Paint Calculator
Estimate paint cans from wall dimensions, openings, coats, and coverage rate.
Written by Golam Rabbani, Founder & Lead Engineer
How to use this paint calculator
- Pick imperial (ft / gallons) or metric (m / litres) using the unit toggle.
- Enter the total wall length, wall height, and how many doors and windows to subtract.
- Set the number of coats — 2 is standard — plus the coverage rate and can size from the paint label.
- Press Calculate to see the number of cans you need to buy.
- Use Copy to save the estimate or Reset to clear all inputs.
About this paint calculator
The paint calculator estimates how many cans of paint you need for a wall-painting job. It starts from the total wall area, subtracts 21 ft² (1.95 m²) for each door and 15 ft² (1.4 m²) for each window, multiplies the result by the number of coats, then divides by the coverage rate printed on the paint can. Defaults assume one US gallon covers ~350 ft² (one litre covers ~10 m²), but you can override the coverage and can size in the inputs.
As a worked example: a room with 40 ft of total wall length and 8 ft ceilings has 320 ft² of gross wall area. Subtract one door (21 ft²) and two windows (30 ft²) and you are left with 269 ft² of paintable wall. Painting two coats means covering 538 ft². At 350 ft²/gal, you need 1.54 gallons — so the tool rounds up to 2 one-gallon cans.
Coverage varies by paint type and surface: smooth drywall gets the full 350 ft²/gal, while porous primer-bare drywall or rough stucco can drop to 200 ft²/gal. Always read the can label and override the default if you are painting an unusual surface, and keep one extra can on hand for touch-ups.
FAQ
- How much paint do I need for an average bedroom?
- A 12 ft × 12 ft bedroom with 8 ft ceilings has 384 ft² of wall area. With one door and one window subtracted, that drops to ~348 ft². Two coats at 350 ft²/gal need ~2 gallons.
- How many coats of paint should I plan for?
- Two coats is the default. Use three when going from a dark wall to a much lighter color, painting bare drywall without a primer, or using a thin paint such as eggshell white over a deep red.
- What coverage rate should I use?
- Manufacturer-published spread rates range from 250–400 ft² per US gallon (7–11 m² per litre). The default of 350 ft²/gal is typical for premium interior latex on smooth, primed drywall. Always check the label.
- Why does the calculator subtract 21 ft² per door and 15 ft² per window?
- Those are typical residential door (3 ft × 7 ft = 21 ft²) and window (3 ft × 5 ft = 15 ft²) areas. The metric defaults are 1.95 m² and 1.4 m², the same standard sizes converted.
- Does the calculator account for ceilings or trim?
- No — it computes wall paint only. For ceilings, run a separate calculation using the ceiling length × width as the area. Trim usually needs a separate can of semi-gloss or trim enamel.
- Is this calculator free to use?
- Yes — completely free, no signup, and everything runs locally in your browser.