The inside dimension of this hoop is 4 3/8″ (4.375″, or 111mm)
Actual sewing area depends on brand of machine approximately 3.6″ or 91mm

This hoop has an inside dimension that is 4.375″ round. That dimension is the same size as a standard Tajima 12cm hoop, as well as many other hoop brands. Please note that your actual sewing area will be determined by the size of your presser foot. Most machines will be able to sew approximately 3 3/8 to 3 5/8″. This Mighty Hoop was designed for items like sleeves and cap backs. It was not made to hold thick and heavy items, like the original Mighty Hoops. It was designed with smaller magnets allowing it to have a smaller outer ring. This makes it easier to get into tight areas. It will not be able to hoop thick bags, workwear, leather, or other difficult fabric. The 4.25″square Mighty Hoop would be a better option for that type of embroidery. The 4.375″ Mighty Hoop will fit most commercial embroidery machines, including the Brother and Baby Lock 6 and 10 needle machines.

Mighty Hoop has a diverse selection of sizes and matches perfectly with Smartstitch embroidery machines.
Youtube video on adding custom frames to the S-1201:  
Youtube video on adding custom frames to the S-1501:  
Youtube video on separate and keep your Mighty Hoop: