How Much Paint Do I Need? (Contractor Formula + Room Examples)

If you’ve ever started painting and realized you’re one gallon short (or bought way too much), you already know the truth: paint math matters. The good news is you don’t need a fancy estimating system to get this right—you just need a clean measurement method, a couple rules of thumb, and a small waste factor.

In this guide, I’ll show you exactly how to calculate how much paint you need for walls and ceilings, how to handle doors/windows, what “paint coverage per gallon” really means, and when primer changes the numbers. If you want to skip the math, you can also use our free paint calculator.

The quick answer (the contractor shortcut)

Most standard interior wall paints cover about 350 sq ft per gallon per coat (some are 300, some are 400—more on that below).

A fast rule-of-thumb for a typical bedroom (10×12 to 12×14 with 8 ft ceilings) is:

• Walls (1 coat): ~1 gallon
• Walls (2 coats): ~2 gallons
• Ceiling (1 coat): ~1 gallon

That gets you close. But if you’re trying to avoid a second trip to the store (or you’re bidding jobs), use the method below.

Step 1: Decide what you’re painting (walls, ceiling, trim?)

Before measuring, make three decisions:

1. Walls only, or walls + ceiling?
2. How many coats? (Most repaints are 2 coats for color changes, 1 coat for same-color refresh.)
3. Are you painting trim/doors? (Trim paint coverage is different and the prep time matters.)

This post focuses on walls + ceilings, because that’s where most of the paint volume goes.

Step 2: Understand paint coverage per gallon (real-world vs label)

You’ll usually see something like “covers 350–400 sq ft per gallon” on the can.

A contractor’s reality check:

• Smooth drywall, good roller technique: 350–400 sq ft/gal/coat
• Textured walls (orange peel / knockdown): 300–350 sq ft/gal/coat
• Porous surfaces / new drywall / patches: 250–325 sq ft/gal/coat (primer helps)
• Deep base colors / bright reds: may require extra coat(s)

For estimating, I recommend:

• Use 350 sq ft/gal as your baseline
• If your walls are textured or rough, use 300 sq ft/gal

Step 3: Measure your room (two methods)

Method A (fast): measure perimeter and ceiling height

1. Measure the room’s length and width.
2. Compute the perimeter:
   • Perimeter = 2 × (L + W)
3. Measure ceiling height.
4. Compute wall area:
   • Wall area = Perimeter × Ceiling height

This works for rectangular rooms.

Method B (accurate): measure each wall

If the room isn’t a perfect rectangle (nooks, bump-outs), measure each wall width and add them.

You can use our square footage calculator for odd shapes to get total floor area and then measure wall lengths.

Step 4: Calculate paintable wall area (subtract doors & windows)

A common question is: do you subtract openings?

For quick estimates, you can either:

• Don’t subtract openings (you’ll slightly overbuy, but you’ll be safe), or
• Subtract with standard approximations:
  • Standard door: ~20 sq ft
  • Average window: ~15 sq ft

Formula:

• Net wall area = (Perimeter × Height) − (Door area + Window area)

If you have big sliding doors or multiple large windows, subtracting becomes worth it.

Step 5: Add ceiling area (if painting ceiling)

Ceiling area is just the floor area of the room:

• Ceiling area = Length × Width

(For vaulted ceilings, treat it like a roof plane—measure the actual surface area if you can.)

Step 6: Convert area to gallons (and then to quarts)

The formula

• Gallons needed (per coat) = Total paintable area ÷ Coverage per gallon
• Total gallons = Gallons needed (per coat) × Number of coats

Then:

• 1 gallon = 4 quarts

Waste factor (don’t skip this)

Paint estimation is like concrete: running short is expensive.

Add 10% waste for:

• roller loading
• touch-ups
• corners and cut lines
• paint left in tray/bucket

Final gallons = Total gallons × 1.10 (round up to practical purchase sizes)

Worked Example #1: 12×14 room, 8 ft ceilings (walls + ceiling, 2 coats on walls)

Given:

• Room: 12 ft × 14 ft
• Ceiling height: 8 ft
• Openings: 1 door, 2 windows
• Coverage: 350 sq ft/gal
• Coats: 2 coats on walls, 1 coat ceiling

1) Perimeter

Perimeter = 2 × (12 + 14) = 2 × 26 = 52 ft

2) Wall area (gross)

