When you pay a professional to paint your house, most of the cost is for labour because the materials are relatively inexpensive and painting can be labour-intensive. If you're painting the outside of your own house, you only have to estimate how much the paint and other materials will cost. It's not rocket science as long as you have a tape measure and a calculator. Once you have a rough idea of how many square feet you will be painting and what your paint will cost, you can come up with a close estimate.
Measure the length of each exterior wall of the house. Write all the measurements down.
Measure the height of each wall. Most houses measure about 10 feet per story. You can also measure a section of boards or panels, count how many there are on the siding and multiply that by the measurement you took. Multiply the height by the length of each wall.
Multiply the total length around your house by the depth of the eaves. Not all homes will have eaves -- they are the projecting overhangs at the bottom edge of the roof. You can "guesstimate" the depth if you don't want to climb a ladder to find out -- measurements don't have to be exact to the square inch.
Measure the height and width of each peak, and divide that in half. Add up all your calculations to find out the square footage. Unless you have an unusual amount of windows and doors, don't bother deducting these from your total--it's better to finish each job with a little left over for touch-up.
Assess the type of siding you have. A gallon of exterior paint typically covers between 250 and 350 square feet per gallon. Rough siding, like clapboard or textured stucco will use up more paint. Vinyl or aluminium siding will require less. Divide the square footage total by your estimated per-gallon coverage for the number of gallons you'll need per coat.
Price the paint you plan to buy. Avoid simply choosing the least expensive paint because it could end up being more costly in the long run if it requires more coats to cover, or if it starts to fade and peel within a year. Multiply the cost of each gallon by the number of gallons you'll need to paint your house.
Double that number for two coats of paint. Most exterior paint jobs look better and will be more durable with two coats.
Add in the trim paint you'll need. Kelly-Moore Paints suggests that 1 gallon of trim paint per 8 gallons of siding paint is a good rough estimate.
Make a list of all other materials -- if you forget these they can "nickel and dime" you way over your original estimate. Other materials may include primer, spackle, caulking, glazing, sandpaper, masking tape and masking paper as well as tools you might need to rent or buy. A visit to a paint or hardware store should give you a good idea of what these will cost.