Cable Span Sag Calculator
Enter a span, the cable’s weight per meter, and the horizontal tension to estimate midspan sag, the tension at the supports, and the cable’s length across the span.
Sag & cable length
How towers, tension, and sag fit together
Every aerial ropeway — a gondola, a chairlift, or a full aerial tramway — carries at least one cable strung between towers under tension. Pull it tighter and it sags less but pulls harder on the towers; let it sag more and the towers see less load but the cable needs more clearance above the terrain below. This calculator applies the standard shallow-sag (parabolic) approximation used across cable engineering to show that trade-off with real numbers.
The same formula also underlies the tool’s support-tension figure: because a real span carries weight along its whole length, the towers feel more force than the flat, midspan tension alone.
Frequently Asked Questions
What is cable sag and why does it matter?
Sag is how far a suspended cable dips below a straight line between its two supports. Every real cable sags at least a little under its own weight — more tension pulls it straighter (less sag), while a heavier cable or a longer span sags more for the same tension. Ropeway and transmission-line engineers size the sag and tension together so the cable clears towers, terrain, and other lines with margin at every temperature and load condition.
What is the parabolic sag approximation?
A hanging cable technically follows a catenary curve (a hyperbolic cosine), but when the sag is small compared with the span — the normal case for a taut ropeway track cable or haul rope — a parabola is a very close and much simpler stand-in. This tool uses that standard approximation: sag ≈ (w·L²) / (8·T), where w is the cable's weight per unit length, L is the span, and T is the horizontal tension.
Why is the tension at the supports higher than the horizontal tension?
The horizontal tension you enter is the pull along the flattest part of the cable, at midspan. At the towers the cable also has to support the weight hanging below that point, adding a vertical component. The resultant tension at the support is √(T² + (wL/2)²) — always a bit more than the horizontal value, and more so on longer or heavier spans.
Does this replace a certified ropeway engineering calculation?
No. This tool is an educational approximation for understanding how span, weight, and tension trade off — it ignores temperature effects, ice/wind loading, cable stretch, and safety factors. Real aerial tramway and gondola cables are designed and certified under ropeway safety standards (such as EN 12929 or equivalent national codes) by a qualified engineer, not estimated with a simple formula.
Educational estimate only, using a shallow-sag approximation — not a certified engineering calculation. Real ropeway cable design accounts for temperature, ice and wind loading, and safety factors, and must be performed by a qualified engineer under the applicable ropeway safety code.