4 to 5 gallons of ceiling paint.
How much trim paint for a whole house interior?
Allow 1 gallon of semi-gloss trim paint per 400 to 500 sq ft of floor area. A 1500 sq ft house typically needs 3 to 4 gallons for all interior trim.
What order should I paint rooms in a house?
Paint from top to bottom and back to front: ceilings throughout first, then walls room by room, then all trim. Start from the room furthest from the front door.