Look at any exchange board: there are two numbers, not one. At the first they buy from you, at the second they sell to you. The distance between them is the spread.
Why it matters more than the fee
Because «zero-fee exchange» says nothing about the price. A service with no fee and a wide spread costs more than a service with a fee and a narrow one. What you should count is the bottom line: how many hryvnia land on your card from a given amount.
What makes it widen
Uncertainty. In a calm market the spread is narrow; in a sharp price move it widens, because the seller prices in the risk of not covering the trade in time. For the same reason the spread is usually wider in rare exchange directions.
How to calculate it yourself
Divide the difference between the rates by the buy rate. If USDT is bought at 45.00 and sold at 45.40, the spread is about 0.9 percent. On a thousand dollars that is nine dollars, and this is the figure worth comparing between services.
What the spread does not show
The network fee, which is paid separately and does not depend on the exchange. And the bank fee on crediting to a card, if there is one.
So what you compare is neither the rate nor the spread on its own, but the bottom line: how many hryvnia end up on the card from a given amount. A good service shows that number before the order is created, not after.
Read more: How to read exchange monitoring sites