How many feet of string lights do I need for a patio? Most guides just add a flat 10–15% for sag. This page instead uses the sag arc length, so a deep drape correctly costs more cord and a taut run costs almost none. It runs entirely in your browser and saves or sends nothing.
True parabola arc length: S = L + 8d² ÷ (3L), where L is the span and d is the sag, both in the same units. Strands needed = ceiling of span cord ÷ strand lit length. Bulbs = ceiling of span cord in inches ÷ bulb spacing. Total cord to buy for one span = span cord + lead run. If you have several spans, run this page once per span and add the strands — do not just multiply one span, because sag differs per span.
With the defaults above: span 20 ft, sag 24 in (2 ft). Arc length = 20 + 8 × 2² ÷ (3 × 20) = 20.53 ft. That needs 1 strand of 25 ft, about 21 bulbs at 12 in spacing, and 30.53 ft of cord in total including the 10 ft lead run. For comparison, a sag of 15% of the span adds about 6% length, and a sag of 22% adds about 13% — illustrative, from the same arc formula, not a flat rule to apply blindly.
Is this a structural calculation? No. Spans over about 20 ft usually need a guide wire and proper anchors; this page does not check load, wind or anchor strength. What about multiple spans or a zigzag? A zigzag is several spans, not one longer span — calculate each run separately. Do bulbs at the ends count? The bulb count here rounds up from spacing, so check your strand's actual first and last bulb positions before buying. This page is not electrical or construction advice.
This is a free, single-purpose calculator. All calculation happens in your browser. No account, no upload, no tracking by this page beyond any ads Google may serve after approval.