Cable Sag & Tension Calculator
Tension in a wire/cable from its span, sag and loading (parabolic).
Tension EXPLODES as you pull the sag out: halve d, double T. A 'tight' clothesline or festoon cable can quietly carry tonnes into its anchors — this is why.
Formula
Cable Sag & Tension Calculator is a free cable tension sag for structural engineers, fabricators and site engineers — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Cable Sag & Tension Calculator
Tension in a wire/cable from its span, sag and loading (parabolic). The calculation implements H = wL²/8d; T_max = √(H² + (wL/2)²) (Parabolic cable theory; transmission line practice). Tension EXPLODES as you pull the sag out: halve d, double T. A 'tight' clothesline or festoon cable can quietly carry tonnes into its anchors — this is why.
How to use Cable Sag & Tension Calculator
- 1Enter Cable load per metre in N/m (Self-weight + ice/garland/lights etc.).
- 2Enter Span in m.
- 3Enter Sag at midspan in m.
- 4Read Horizontal tension, Max tension (at support) instantly — no submit button needed.
- 5Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Cable Sag & Tension Calculator?
- ✓Implements the standard formula — H = wL²/8d; T_max = √(H² + (wL/2)²)
- ✓Reference cited on-page: Parabolic cable theory; transmission line practice
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Live worked example: the substitution recomputes from your numbers
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Cable Sag & Tension Calculator use?+
It computes H = wL²/8d; T_max = √(H² + (wL/2)²), per Parabolic cable theory; transmission line practice. The formula is displayed under the result along with a worked example substituted with your own inputs.
What should I keep in mind when using this calculator?+
Tension EXPLODES as you pull the sag out: halve d, double T. A 'tight' clothesline or festoon cable can quietly carry tonnes into its anchors — this is why.
Can I use this for real structural design?+
It implements the exact textbook/code formula cited below the result and is ideal for sizing, checking and learning. Final designs should be verified by a qualified engineer against the full code with all load cases.
Is the Cable Sag & Tension Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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