Paint Quantity and Cost Calculator

How to Use This Tool

Enter the room, list the doors and windows, and set the coverage from your tin. The default figures are for primed drywall with a roller.

Why the two coats are calculated separately

Coverage is not a property of the paint alone. It is a property of the paint and what it is going onto. On bare or freshly primed drywall, part of the first coat is drawn into the surface. The second coat lands on sealed paint film and spreads further.

Typical figures are around 320 sq ft per gallon for the first coat and 400 sq ft for later ones. Both are inputs here, because the number on the tin assumes a smooth surface and a specific application method.

gross wall area   2 x (12 + 14) x 8   = 416 sq ft
one door          3 x 6.67            =  20.0 sq ft
two windows       2 x (3 x 4)         =  24.0 sq ft
net area                              = 372.0 sq ft

naive    372 x 2 / 350                = 2.13 gal
split    372/320 + 372/400            = 2.09 gal
tins (1 gal)                          = 3, with 0.91 gal left

A 12 x 14 ft room, 8 ft ceiling, one door and two windows

Deduct openings individually, not as a percentage

Some calculators subtract a flat 15 per cent for openings. That is fine for an average room and wrong for the two rooms where it matters: a bathroom with one small window, and a living room with patio doors. In the example above the openings are 44 sq ft, which is 10.6 per cent — a flat 15 per cent would have under-ordered.

The usual advice is to skip openings smaller than about 15 sq ft, on the grounds that the cutting-in around them uses the paint you saved. That is a reasonable rule, and it is why the door and window areas are editable rather than fixed.

Rough surfaces change the answer more than anything else

Knockdown texture, brick, block and rough render can take 30 to 50 per cent more paint than smooth drywall, because the real surface area is larger than the measured rectangle. If you are painting any of those, reduce both coverage figures before trusting the total.

Buy in whole tins, and expect leftovers

The output that matters is not litres or gallons, it is tins. Paint is sold in fixed sizes, so the answer is always rounded up, and the leftover is shown because that is your touch-up supply. Running out mid-wall is worse than over-buying: a second batch can differ slightly in shade, and a join in the middle of a wall shows.

Frequently Asked Questions

How much paint do I need for a 12x14 room?

With an 8 ft ceiling, one door and two windows, the net wall area is 372 sq ft. At 320 sq ft per gallon for the first coat and 400 for the second, that is 2.09 gallons, so three one-gallon tins with about 0.9 gallons left over for touch-ups.

Does one gallon really cover 400 square feet?

On a second coat over sealed paint, often yes. On a first coat over bare or newly primed drywall, closer to 320. The label figure assumes a smooth surface and ideal application, so treat it as the best case rather than the expected case.

Should I subtract doors and windows?

Subtract anything over about 15 sq ft. Smaller openings are usually left in, because cutting in around a small window uses roughly the paint you would have saved. Large openings such as patio doors must be deducted or you will over-buy by a tin.

Why does the second coat use less paint?

Coverage depends on the surface as much as the paint. The first coat is partly absorbed into porous substrate; the second lands on a sealed film and spreads further. This is also why a repaint uses less than new drywall for the same room.

Do I need primer, and does it count as a coat?

Primer is a separate product with its own coverage, so calculate it separately. On new drywall, patched areas or a big colour change it usually saves a topcoat, which makes it cheaper overall rather than an extra expense.

How much extra paint for textured walls?

Between 30 and 50 per cent more, because the actual surface area is larger than the rectangle you measured. Rather than adding a blanket margin, lower the coverage figures: 320 becomes roughly 220 on heavy texture.

Two Coats Is Not Twice the Paint, Because the Second Goes Further

How the calculation works