A co-pay is a fixed amount that a patient pays for a covered healthcare service, while coinsurance is a percentage of the cost of a covered healthcare service that a patient pays. Knowing these differences is important in billing and coding functions because it helps ensure that patients are charged the correct amount for their healthcare services. For example, a patient may have a co-pay of $20 for a doctor's visit, which means they pay $20 at the time of the visit and their insurance pays the rest. On the other hand, if a patient has a coinsurance of 20% for a medical procedure that costs $1,000, they would be responsible for paying $200 (20% of $1,000) and their insurance would pay the remaining $800.