Cable Sag Calculator
Estimate mid-span sag using a simple parabolic approximation. Great for quick planning of overhead cables, ropes, or utility lines.
Formula used: sag = (w × L²) / (8 × T), where w is weight per unit length (N/m), L is span (m), and T is horizontal tension (N).
What Is Sag and Why It Matters
In overhead systems, sag is the vertical drop of a cable at mid-span compared with the support points. Every suspended cable has some sag, and that sag directly affects clearance, safety, and long-term reliability.
Whether you are hanging a service line between poles, planning a zip line, setting a messenger wire, or running temporary rigging, a quick sag estimate helps answer a critical question: Will this line stay high enough under expected load?
How This Calculator Works
This tool uses a classic engineering shortcut known as the parabolic approximation. For many practical spans with moderate sag, it gives a fast and useful estimate without requiring advanced catenary analysis.
sag (d) = (w × L²) / (8 × T)
where:
• w = cable weight per unit length in N/m
• L = span length in meters
• T = horizontal tension in Newtons
In addition to sag, this calculator estimates cable length along the curve and optional mid-span clearance if you provide support height.
Input Guide
1) Span Length
Measure the horizontal distance between supports. Use realistic field values, not route length over terrain.
2) Cable Mass per Meter
Enter manufacturer data if available. If you only know total mass and cable length, divide total mass by length to get kg/m.
3) Horizontal Tension
Use your design tension, not breaking strength. The calculator expects kN input and converts internally to Newtons.
4) Support Height (Optional)
If both ends are roughly equal height, enter that value to estimate lowest-point clearance: clearance ≈ support height - sag.
Practical Notes Before You Build
- Temperature: Cables elongate as temperature rises, increasing sag.
- Wind loading: Side load can change the cable shape and stress profile.
- Ice and rain: Added weight increases sag significantly.
- Creep over time: Some materials permanently elongate under sustained tension.
- Safety codes: Always confirm local utility, electrical, and structural standards.
Example Scenario
Suppose you have an 80 m span, cable mass of 1.2 kg/m, and horizontal tension of 18 kN. The calculator returns an estimated sag around 0.52 m. If your pole attachment is 12 m high, the modeled mid-span clearance is roughly 11.48 m.
That seems generous—but if winter ice doubles effective line weight, sag could increase sharply. This is why quick math is useful early, but final design should include worst-case loading.
When to Use a More Advanced Model
Use full catenary calculations or engineering software when:
- Spans are very long
- Support elevations are different
- Dynamic loads or vibration are expected
- Regulatory clearance margins are tight
- Public safety or high-value assets are involved
Bottom Line
A sag calculator is a fast decision tool. It helps you size tension, check likely clearances, and avoid obvious layout mistakes. Use it during planning and communication, then validate with code-compliant engineering before installation.