From Bradley Mitchell,
Your Guide to Wireless / Networking.
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A. The range of a home Wi-Fi LAN depends on the wireless access point (WAP) or wireless router being used. Factors that determine a particular WAP or wireless router's range are:

• the specific 802.11 protocol employed
• the overall strength of the device transmitter
• the nature of obstructions and interference in the surrounding area

A general rule of thumb in home networking says that 802.11b and 802.11g WAPs and routers support a range of up to 150 feet (46 m) indoors and 300 feet (92 m) outdoors.

Another rule of thumb holds that the effective range of 802.11a is approximately one-third that of 802.11b/g.

Both of these rough estimates fall on the high end of the range seen in practice. Obstructions in home such as brick walls and metal frames or siding greatly can reduce the range of a Wi-Fi LAN by 25% or more. Because 802.11a employs a higher signalling frequency than 802.11b/g, 802.11a is most susceptible to obstructions. Interference from microwave ovens and other equipment also affects range. 802.11b and 802.11g are both susceptible to these.

Of course, it's possible to extend a Wi-Fi LAN to much longer distances by chaining together multiple wireless access points or routers.