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t12 12/8ths.. T5 (any variety) 5/8th inches in diameter..A T8 lamp (on the right in the figure below) is eight-eighths of an inch, or one inch (2.54 cm), in diameter. Both T12 and T8 lamps use the medium bi-pin base, which allows T8 lamps to fit into the same luminaires as T12 lamps of the same length