Determine the length of the fascia (the edge of the roof that is horizontal and where the gutter will be installed). Next, measure the width from the fascia to the ridge (the top edge of the roof area).
Find the pitch of the roof. Hold a level against the side of the house where the roof slopes upward so one end of the level touches the roof surface. Mark a point 12 inches along the level, starting from the spot where the level touches the roof. Measure from the 12-inch point straight up to the surface of the roof. Write this as the number of inches per 12 inches. For example, if you measure five inches up, the pitch is 5 in 12.
Find the watershed area by first multiplying the length of the fascia by the width. Multiply your answer by the appropriate pitch factor. For a pitch of 3 in 12 or less, the pitch factor is 1.0. For 4 or 5 in 12, the pitch factor is 1.05, for 6 to 8 inches the pitch factor is 1.1, and for 9 to 11 inches the factor is 1.2. A pitch of 12 in 12 has a pitch factor of 1.3 For example, if the roof is 30 feet long and 15 feet wide with a pitch of 5 in 12, you have 30 feet x 15 feet x 1.1 = 495 square feet of watershed area.
Determine the probable rainfall intensity in your area. This is the maximum rate of rainfall you are likely to have. State-by-state rainfall intensity information is available on the National Climate Data Center website (see Resources below).
Multiply the watershed area by the rainfall intensity in inches per hour. This gives you the maximum area a gutter can handle with a rainfall rate of one inch per hour (this is how gutter capacity is normally rated). For example, if the maximum rainfall intensity in your area is eight inches per hour and the watershed area is 495 square feet, you have 495 square feet x 8 inches = 3,960 square feet at one inch of rainfall per hour.
Check the capacity rating of gutters your building supply store carries. If a particular gutter design has a capacity less than your calculated maximum (from Step 4) the gutter is too small and you'll need at least one size larger. Select a gutter size that has a capacity at least equal to your requirements.
Calculate downspout size. Downspouts can handle about 100 square feet of watershed area per cross-sectional square inch. Round the watershed up to the next even 100 then divide by 100 to determine the square inches of downspout cross section required. For example, a 1,550 square foot watershed area needs at least 16 square inches of downspout cross section. A 2-inch by 3-inch downspout has a 6-square inch cross section (2 inches times 3 inches) while a 3-inch by 4-inch downspout has a 12-square inch cross section. In this example, you need three 2 by 3-inch downspouts (a total 18 square inches cross section) or two 3 by 4-inch downspouts (24 square inches total cross section) to at least equal the minimum 16 square inches required.