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Paint Calculator

Calculate paint needed and costs for your room

Paint Calculator Guide

A paint calculator estimates gallons (or liters) of paint you need by combining wall area, coverage per gallon, and number of coats. It also helps with primer, ceilings, and trim so you buy enough without overstocking.

What is Paint Calculator?

The paint calculator turns room measurements into a shopping list. It supports imperial and metric units, subtracts door/window openings, and lets you apply different coverages for primers vs finish paints.

How to Use the Paint Calculator

  1. Measure room dimensions: length, width, and height for each wall; include ceilings if painting them.
  2. Subtract openings: doors and windows (width x height).
  3. Enter coverage: typical 350-400 sq ft/gal (9-10 m²/L) for smooth interior walls.
  4. Choose coats for primer and finish.
  5. Calculate gallons/liters needed and add a waste buffer (5-10%) for edges and touch-ups.

Formulas & Methods

  • Wall area: A_walls = 2*(L + W)*H (rectangular room); otherwise sum each wall width*height.
  • Ceiling: A_ceiling = L*W.
  • Openings: A_net = A_walls + A_ceiling - sum(A_openings).
  • Paint needed: Gallons = (A_net / Coverage_per_gal) * Coats.
  • Trim: estimate linear feet * trim height to get area, or treat as a coverage add-on (e.g., +10%).

Assumptions & limitations

  • Coverage varies by paint type, surface texture, temperature, and technique.
  • Porous or dark-to-light color changes may require a tinted primer.
  • Sprayers vs rollers change transfer efficiency; sprayers may need more.

Examples

Example A — Standard room
Room 12 ft x 15 ft, height 8 ft, one door 3x7 ft, two windows 3x4 ft each.
A_walls = 2*(12+15)*8 = 432 sq ft.
Openings = 21 + 2*12 = 45 sq ft.
Net walls = 387 sq ft. Coverage = 375 sq ft/gal. Two coats -> Gallons = 387/375*2 = 2.06 → Buy 3 gallons (allows touch-ups).

Example B — Ceiling & trim
Add ceiling = 12*15 = 180 sq ft with one coat at same coverage -> 180/375 = 0.48 gal.
Trim estimated = 120 linear ft * 0.25 ft = 30 sq ft -> 30/375 = 0.08 gal.

| Surface | Area | Coats | Gallons | |---|---:|---:|---:| | Walls (net) | 387 sq ft | 2 | 2.06 | | Ceiling | 180 sq ft | 1 | 0.48 | | Trim | 30 sq ft | 1 | 0.08 |

Pro Tips & Best Practices

  • Buy one extra quart for future touch-ups; color batches can vary.
  • Use primer on new drywall, stains, or big color swings.
  • Keep a wet edge and consistent roller nap to avoid lap marks.
  • Record brand, color code, and sheen for reorders.
  • Ventilate and follow VOC guidance when choosing interior paints.

Related Calculators

FAQ

Q: How much paint do I need?

A: Multiply total paintable area by the coverage per gallon (or liter), then divide and multiply by the number of coats.

Q: What is typical paint coverage?

A: A common rule is 350-400 sq ft per gallon for smooth interior walls; rough or unprimed surfaces cover less.

Q: Do I subtract doors and windows?

A: Yes—subtract the area of large openings. Trim and edges add back a small amount for overlap.

Q: How many coats should I plan for?

A: Most projects need two coats; deep color changes or spot repairs may need a primer plus two coats.

Q: How does primer affect coverage?

A: Primer seals porous surfaces so finish coats cover closer to the rated value.

Call to Action

Enter room sizes, coverage, and coats to get a precise paint list—then add a small buffer so you finish without last-minute runs.