Cable Sag Calculator

ft
lbs/ft
lbs
Construction

Calculate the sag of a cable or wire rope under its own weight using parabolic approximation.

Formula

Sag = w × L² / (8H); Cable length ≈ L + 8s²/(3L); Max tension = √(H² + (wL/2)²)
  • Parabolic approximation; valid when sag < 10% of span.
  • w = cable unit weight (lbs/ft); L = horizontal span (ft); H = horizontal tension component (lbs).
  • Max tension occurs at the support points.

100 ft span, 0.5 lb/ft cable, 500 lb tension

Inputs
  • Horizontal Span: 100 ft
  • Cable Weight per Foot: 0.5 lbs/ft
  • Horizontal Tension: 500 lbs

Sag = 0.5 × 10000 / (8 × 500) = 5000/4000 = 1.25 ft. Max tension = √(500² + 25²) = 500.6 lbs.

Frequently asked questions

When is the parabolic approximation valid?
The parabolic approximation is accurate when sag/span < 10%. For heavy sag (e.g. power lines), use the exact catenary formula: y = (H/w) × (cosh(wx/H) − 1).