A copayment is usually a set amount, rather than a percentage. Copayments (sometimes called “copays”) can vary for different services within the same plan, like prescription drugs, lab tests, and visits to specialists. Generally, plans with lower monthly premiums have higher copayments. Plans with higher monthly premiums usually have lower copayments.

Here’s one example of how a copay could work: Let’s say your health insurance plan’s copayment for a doctor visit is $20, and the plan’s allowable cost for a doctor’s office visit is $100. If you’ve paid your deductible, you would pay $20, usually at the time of the visit. If you haven’t met your deductible, you would pay $100, the full allowable amount for the visit.

References

https://www.medicare.gov/glossary/c.html
https://www.medicare.gov/find-a-plan/staticpages/glossary/planfinder-glossary.aspx
https://www.healthcare.gov/glossary/co-payment/