Paint Calculator

Enter the room dimensions and ceiling height, say how many coats you are applying, deduct the doors and windows, and get the quantity in US gallons and litres.

Ceiling and trim can be included separately, and the coverage rate is adjustable so you can use the figure printed on the tin you are actually buying.

How to use the paint calculator

Enter the room dimensions and ceiling height.

Set coats, doors, windows and coverage.

Read gallons and litres for walls, ceiling and trim.

Why use the Build Tally paint calculator

Openings deducted

Doors and windows come off the wall area automatically.

Adjustable coverage

Use the figure from the tin you are buying.

Ceiling and trim

Included as separate line items.

Gallons and litres

Both units shown for every result.

How paint quantity is worked out

Wall area is the perimeter times the ceiling height: (length + width) × 2 × height. Doors are typically 20 sq ft each and windows around 15 sq ft, and those come off the total. Multiply by the number of coats, divide by the coverage rate, and that is your quantity. A US gallon covers roughly 350–400 sq ft per coat on a smooth, previously painted wall — so a 12 × 14 ft room with 8 ft ceilings takes a little over a gallon per coat.

Coverage varies more than the tin suggests

The number on the tin assumes a smooth, sealed, similar-coloured surface. Bare drywall soaks up noticeably more on the first coat, textured or rough-rendered walls can drop coverage by a third, and a dark-to-light colour change often needs three coats or a tinted primer instead of two. Deep bases and some strong colours cover poorly by nature. If you are working on new or patched drywall, prime first — it is cheaper than an extra coat of finish paint and gives a more even result.

Buying and storing sensibly

Round up to whole tins, and where the calculator lands just over a boundary consider whether a gallon plus a quart is genuinely cheaper than two gallons — often it is not, and the leftover is useful for touch-ups. Buy all of one colour in a single purchase so the tint batch matches, or ask for the tins to be boxed together and mixed. Note the colour code and base on the lid before you store it; matching a colour from a dried drip years later is a miserable job.

Read the guide: How Much Paint Do I Need? Gallons Per Room, Worked Out

Frequently asked questions

How much paint do I need for a 12x12 room?

With 8 ft ceilings, about 384 sq ft of wall before deductions — roughly one gallon per coat, so two gallons for two coats.

How much does a gallon of paint cover?

Around 350–400 square feet per coat on smooth, sealed walls. Bare or textured surfaces cover less.

How many coats of paint do I need?

Two is standard. Three if you are going from a dark colour to a light one, or use a tinted primer instead of the third coat.

Do I subtract doors and windows?

Yes — the calculator deducts about 20 sq ft per door and 15 sq ft per window, and you can set the counts above.

How much paint for a ceiling?

Ceiling area is length × width, so a 12 × 14 ft ceiling is 168 sq ft — well under a gallon per coat. Tick the ceiling option to include it.

Should I prime first?

On bare drywall, patched areas, or a big colour change, yes. Primer seals the surface so the finish coat covers evenly and you often save a coat.