The "mid-market rate" — the number shown here and on most financial sites — is the midpoint between what currency traders are willing to buy and sell a currency pair for at that moment. It's a real number, but it's not a number any individual can actually transact at. It's the reference point everyone else prices around.
Banks, card networks, and transfer services all add a spread on top of that reference rate. The spread is the difference between the rate they give you and the mid-market rate, and it's how most of these services make money even when they advertise "no fees" or "0% commission." If a bank's screen shows you 1 GBP = 1.24 USD while the mid-market rate is 1.27, that 3-cent gap on every dollar is the fee — it's just not labeled as one.
This is worth checking specifically because the spread varies enormously by provider and by currency pair. Major pairs like EUR/USD or GBP/USD tend to have tight spreads because so much volume trades through them. Convert into a less commonly traded currency — a Southeast Asian or African currency, for example — and the spread can widen substantially, sometimes past 5%, because there's less liquidity for the provider to work with.
A practical way to check what you're actually being charged: look up the pair here first, note the rate, then compare it to the rate your bank or transfer app is offering for the same pair at roughly the same time. The difference, as a percentage of the amount you're converting, is your effective fee — regardless of what the provider's fee disclosure says.
Card payments abroad add a second layer. Many cards apply their own currency conversion markup on top of whatever the merchant's payment processor charges, and that markup usually isn't itemized on your statement — it's baked into the exchange rate the transaction posts at. If a card explicitly advertises "no foreign transaction fees," that's addressing one layer of cost, not necessarily the conversion spread itself.
None of this means every provider is trying to hide something. Fixed, transparent fees are a legitimate business model. The point is that "fee-free" and "commission-free" are marketing terms, not evidence of a good rate — the only way to know if you're getting a fair deal is to compare against the mid-market rate at the time of the transaction, which is exactly what a live reference rate is for.