The Opening. There should be no obstacles in the gate opening. Make sure there are no objects to hit when the gate swings. The opening's width at the top should be within 1/2" of the width at the bottom, or your gate will hang really funny because your gate posts are a bit tilted!
For a One Panel Gate. Order a gate 3/4" less wide than the opening, and make sure to measure where the opening is tightest. For the height, make sure you can clear the ground or floor by 1-2",and that your gate can fit under any post caps, if they exist.
For a Two panel Gate. Make sure that the width of the two panels, when added together, are 1" less wide than the opening. You need these tolerances for your hinges, for your panels to swing, and for any slight tilting that may occur in the gate posts over time.
Gate Height and Gate Sides. Take into account that the sides of the gate will be 1" longer than the height of the gate that you buy. Height is measured from the bottom of the footrail to top of the handrail.
Solid posts. Gates must hinge to a 4x4 post or to a similar structure that is well-anchored into the ground or deck. We recommend a 4x4 because the strap hinge needs a surface a few inches across for the base of the hinge.
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Need guidance on child safety features? In general, air gaps should be under 3 inches. More information at Consumer Reports Buying Guide.