— Estimation Tool
PaintCalculator
Enter your room dimensions and get an exact gallon count with cost estimate. No guesswork. No wasted paint.
Room Dimensions
Windows & Doors
Windows
Doors
Paint & Surface
Calculation Results
Fill out the form and
click "Calculate" to see
your paint estimate.
What is a paint calculator?
A paint calculator is a digital tool that estimates the number of gallons of paint required to cover a room’s walls. It takes room length, width, and ceiling height, subtracts the area of windows and doors, then divides the net paintable surface by the paint’s coverage rate — typically 300 to 400 square feet per gallon depending on finish type. The result is adjusted for the number of coats, surface porosity, and an optional waste allowance. This calculator applies professional-grade coverage rates and surface multipliers used by painting contractors to produce estimates accurate to within 10 percent.
Published by the CleverSpaceSolutions editorial team. Coverage rates referenced from professional painting contractor industry standards.
Paint Finish Reference
Coverage · Pricing · UseCoverage rates and price ranges below reflect industry averages for latex interior paint on smooth, previously painted drywall. Source: professional painting contractor standards. Porous or textured surfaces reduce effective coverage by 15 to 40 percent.
| Finish | Coverage | Price / Gal | Best For | Sheen |
|---|---|---|---|---|
| Flat / Matte | 400 sq ft / gal | $25 – $60 | Bedrooms, ceilings | 1/5 |
| Eggshell | 350 sq ft / gal | $30 – $65 | Living rooms, dining rooms | 2/5 |
| Satin | 350 sq ft / gal | $32 – $70 | Hallways, kids' rooms | 3/5 |
| Semi-Gloss | 300 sq ft / gal | $35 – $75 | Kitchens, bathrooms | 4/5 |
| High Gloss | 300 sq ft / gal | $38 – $80 | Trim, cabinets, doors | 5/5 |
The Formula
- 1Measure wall area: (L + W) × 2 × Height
- 2Subtract window and door areas
- 3Adjust for surface type (textured walls absorb more)
- 4Divide by paint coverage (sq ft per gallon)
- 5Multiply by number of coats
- 6Add 15% for waste and touch-ups
Before You Buy
Round up
Always purchase full gallons. Partial cans in matched colors are rarely available.
Same dye lot
Buy all cans in one purchase. Different batches can vary slightly in tone.
Keep a quart
Set one quart aside from the final batch for future touch-ups.
Prime first
New or unpainted drywall absorbs significantly more paint. Always prime.
Read the complete paint guide — room-by-room estimates, finish selection, and common mistakes.
Questions
How many gallons for a 12×12 room?
For a standard 12×12 room with 9-foot ceilings, 2 windows, and 1 door, plan on 2 to 2.5 gallons for two coats on smooth drywall. Choosing eggshell or satin instead of flat adds roughly 0.3 gallons due to the lower coverage rate (350 sq ft/gal vs. 400). Use the calculator above for an exact figure based on your specific dimensions, finish, and surface type.
How accurate is this calculator?
It uses the same coverage rates cited by professional painters — 400 sq ft/gal for flat, 350 for eggshell and satin, 300 for semi-gloss and high gloss. Surface type multipliers are applied automatically for textured, raw wood, and masonry walls. The optional 15% waste buffer accounts for roller absorption and edge losses. Expect ±10% variance in real-world conditions depending on application technique and wall condition.
Does one gallon really cover 400 square feet?
Only for flat and matte finishes on smooth, previously painted drywall. Eggshell and satin cover around 350 sq ft per gallon because their formulations are denser. Semi-gloss and high gloss cover approximately 300 sq ft. Textured walls, raw wood, and brick are porous and can reduce effective coverage by 25 to 40 percent compared to smooth surfaces. This calculator adjusts automatically when you select a surface type.
Which finish should I choose?
Flat for bedrooms and ceilings — it hides surface imperfections and reflects no light. Eggshell for living and dining areas where light washability matters. Satin for hallways and children's rooms because it resists scrubbing. Semi-gloss for kitchens and bathrooms where moisture and grease are common. High gloss for trim, cabinets, and doors — it creates a hard, durable surface that can be wiped clean repeatedly.
Is the 15% waste allowance necessary?
Yes, in almost every case. The buffer accounts for roller absorption (which can consume 5 to 10% of paint before it reaches the wall), spills, cutting-in at edges, and storing a small reserve for future touch-ups. The more critical reason: running out mid-job and buying an additional can from a different production batch risks color variation between dye lots, which may be visible even in the same paint color.
Does this calculate ceiling paint?
Not currently — this tool calculates wall surfaces only. For ceilings, the formula is straightforward: divide room area (length × width) by your paint's coverage rate. A 12×12 ceiling is 144 sq ft, which requires less than half a gallon of flat ceiling paint per coat. Most ceiling paints are flat finish and cover 400 sq ft per gallon. Add one extra coat if the ceiling is new drywall or stained.