A CFL light bulb can produce the same amount of bright light, measured in lumens, as a tungsten light bulb, but uses 75 to 80 percent fewer watts to do so. According to Investment University, by 2012, tungsten light bulbs will be banned by the U.S. government, so it's best to get used to the conversion now.
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Determine the wattage of your old (tungsten) light bulb. Standard household lighting frequently uses 60 watt tungsten light bulbs.
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Multiply the old wattage by .25. For example, 60 times .25 equals 15, so you will want a 15 watt CFL bulb.
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If you're wondering where the .25 percent came from, remember that CFL bulbs use approximately 75 percent less wattage than tungsten. This means their actual wattage needed is 25 percent of a tungsten bulb's wattage, so 100 percent (full wattage) minus the 75 percent reduction is 25 percent, or .25.
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Look for CFL bulbs near the range of your wattage result, as actual brightness can vary. A typical 60 watt old light bulb can be replaced by a 12 to 15 watt CFL, although 13 to 15 watts is recommended by CFL manufacturers.