Post Stock Dimensions
Common fence post lumber dimensions are 4-by-4 inches and 6-by-6 inches. Common standard lengths are 8, 10 and 12 feet. For outdoor construction, including fences and gate posts, rot-resistant lumber should be used. Cypress, cedar, redwood and pressure-treated fir or pine are popular choices. For a light to midweight gate, 4-by-4-inch stock is strong enough to support the gate. Especially large or heavy gates, such as privacy-fence gates greater than 6 feet tall, require 6-by-6-inch posts.
Gate post above-ground height depends on the fence style and personal preference. Many builders trim the gate posts to the same height as adjacent fence posts. Gate posts are often used to support overhead trellises to create an ornate entryway or garden focal point. If used to support an arch or trellis, the posts are typically trimmed to 7 to 10 feet above ground.
Because of lateral pressure exerted on the gate post that supports the gate and hinges, it must be buried deeper than fence posts. The gate post that's used to latch the gate must be installed securely enough to withstand the potential lateral force of repeated slamming of the gate. When installing fence posts, as a general rule, 1/4 of the overall length of the post is buried underground. To securely install gate posts, 1/3 of the post's length should be buried.
To calculate the total necessary length of the gate post, multiply the desired above-ground height by 1.5. Measure and mark the post 1/3 of its length from the bottom end. The portion between the bottom of the post and the mark is to be buried.