Wall area = 52 × 8 = 416 sq ft

3) Subtract openings

Door: 20 sq ft Windows: 2 × 15 = 30 sq ft Total openings: 50 sq ft

Net wall area = 416 − 50 = 366 sq ft

4) Ceiling area

Ceiling area = 12 × 14 = 168 sq ft

5) Gallons for walls

Per coat gallons (walls) = 366 ÷ 350 = 1.05 gal Two coats = 2.10 gal

6) Gallons for ceiling

Ceiling gallons (1 coat) = 168 ÷ 350 = 0.48 gal

7) Total + waste

Total gallons before waste = 2.10 + 0.48 = 2.58 gal Add 10% waste: 2.58 × 1.10 = 2.84 gal

What to buy:

• 3 gallons total (likely 2 gallons wall color + 1 gallon ceiling)

Worked Example #2: 18×20 room, 9 ft ceilings (walls only, 2 coats)

Given:

• Room: 18 ft × 20 ft
• Ceiling height: 9 ft
• Openings: 2 doors, 3 windows
• Coverage: 350 sq ft/gal
• Coats: 2

1) Perimeter

Perimeter = 2 × (18 + 20) = 2 × 38 = 76 ft

2) Wall area (gross)

Wall area = 76 × 9 = 684 sq ft

3) Subtract openings

Doors: 2 × 20 = 40 sq ft Windows: 3 × 15 = 45 sq ft Total openings: 85 sq ft

Net wall area = 684 − 85 = 599 sq ft

4) Gallons for walls

Per coat gallons = 599 ÷ 350 = 1.71 gal Two coats = 3.42 gal

5) Total + waste

Add 10% waste: 3.42 × 1.10 = 3.76 gal

What to buy:

• 4 gallons (walls only)

How to measure walls for paint (without overthinking it)

If you want this to be painless, here’s the workflow:

1. Measure L, W, ceiling height
2. Compute perimeter × height
3. Subtract openings if there are a lot of them
4. Add ceiling area if painting ceiling
5. Divide by 350 (or 300 for textured)
6. Multiply by coats
7. Add 10% and round up

Or use the free paint calculator and save the math.

Primer: when it changes your paint math

Primer doesn’t always reduce paint used—but it often prevents extra coats.

Use primer when:

• New drywall or heavy patching
• Stains (water, smoke, tannins)
• Switching from oil to latex (or questionable existing finishes)
• Extreme color change (dark → light or light → dark)

How primer affects the estimate:

• Primer has its own coverage rate (often similar to paint, sometimes less)
• The real win is that a primed surface can make 2 top coats behave like 2 coats, not 3

If you’re on the fence, primer is usually cheaper than an extra coat of finish paint—and it makes the job look better.

Pro tips that prevent paint shortages

• If you’re between two numbers, round up. Touch-ups always happen.
• Plan for more paint on textured walls.
• Deep colors may require extra coats.
• Keep 1 quart for future touch-ups if it’s a rental or high-traffic room.

Ready to make estimating easier?

If you’re doing this often (as a contractor or remodeler), the next step is getting the whole estimate built fast—materials, labor, overhead, markup, and a clean PDF you can send. If you’re setting your painting labor rates, our guide on how much to charge for labor covers the full formula for burdened rates, overhead, and profit.

Try EstimationPro free

FAQs

1) How many square feet does one gallon of paint cover?

Most interior paints cover about 350 sq ft per gallon per coat, but textured/porous surfaces can drop closer to 300 sq ft.

2) Should I subtract doors and windows when estimating paint?

If you want accuracy, yes—use 20 sq ft per door and 15 sq ft per window as quick deductions. For quick jobs, skipping deductions is fine.

3) How much extra paint should I buy?

Add 10% for waste and touch-ups. If you’re between gallons, round up.

4) Do I need primer, and does it change how much paint I need?

Primer can prevent extra finish coats on porous surfaces and major color changes. You may buy primer, but you often save a full coat of finish paint.

5) How do I estimate paint for vaulted ceilings?

Measure the actual ceiling surface area (it’s larger than floor area). If you can’t, estimate conservatively and round up.

For exterior painting projects, our guide on how to estimate paint for a house exterior covers surface area methods, texture waste factors, and real pricing by siding type. If you want to see how paint quantities translate into full project costs by room, see interior painting cost per room for professional vs DIY breakdowns. And if you need to estimate painting labor, see painting labor cost per square foot for contractor rate ranges by project type.

Use the Painting Estimate Calculator to build a full line-item painting estimate with materials and labor combined.