SAG (Cable Sag) Calculator

Find how much a cable or wire sags between two supports.

How the Math Works

The SAG calculator uses the fundamental catenary approximation formula: sag = w·L² ÷ (8·T). This equation relates the maximum vertical displacement (sag) of a cable to four key parameters: w (uniform weight per unit length), L (horizontal span length), and T (horizontal tension). The weight and span squared in the numerator show that heavier cables and longer spans create significantly more sag, while the tension in the denominator demonstrates how increased tightness reduces sag. The factor of 8 arises from the parabolic approximation of the cable's shape under uniform loading.

Practical Applications

To use this calculator for practical installations, first determine the cable's weight per meter from manufacturer specifications or measurement. Measure the exact horizontal distance between attachment points (span length). Calculate or specify the desired tension force, typically based on engineering requirements or manufacturer recommendations. Input these three values to find the expected sag, then adjust tension or cable selection to meet clearance requirements. This is essential for power lines, suspension bridges, stage rigging, and any suspended structural element.

Day-to-Day Use

This calculation directly impacts the safety and reliability of infrastructure around us. When power lines sag too low, they can contact trees and cause outages or fires. Understanding cable sag helps utility workers maintain safe clearances and plan vegetation management. It also affects the design of suspension bridges, where proper cable tension ensures structural stability. Even in everyday settings like stage performances or decorative lighting, knowing how much a cable will droop prevents accidents and ensures professional results.

Worked example

0.5 lb/ft over 100 ft at 500 lb → 1.25 ft sag.

FAQ