How to use Paint Calculator
The paint calculator estimates litres or gallons of paint required for walls, ceilings, trim or an entire room based on dimensions, number of coats and paint coverage rate. It subtracts door and window area automatically and adds a recommended overage so you have enough for touch-ups without ordering a second tin.
- Enter room dimensions (length, width, ceiling height).
- Enter the number of doors and windows to subtract.
- Set the coverage rate of your chosen paint (shown on the tin).
- Choose the number of coats.
- Review litres or gallons needed, plus recommended purchase quantity.
Your data never leaves your device — 100% private processing.
Paint coverage and coats
Premium paints cover 350–400 sq ft per gallon; budget paints 300–350 sq ft. Two coats are standard for a colour change; one coat works for touch-ups or same-colour repaints. The formula: gallons = total area ÷ coverage rate × coats. Always buy in round-up increments (quart, gallon) — leftover paint is useful for touch-ups for years.
| Finish | Coverage (sq ft/gal) | Best used for | Washability |
|---|---|---|---|
| Flat / matte | 350–400 | Ceilings, low-traffic walls | Low |
| Eggshell | 330–380 | Living rooms, bedrooms | Moderate |
| Satin | 300–360 | Kitchens, bathrooms, trim | Good |
| Semi-gloss | 300–350 | Trim, doors, cabinets | Very good |
| Gloss | 280–330 | Doors, furniture, metal | Excellent |
Surface preparation — the most important step
Paint adhesion and finish quality depend almost entirely on preparation: fill holes and cracks with spackle, sand rough surfaces, clean off grease with TSP solution, and prime bare drywall or repaired spots before painting. Skipping primer on new or repaired drywall leads to uneven sheen and poor adhesion. On dark or stained surfaces, a stain-blocking primer prevents bleed-through.
Worked examples
12×12 room, 8 ft ceilings, 2 coats
Inputs: perimeter 48 ft × 8 ft = 384 sq ft × 2 coats = 768 sq ft
Result: 768 ÷ 350 ≈ 2.2 gallons (buy 3 incl. touch-ups)
Single 12×8 ft accent wall, 2 coats
Inputs: 96 sq ft × 2 = 192 sq ft ÷ 350
Result: ≈ 0.55 gallon — two quarts cover it
12×12 room minus door + window
Inputs: 384 − 36 sq ft openings = 348 × 2 coats = 696
Result: 696 ÷ 350 ≈ 2.0 gallons
Recommended coats by painting scenario
| Painting scenario | Primer? | Finish coats |
|---|---|---|
| Same-colour refresh | No | 1 |
| Light over light (new colour) | Optional | 2 |
| Dark over light | Yes (tinted) | 2 |
| Light over dark | Yes (blocking) | 2–3 |
| Bare / new drywall | Yes | 2 |
Glossary
- Sheen
- The reflectivity/gloss level of a paint finish, ranging from flat to high-gloss.
- Coverage rate
- Square feet a gallon of paint covers at the recommended film thickness.
- Primer
- A base coat applied before paint to improve adhesion and hide stains.
- TSP
- Trisodium phosphate — a heavy-duty cleaner for degreasing walls before painting.
Related reading
Frequently Asked Questions
Why use Paint Calculator?
- Automatically subtracts door and window area to prevent over-ordering paint
- Adjust coat count to compare one-coat versus two-coat total paint volumes
- Switch between litres and gallons to match the tin sizes sold at your local store
- Built-in overage recommendation ensures enough paint remains for future touch-ups
Common use cases
- Work out how many gallons to buy before repainting a master bedroom
- Plan primer and topcoat quantities for a full interior repaint across multiple rooms
- Estimate paint needed for an exterior repaint ahead of getting supplier quotes
- Check whether one quart covers a single accent wall without leftover waste
- Calculate how much semi-gloss is needed to paint all the trim in a new build
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