Sag Calculator: Cable, Wire, and Rope Sag Under Tension
Calculate the sag of a suspended cable, wire, or rope from span length, unit weight, and horizontal tension using the parabolic approximation of the catenary curve. Solve for sag, required tension, or maximum span, then check clearance against your site conditions.
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View Chart →How the Sag Calculation Works
Measure the Span
Determine the straight-line horizontal distance between your two support points.
Find Unit Weight
Look up or weigh a sample length of your cable, wire, or rope per foot.
Apply the Formula
Sag equals weight times span squared, divided by eight times horizontal tension.
Check Clearance
Compare the result against ground clearance or obstacle height requirements.
Sag Reference Values
Formula
Parabolic approximation, accurate for sag under 10% of span.
Typical Power Line Sag
Sag ratio for distribution conductors at final loading condition.
Extra Cable Length
Approximate added length a sagged cable needs over straight span.
Thermal Effect
Sag increases in hot weather, decreases in cold; not modeled here.
What Cable Sag Means for Your Project
Sag is the vertical distance a suspended cable, wire, or rope drops below a straight line connecting its two support points. Every cable sags to some degree under its own weight unless tension is infinite, which is physically impossible.
The exact shape of a hanging cable is a catenary curve, described by hyperbolic cosine functions. For most construction and utility work where sag stays under roughly 10 percent of the span, engineers substitute a simpler parabolic approximation: Sag = (w × L2) / (8 × T). This calculator uses that parabolic formula because it matches catenary results within about 1 percent error in the sag ranges typical of guy wires, fence lines, temporary power drops, and low-voltage overhead runs.
Getting sag right matters for three reasons. First, ground clearance: national and local electrical codes set minimum clearance heights for overhead conductors, and excess sag can put a line below code minimums. Second, structural load: the tension you calculate feeds directly into pole, anchor, and bracket design. Third, material ordering: a sagged cable is physically longer than the straight-line span, so ignoring sag when ordering wire or rope leads to running short on-site.
Sample Calculation Scenarios
Scenario: Temporary Power Drop
Span: 100 ft
Weight: 0.5 lb/ft
Tension: 500 lb
A 1.25 ft sag on a 100 ft span is a 1.25 percent sag ratio, well within the 2-5 percent range typical for distribution lines.
Scenario: Solving for Required Tension
Span: 60 ft
Weight: 0.15 lb/ft (guy wire)
Target sag: 0.5 ft
A guy wire installer needing no more than 0.5 ft of sag over a 60 ft run must pull to at least 135 lb of horizontal tension, before applying any safety design factor.
Common Mistake: Confusing Slope Length with Span
Error: Using cable length instead of horizontal span
Correct input: Horizontal distance only
Always measure the horizontal distance between anchor points, not the physical cable length, which is always slightly longer once sagged.
Errors That Skew Sag Calculations
Mixing Total Tension with Horizontal Tension
On an inclined span, the tension measured along the cable at the support is higher than the horizontal component used in this formula. Southwire's inclined span sag reference resolves this with separate horizontal and along-cable tension terms; using the wrong one overstates or understates sag.
Ignoring Ice or Wind Load Additions
Unit weight should include ice accretion or wind-induced load for outdoor spans in freezing climates. Utilities calculate separate sag-tension cases for bare, ice-loaded, and wind-loaded conditions per IEEE sag-tension guidance rather than a single static weight.
Applying the Parabolic Formula Beyond Its Range
The parabolic approximation loses accuracy once sag exceeds roughly 10 percent of span. Very slack cables, hammocks, or heavily loaded spans need the full catenary equation with hyperbolic cosine terms instead of this simplified formula.
Forgetting Temperature and Loading Case Variation
A single sag value only describes one condition. Final sag after long-term creep, hot-weather sag, and cold-weather sag can all differ meaningfully for the same cable and span, as documented in southwire and IEEE sag-tension references.
Where Sag Calculations Apply on Site
Electrical Contractors
Overhead service drops and temporary power runs must clear driveways, roads, and pedestrian areas per NESC ground clearance rules.
Structural & Rigging Crews
Guy wires, cable bracing, and temporary rigging lines need predictable sag to avoid interference with equipment and structures.
Fencing Contractors
Chain link top rail wire and high-tensile fence wire sag between posts affects spacing decisions and post height requirements.
Telecom & Utility Installers
Drop wires and aerial cable runs use sag-tension charts to set installation tension that keeps final sag within design limits.
Frequently Asked Questions
The standard parabolic approximation is Sag = (w × L2) / (8 × T), where w is weight per unit length, L is span, and T is horizontal tension. This formula is accurate within about 1 percent for sag ratios under 10 percent of span, per Engineering Toolbox cable load references.
It stays accurate for sag ratios under roughly 10 percent of span length. For long overhead line spans or higher sag ratios, utilities switch to the full catenary equation using hyperbolic cosine functions for better precision.
There is no single universal target. Overhead power conductors commonly run 2 to 5 percent sag ratio depending on loading case, while fence and low-voltage lines vary more widely. Ground clearance requirements under NESC Rule 232 ultimately set the ceiling on allowable sag for a given pole height and span.
Yes. Cables expand and sag more in heat and contract with less sag in cold. Utility sag-tension studies calculate separate results for initial, final, hot, cold, and ice-loaded conditions. This calculator returns a single-condition estimate and does not model thermal expansion coefficients directly.
Sag is the vertical drop at midspan in feet or inches. Percent sag divides that value by span length and multiplies by 100, which lets you compare relative slackness across spans of different lengths.
Rearrange the parabolic formula to T = (w × L2) / (8 × Sag). Use the Find Tension mode above to run this calculation directly by entering your target sag instead of a known tension value.
A sagged cable follows a curved path, so it always measures longer than the straight-line span. The approximate extra length is 8 × Sag2 / (3 × Span), per the standard catenary length approximation used in Engineering Toolbox cable load calculations. Account for this when ordering material.
Sources & Methodology
- Parabolic sag formula, Sag = wL²/8T: Engineering Toolbox, Cable Loads.
- Approximate sagged cable length, s = L + 8h²/3L: Engineering Toolbox, Cable Loads.
- Inclined span sag-tension methodology and horizontal versus along-cable tension: Southwire Overhead Conductor Engineering Data, Appendix J.
- Catenary curve mathematical basis (hyperbolic cosine formulation): Omni Calculator, Catenary Curve Calculator.
- Sag-tension calculation conditions (initial, final, hot, cold, ice): IEEE Transmission and Distribution Committee, Sag-Tension Calculations tutorial.
- Ground clearance requirements for overhead conductors: National Electrical Safety Code (NESC), Rule 232.
Last reviewed: July 2026.
Engineering Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 §1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
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