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SAG Calculator

Construction

Calculate the sag of a horizontally spanned cable or wire under its own weight and tension. Enter span, weight per foot, and tension for an instant estimate.

Reviewed by the thecalcu.com team · Last updated July 18, 2026

52,000
0.0120
1050,000

Sag (feet)

1.25
Sag (inches)
15

This calculator computes your Sag (feet), Sag (inches) from the values you enter.

Inputs
SpanWeight per FootTension
Outputs
Sag (feet)Sag (inches)

What is a SAG?

A SAG Calculator computes the vertical droop of a horizontally spanned cable or wire caused by its own weight acting against the tension holding it taut. Sag is a fundamental consideration in overhead power line design, guy wire installation, fence construction, and any application where a cable spans a distance between two fixed support points.

Using the standard parabolic cable approximation, accurate for the vast majority of practical spans where sag is small relative to span length, this calculator converts your span, cable weight per foot, and applied tension directly into a sag figure in both feet and inches. It's a useful companion to the Rolling Offset Calculator for other trade-specific geometry calculations used in construction and installation work.

Why Use a SAG Calculator?

Getting sag wrong in a cable or wire installation creates real problems: too much sag risks inadequate ground clearance or contact with structures, while too little sag (over-tensioning) can stress the cable, its supports, or its termination hardware beyond safe limits. Manually working through the parabolic sag formula by hand is error-prone, especially when checking multiple tension or span scenarios.

This calculator solves that by taking your Span, Weight per Foot, and Tension directly and returning the expected sag instantly, letting you compare different tension settings or span lengths before committing to an installation.

Who Should Use This Calculator?

Electrical linemen and utility engineers planning overhead distribution or transmission line spans use it to verify clearance meets code minimums. Fence contractors installing high-tensile wire fencing check expected sag between posts to plan post spacing. Riggers and installers setting guy wires for towers or temporary structures use it to confirm adequate tension. Telecommunications technicians running aerial cable between poles reference sag calculations for clearance planning. Engineering students studying cable statics can use it to verify hand-worked catenary and parabolic approximation problems.

What Insights Does the SAG Calculator Give You?

Sag (feet) is the primary result, the vertical droop of the cable at its midspan, expressed in feet. This is the figure to compare against required ground clearance or code minimums for your application.

Sag (inches) presents the same result converted to inches, which is often the more practical unit for shorter spans like fence wire or guy wires where feet would be an awkwardly small number.

How to use this SAG calculator

  1. Enter the Span in feet, the horizontal distance between the two support points.
  2. Enter the Weight per Foot of your cable or wire, in pounds per foot, from the manufacturer's spec sheet.
  3. Enter the Tension applied to the cable, in pounds.
  4. Read the Sag (feet) result to check against required clearance for your installation.
  5. Reference Sag (inches) if you need a finer-grained measurement for shorter spans like fence wire or guy wires.
Show formula & methodology ↓Show less ↑

Formula & Methodology

The calculator uses the standard parabolic cable approximation, valid for spans where sag is small relative to span length:

Sag (feet) = (Weight per Foot x Span²) ÷ (8 x Tension)

Sag (inches) = Sag (feet) x 12

Worked example: A 100 ft span with a cable weighing 0.5 lb/ft under 500 lb of tension:
- Sag (feet) = (0.5 x 100²) ÷ (8 x 500) = 5,000 ÷ 4,000 = 1.25 ft
- Sag (inches) = 1.25 x 12 = 15 in

This 1.25 ft of midspan sag would need to be checked against the minimum ground clearance required for the specific application and local code before finalizing the installation tension.

Frequently Asked Questions

What is sag in a cable or wire span?
Sag is the vertical distance a cable or wire droops below a straight line between its two support points, caused by its own weight pulling it downward under tension. It's a critical factor in overhead power lines, guy wires, fence lines, and any horizontally spanned cable where clearance and tension both matter.
What is the formula for calculating cable sag?
The parabolic approximation formula is sag in feet equals weight per foot times span squared, divided by 8 times tension. This approximation works well for cables where the sag is small relative to the span, which covers most practical wire and cable installations.
What is the difference between sag and tension in a cable?
Tension is the pulling force holding the cable taut between its supports, while sag is the resulting droop caused by the cable's weight acting against that tension. Increasing tension reduces sag, but excessive tension raises the risk of cable failure or support damage, so the two must be balanced.
Why does span length affect sag so much?
Sag increases with the square of the span length in the parabolic formula, meaning doubling the span quadruples the sag for the same weight and tension. This is why long spans in power transmission lines require either much higher tension or intermediate support towers to keep sag within safe limits.
Is the parabolic sag formula accurate for all cable spans?
The parabolic approximation is accurate for spans where sag is small compared to span length, which describes the vast majority of practical installations like fence wire, guy wires, and moderate-span power lines. Very long or very slack spans, such as suspension bridge main cables, require the more complex catenary formula instead.
How much sag is normal for a power line?
Sag varies widely by span length, conductor weight, and utility design standards, but typical distribution line spans often sag 1-3 ft over a 100-150 ft span. Transmission lines spanning much longer distances between towers are engineered with specific sag-tension charts by the utility rather than a single rule of thumb.
How do I measure weight per foot for my cable?
Weight per foot is usually printed on the cable or wire spec sheet from the manufacturer, listed in pounds per foot or sometimes pounds per 1000 feet. If unavailable, weigh a known length of the cable on a scale and divide by that length to calculate the value.
What happens if sag is too large in a span?
Excessive sag reduces ground clearance below power lines, increases the risk of contact with vegetation or structures, and can violate utility and safety code minimum clearance requirements. It also indicates the cable may be under-tensioned relative to its design load, which can affect long-term performance.
Can I use this calculator for fence wire sag?
Yes, the same parabolic formula applies to fence wire, guy wires, and any horizontally spanned cable under tension, not just power lines. Enter your fence wire's weight per foot and the tension you're applying to estimate the expected sag between posts.
Does temperature affect cable sag?
Yes, cables expand and sag more in hot weather and contract with less sag in cold weather, which is why utilities calculate sag at multiple design temperatures. This calculator computes sag for a single fixed tension value, for temperature-dependent analysis, consult utility sag-tension engineering tables.
Also known as
cable sag calculatorwire sag calculatorcatenary sag calculatorpower line sag calculatorsag tension calculator