Measuring a house exterior for paint

The shape of the problem

An exterior paint estimate needs the same four inputs as an interior one: area, coverage, coats and an allowance. The area is harder to get, because the walls are tall, the gables are triangles and you are standing on the ground. The good news is that exterior paint is bought in 5-gallon pails and gallons, so a measurement within a few percent is plenty. This guide measures the walls, the gables, the openings and the trim, and finishes with two worked houses.

Step 1: the perimeter

Walk round the house with a long tape, or a measuring wheel, and measure every wall along the foundation. Add them up. A simple rectangular house is twice the length plus twice the width; a house with wings, bays and an L-shape is easier to measure wall by wall. Include the walls of an attached garage if you are painting them.

Check the total against the footprint: a rectangle 50 × 30 ft has a perimeter of 160 ft, and an L-shaped house of the same floor area has a longer one. If the total does not fit the shape you walked round, look for a missed wall.

Step 2: the wall height

Measure, or estimate, the height from the bottom of the siding to the eave, the line where the roof meets the wall. For a one-storey house this is usually about 8 to 10 ft; for two storeys, about 18 to 20 ft. The exterior house paint calculator asks for the height per storey and the number of storeys, and its default of 9 ft per storey includes the floor structure between them.

From the ground, count the siding courses on a lap-sided house: measure the exposure of one board, often 4 to 7 in, and multiply by the number of boards. Or measure a door, which is 80 in, and estimate the wall height in door heights.

Step 3: the gables

A gable is the triangular wall at the end of a pitched roof, between the eave line and the peak. Its area is half the width times the height:

The width is the width of that end of the house. The height, from the eave line to the peak, can be estimated from the roof pitch: on a common 6/12 roof, the peak rises half a foot for every foot of horizontal run, so a 30 ft wide gable with a 6/12 roof rises about 7.5 ft. Or count the siding courses in the gable. Hip roofs have no gables; dormers have small ones of their own.

Step 4: the openings

Take out the doors and windows, using standard sizes: 20 sq ft for a door and 15 sq ft for a window. A single garage door counts as about three doors, a double as six, if it is not being painted in the siding colour. A large picture window or a sliding patio door is about two standard openings. Do not subtract small vents, meters or lights. The full list is in the standard sizes reference.

Step 5: the siding factor

Lap siding has more surface than its flat face, because of the board edges and the shadow lines, and rough or textured surfaces have far more. The calculator multiplies the net area by a factor for the siding: 1.0 for smooth panels, 1.1 for lap or clapboard, 1.3 for rough-sawn wood, 1.4 for stucco and 1.5 for brick. Siding types and paint usage explains each.

Two worked houses

Step 6: trim, soffits and the rest

Trim is a separate product, usually in a contrasting colour and often a higher sheen, and it is measured separately:

• Fascia: the board along the eaves, the perimeter times its width.
• Soffit: the underside of the eaves, the perimeter times the overhang depth. Soffits are often the largest trim area on the house.
• Rake boards: the trim along the sloping edges of the gables.
• Corner boards, door and window casings: counted.

A one-storey house with a 160 ft perimeter has about 502 sq ft of trim and soffit, 3.16 gallons at two coats, from the exterior trim, fascia and soffit calculator. Garage doors have their own calculator, the garage door paint calculator, because their panels change the area. Sheds and outbuildings, with their own small gables and trim, are covered by the shed paint calculator.

Checking the estimate

Two sanity checks catch most errors. First, the wall area should be close to the perimeter times the total wall height, plus a little for the gables; if the net area is far from that, an input is wrong. Second, compare with the worked houses above: a 160 ft one-storey ranch took about two pails for two coats, and a two-storey house with a smaller footprint took more than three. If your figure is far from what a house like yours suggests, recheck the perimeter and the storeys. If you have a painter’s quote, compare their gallons with yours; what painters charge explains how quotes are built.

Measuring from plans or online maps

If you have the house plans, the perimeter and wall heights are on them. Online satellite maps can give the footprint, but they show the roof, which overhangs the walls, so the perimeter they suggest is larger than the walls by twice the overhang on each side. Subtract the overhang, or measure on the ground. The same principle, measuring the walls rather than the roof, applies to measuring a room indoors.

Common mistakes

• Forgetting the gables. On the ranch above, they add 240 sq ft to 1,440 sq ft of wall.
• Measuring the roof outline. Eaves overhang; measure the walls.
• Counting a two-storey house as one tall storey of the wrong height. Use the storeys.
• Subtracting the garage door when it is painted with the siding. Only subtract what you are not painting.
• Lumping the trim in with the siding. Different product, different colour, different calculator.
• Ignoring texture. Stucco and brick need far more paint than lap siding.