Key limes

The Key Lime lime (Citrus aurantifolia Swingle), also referred to as a Mexican Lime, bears a profuse amount of small, thin-skinned, greenish-yellow limes. This juicy lime is known for it's distinctive aroma high-acid content. The image shows the size and color difference between the more common Persian lime and the Key Lime.  The average Persian lime weighs about 3 ounces and is about 2" in diameter as opposed to the small Key lime which averages 1 ounce each and is about 1 1/2" in diameter.

Persian lime (grocery store limes) equal amounts

1lb = 16 key limes
12 key limes = about 1/2 cup lime juice