I’ve just upgraded to the 30 MB/s internet plan at Charter cable (and added HBO so we can watch Game of Thrones), so here’s the obligatory speedtest results.

1 GB refers to a gigabyte (10^9 bytes) in this context, since we are talking about file sizes and network speeds. If we were talking about RAM, a GB would actually refer to a gibibyte. However, 1 Mb is a megabit (10^6 bits), not a megabyte (10^6 bytes), because of the small-case b. So 1 Mb is actually 1/8 MB (since there are 8 bits per byte).

(size) / (speed) = (time)

(18.3 x 10^9 bytes) / ( (30 x 10^6 bits / sec) x (1 byte / 8 bits) = 18.3 x 10^9 * 8 / 30 x 10^6 = 4880 seconds = 81.3 minutes

Wolfram Alpha gets the answer right, too (and I like teh natural language query – very intuitive).

Now, suppose I’m rocking 300 KB/s according to a certain beta software download client. How am I really doing? The capital B means it is kilobytes, so that’s actually 300 x 10^3 x 8 = 2400 x 10^3 = 2400000 = 2.4 Mbps. Wait, what??

This is why it’s important to do the math. Of course, the download speed may be limited by a lot of other factors, most notably how fast the server at the other end can deliver the data. I clocked almost 40 Mbps doing a speedtest with some local, low-ping server somewhere, but for downloading this big file I’m probably going a lot further and their server has a lot more to do than humor my ping requests. I guess I should be satisfied.

(But, I’m not. grrr….)

1. I usually use 10:1 as the bit:byte conversion rate, with the extra 2 being for overhead of various kinds. In other words, if I have 1 megabit of download, then I’ll get about 100 kilobytes per second.

2. Pete Zaitcev says:

This may be hazardous on high-overhead networks, such as radio (WiFi) and USB.

3. My preference is to calculate the exact number i should be getting, then compare to measured reality. That gives me a sense of my performance – bundling in “overhead” as a rule of thumb obscures the information I am trying to divine for my purposes.

Pete, I should have mentioned that yeah my PC on which I am doing the test is on WiFi, not directly connected to the router. Good point. Since I am using 802.11n 5GHz, I am assuming that the speed of that connection is not the bottleneck (though I havent carefully parsed the table here to verify)

at any rate, the more important takeaway here is that your max download speed is more a measure of your simultaneous download usage capability than for any one file. There’s a plateau on how much bandwidth any one device can actually use, even if for HD-quality streaming (did someone calculate the BW requirement of an iPad HD streaming a movie somewhere?). The more BW you have the more devices you can support simultaneously, so I can game on my PC while my kids both watch Netflix on separate iPods and iPads. I think we have 5 devices in the house that support video streaming, not including the PCs, so this matters.