Three feet make one yard, exactly. It is the simplest conversion in the imperial system and the one that reveals its structure most clearly: the yard is the defined unit, and the foot exists as a third of it rather than the other way round.
About the units
Since the International Yard and Pound Agreement of 1959 the yard has been exactly 0.9144 metres, and every other imperial length follows from that number. The foot is a third of a yard, the inch a thirty-sixth, the mile 1,760 yards. This hierarchy is worth knowing because it explains why the foot's metric value, 0.3048 m, has four decimal places while the yard's has four as well and the two are related by an exact factor of three. The yard's own origins are disputed, with the double cubit and the standard cloth rod both proposed, but its function has always been the same: a convenient length for measuring fabric and marking out ground.
The exact factor
The factor is exactly 3 in one direction and exactly one third in the other. That means yards to feet is clean — multiply by three — while feet to yards produces repeating thirds for any measurement not divisible by three. Ten feet is 3.333... yards, which is why materials sold by the yard are ordered in whole yards and cut down rather than measured in feet and converted.
Where you meet this conversion
Construction materials are the main case. Concrete is ordered by the cubic yard in the United States while formwork and slab dimensions are set out in feet, so a slab 20 by 30 feet at 4 inches thick needs the whole calculation done in feet and then divided by 27 to reach cubic yards. Carpet, turf and topsoil are sold the same way. In sport, American football marks the field in yards while player heights and goalpost dimensions are in feet, and track and field mixes both in older records.