Running out of paint halfway through the second coat is one of the most frustrating DIY moments — and buying far more than you need is just money sitting in tins in the garage. This free paint calculator takes the guesswork out of buying paint by working out exactly how much you need for walls, ceilings, or a full room, based on your actual measurements.
It handles the boring-but-important maths for you: measuring wall area, subtracting doors and windows, and adjusting for the number of coats and the coverage rate of your paint. Whether you’re freshening up a bedroom with a single accent wall or repainting a whole house, you’ll get an estimate in gallons and litres that keeps you from making a second trip to the hardware store.
I’m Dave Collins, and I’ve painted more rooms than I can count — including two where I confidently bought “plenty” and still ran short. That experience is exactly why tools like this exist. This calculator is designed for DIYers, landlords, and first-time decorators who want a sensible buying guide before they head to the paint aisle.
How to Use the Paint Calculator
- Enter your room dimensions: measure the length and width of the room (or each wall section) and the wall height in feet or metres. A tape measure and a helper make this a two-minute job.
- Count doors and windows: enter how many doors and windows are in the room. The calculator subtracts their typical area so you don’t buy paint for holes you won’t cover.
- Set your number of coats: most jobs need 2 coats, but going over a dark colour — or using a cheaper paint — may need 3. Primer coats count as coats here.
- Check the coverage rate: most interior paints cover around 350–400 sq ft per gallon (about 10–12 m² per litre). The tool uses a standard default, but if your tin states a coverage figure, use it for a tighter estimate.
- Read your results: you’ll see the net paintable area plus the gallons (and litres) needed. The result rounds up to whole tins — you can only buy full tins, after all.
How the Calculation Works
The method is simple and transparent. First, the calculator finds the total wall area: for a rectangular room it’s 2 × (length + width) × height. Then it subtracts a standard allowance for each door (about 21 sq ft / 1.9 m²) and each window (about 12 sq ft / 1.1 m²). The remaining net area is divided by the paint’s coverage rate to give the amount per coat, multiplied by the number of coats, and rounded up to the next whole tin.
Worked example: a 12 × 14 ft room with 8 ft ceilings has a wall area of 2 × (12 + 14) × 8 = 416 sq ft. Subtract one door (21 sq ft) and two windows (24 sq ft) for a net of 371 sq ft. At 350 sq ft per gallon, that’s 1.06 gallons per coat — 2.12 gallons for two coats — which rounds up to 3 gallons. For one coat on a ceiling in the same room: 168 sq ft ÷ 350 ≈ 0.48 gallons, rounding up to 1 gallon.
Keep in mind this is an estimate. Rough or porous surfaces like new drywall, stucco, or old chalky paint can drink 10–20% more than smooth, previously painted walls, so budget a little extra when in doubt. Textured walls typically add about 10% to usage compared with smooth walls.
Tips for Accurate Results
- Measure twice, buy once: double-check each wall length before entering it — a 2 ft error on a long wall can shift the result by nearly half a gallon over two coats.
- Measure bay windows and alcoves separately: irregular rooms aren’t rectangles, so break the space into rectangles, enter them as separate sections, and add the results.
- Don’t forget the ceiling: ceilings often need the same two coats as walls, especially if you’re covering a dark colour — include them if you’re painting them too.
- Check your paint’s stated coverage: budget paints may only cover 250–300 sq ft per gallon, while premium ones hit 400+. The tin’s label is the most accurate figure for your estimate.
- Round up, not down: a partial gallon can’t be bought, and leftover paint is perfect for touch-ups. Running short mid-project risks a visible colour mismatch between batches.
- Prime thirsty surfaces first: bare drywall, wood, and patches of filler absorb paint fast. A coat of primer seals them so your topcoats cover evenly — and it usually saves paint overall.
Frequently Asked Questions
How much paint do I need for a 12×12 room?
Typically around 2 gallons for two coats on the walls of a 12 × 12 ft room with 8 ft ceilings, after subtracting one door and one window — roughly 1 gallon per coat. Use the calculator with your exact measurements for a firmer number.
Should I buy paint in gallons or quarts?
For most full rooms, gallons are far better value per unit. Quarts (or litres in the UK) are useful only for small accent walls, trim, or touch-ups — or when you need to tweak a custom colour mix in small steps.
Does the calculator account for primer?
It accounts for coats, so enter a primer coat as one of your coats if you’re using a separate primer. Dedicated primer tins often have different coverage rates than topcoat paint, so check the label and adjust the coverage figure accordingly.
How accurate is this estimate?
For smooth, previously painted walls it’s typically very close — within about 10%. Porous surfaces, heavy textures, and dramatic colour changes can increase usage, which is why the tool rounds up and why we recommend adding a small buffer on trickier jobs.
What about painting trim, skirting and doors?
Trim is best calculated separately because trim paint (usually satin or gloss) comes in smaller tins and covers differently. As a rough guide, allow one quart per average room’s worth of skirting and door frames — or see our guide to painting a room like a pro for a full materials checklist.
Once you know your numbers, make sure the rest of the job goes smoothly — read how to paint a room like a pro for the step-by-step technique, and how much paint you need for a room for a deeper look at the maths behind this calculator.