Estimate the cord length for an evenly spaced zigzag over a rectangular patio. The calculation models a specific path: a chosen number of diagonal spans crossing the patio width while progressing along its length. It then adds your sag allowance and the lead from the outlet. A visual layout and an on-site measurement should confirm the route before purchase.
Decide what counts as a span
Measure the patio length and width in feet. In this tool, each span is one diagonal crossing from one long side to the other. Two spans make a back-and-forth pair; they are not the same as two complete X shapes. Mark the starting anchor, alternating side anchors, and finishing anchor on a sketch before entering the count.
The model advances the same distance along the patio on every crossing: total patio length divided by the number of spans. All anchors are assumed to lie in one horizontal plane. A pergola with different anchor heights, an off-center pole, or a route around an obstacle will need direct segment measurements instead of this uniform model.
How the route length is calculated
For N spans, longitudinal advance is patio length ÷ N. Each straight diagonal is the square root of width² + advance². Multiplying by N gives the modeled straight route. The sag percentage increases that route by a simple percentage; the lead length is then added separately. This is a buying allowance, not a physical catenary or structural load calculation.
Keep the outlet-to-first-anchor lead distinct from the illuminated route. Some products have a long unlit lead, while others place a socket near the plug. Advertised total string length and illuminated length are not always interchangeable. The calculator cannot know the exact spacing, end connectors, or plug-to-first-bulb dimensions of your selected product.
Worked example: a 7-by-18-foot patio
For a patio 18 feet long and 7 feet wide, choose 6 diagonal spans. The lengthwise advance is 18 ÷ 6 = 3 feet. Each span is √(7² + 3²), approximately 7.62 feet, giving approximately 45.69 feet of straight route. A 10% sag allowance increases that to approximately 50.26 feet.
Add a 6-foot lead and the estimated total is approximately 56.26 feet. A single 48-foot string would not cover that modeled route. A longer product or permitted connected strings may work, but verify the usable length, connection limits, and actual anchor route. Choosing fewer crossings substantially changes the result and the appearance.
Check the hardware and product instructions
Measure the proposed path with a nonconductive measuring line before ordering, including changes of height and the outlet route. Check that the product is rated for the intended outdoor use and mounting conditions. Follow its instructions for supports and connections; the light cord should not be assumed to serve as a structural support cable.
Connection limits are product-specific. Two strings of the same length can have different bulbs, power draw, and maximum permitted runs. Read the manufacturer’s instructions rather than infer a safe connected length from this calculator. The tool does not assess electrical capacity, select anchors, establish safe clearances, or verify whether a building attachment can take the load.
Common questions
Does this calculate an X layout or perimeter lighting?
No. It models evenly advancing diagonal spans along a rectangle. For an X, perimeter, fan, or irregular layout, measure and sum the intended segments separately.
Is 10% sag a safety recommendation?
No. It is only an example estimating allowance. Appropriate mounting and actual sag depend on the product, supports, and installation instructions.