Paint Calculator Guide: How to Estimate Paint and Cost in 3 Minutes

Buying paint without doing the math is how most people end up with three half-empty gallons in the garage — or worse, running out mid-wall. A paint calculator solves both problems in about three minutes. Here's the entire formula, every input that matters, and how to use it.

What a paint calculator does

It computes paintable square footage from your room dimensions, subtracts your doors and windows, multiplies by the number of coats, and divides by coverage per gallon. The result is rounded up to the nearest gallon, then multiplied by your price per gallon to give you a real material cost.

The inputs you need

• Room length, width, and ceiling height (in feet)
• Number of doors (each ~21 sq ft) and windows (~15 sq ft)
• Coats — almost always 2
• Coverage per gallon — 350 sq ft is the industry standard
• Price per gallon — premium runs $50–$70, contractor grade $25–$35

Example 1: a 12 × 14 bedroom

Perimeter: 52 ft. Walls: 52 × 9 = 468 sq ft. Subtract 1 door (21) + 2 windows (30) = 417 sq ft of paintable wall. Two coats = 834 sq ft. At 350 sq ft/gal: 2.4 → round up to 3 gallons. At $45/gal: $135 in paint.

Example 2: an exterior, 1,800 sq ft single-story

Wall area roughly 1,500 sq ft after subtracting windows and doors. Two coats = 3,000 sq ft. At 300 sq ft/gal (exterior is more porous): 10 gallons. Add 2 gallons of primer. At $55/gallon: about $660 in materials.

Three things calculators get wrong

1. Assuming smooth walls. Textured or porous walls drop coverage 20–30%.
2. Ignoring trim. Trim and doors need separate gallons (semi-gloss vs eggshell on walls).
3. Single coat. Almost no project actually needs one coat — even premium 'one-coat' paints look better with two.

How accurate is the result?

Within about 10% if you measure carefully. Always buy 10% extra — touch-up paint is invaluable, and storing a sealed gallon makes future repairs blend perfectly.