SAG Calculator
ConstructionCalculate 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.
Sag (feet)
1.25
Sag (inches)
15
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.
How to use this SAG calculator
- Enter the Span in feet — the horizontal distance between the two support points.
- Enter the Weight per Foot of your cable or wire, in pounds per foot, from the manufacturer's spec sheet.
- Enter the Tension applied to the cable, in pounds.
- Read the Sag (feet) result to check against required clearance for your installation.
- Reference Sag (inches) if you need a finer-grained measurement for shorter spans like fence wire or guy wires.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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