BitPay consolidates market depth from multiple exchanges to provide buyers with a Bitcoin Best Bid (BBB) exchange rate. BitPay currently calculates BBB based on Bitcoin/US Dollar rates because of the maximum liquidity.
To calculate the exchange rate for US Dollars, we pull the market depth from exchanges with adequate liquidity and withdrawal capability in USA and the Eurozone. The exchange order books are merged into a Consolidated Level II table.
The BBB is calculated by simulating an auto-routing market sell order, across all exchanges, with zero commission fees. Buyers will always get a better value by spending their bitcoins at a BitPay merchant than by selling them on an exchange.
The BBB is available via JSON API at https://bitpay.com/api/rates. Rates are updated every 1 minute.