Orb-weaving spiders modulate their web-building behavior in response to changes in their situation (internal or external state), and this capacity is expected to directly influence fitness. To understand the rules underlying the investment devoted to web construction, we need to quantify precisely the costs and benefits associated with web-building behavior. The sticky silk (capture spiral) is costly to produce and to set up and is a limiting factor in web building. Thus, it would be useful to have an efficient formula to calculate the total capture thread length (CTL) produced, which reflects the investment devoted to foraging activities. On the basis of web photographs we tested nine formulae on webs built by three spider species: Zygiella x-notata (Clerck), Araneus diadematus Clerck, and Larinioides sclopetarius (Clerck). We compared the calculated lengths with lengths directly measured from photographs. We showed that the most efficient formula (CTLvh) involved data recorded along both the horizontal and vertical axes. Because of its accuracy for all species tested, together with its ease of use in the field, this formula represents a useful tool for behavioral ecology studies on spiders.