SAG Calculator
Estimate how much a cable, wire, or rope will sag between two support points based on span length, weight, and tension.
What this calculator does
The SAG Calculator estimates the vertical droop (sag) that develops at the midpoint of a cable, wire, or rope strung between two supports of equal height, under its own weight and a given horizontal tension.
The formula
For a span that is much longer than its sag (a common condition for fences, guy wires, and utility spans), the cable shape closely follows a parabola. The standard approximation is Sag = (w × L²) / (8 × T), where w is the weight per unit length, L is the span between supports, and T is the horizontal tension applied to the cable.
The calculator also estimates the extra cable length consumed by the sag itself, which is useful when ordering material for a run.
- Higher tension reduces sag.
- Longer spans and heavier cable increase sag rapidly (sag grows with the square of the span).
This tool is for general planning and educational purposes. For safety-critical spans (overhead power lines, structural guy wires, elevated crossings) always follow the applicable engineering codes and have the installation reviewed by a qualified professional.