The best way to determine the correct length for your flat back piercing is to measure the post of a piece of body jewelry you already have that fits you well. You can also measure your piercing itself by measuring the length between the two holes using a ruler or caliper. Otherwise check with your piercer for which length suits your needs.
All bodies and piercings are different
If you are unsure where to start we suggest 10mm for lobe, 8mm for upper lobe, 6mm for tragus or lip, and 4mm for conch and helix based on average measurements for these piercings.