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Since effective ISP speed is based on bandwidth, load (both global and service-based), and latency, it's not the most easy thing to calculate. There is a special ISP technology for performing such measurements; it's basically a stress test. There are different tests for different ISP services. One popular one is called Apache Bench. It is used for measuring an ISP's speed for serving Web pages. Note though that these ISP speed tests are used by ISPs on themselves; using one on an unsuspecting ISP will probably be considered a denial-of-service attack and can potentially get one in legal trouble.