Sag Ratio Calculator
Every cable, wire, or conductor sags under its own weight. This calculator uses the parabolic sag formula to find the midspan sag distance, sag-to-span ratio, and total cable length you need between two support points. Essential for power line design, rigging, zip lines, and catenary lighting.
The parabolic sag formula is Sag = (w x L²) / (8 x T) where w is the cable weight per linear foot, L is the span between supports, and T is the horizontal component of tension. This approximation is accurate when the sag ratio (sag / span) is below 0.1, which covers most practical applications.
The sag ratio (sag divided by span) is the key design parameter. For power transmission lines, typical sag ratios are 2-5% depending on temperature and ice loading. For catenary lighting or decorative cables, 3-5% gives a pleasing drape. For structural cables (bridges, guy wires), sag ratios are usually kept under 2%.
Temperature affects sag significantly. Cables expand in heat and contract in cold, which changes the sag. A power line designed for 2% sag at 60°F might sag 3-4% at 120°F. This is why power line clearances are calculated at the maximum expected conductor temperature, not at installation temperature.