# Sag Ratio Calculator

Calculate cable sag from span, weight, and tension. Get sag distance, sag ratio, and actual cable length. Free sag calculator for power lines and rigging.

## What this calculates

Every cable, wire, or conductor sags under its own weight. This calculator uses the parabolic sag formula to find the midspan sag distance, sag-to-span ratio, and total cable length you need between two support points. Essential for power line design, rigging, zip lines, and catenary lighting.

## Inputs

- **Span Length** (ft) — min 1 — Horizontal distance between the two support points
- **Cable Weight Per Foot** (lb/ft) — min 0.01 — Linear weight of cable, wire, or conductor
- **Horizontal Tension** (lbs) — min 1 — Horizontal component of cable tension

## Outputs

- **Sag Distance** (ft) — Maximum vertical drop at midspan
- **Sag Ratio** — Sag / Span (typical: 0.02-0.05)
- **Sag Percentage** (%)
- **Actual Cable Length** (ft) — Total cable length accounting for sag

## Details

The parabolic sag formula is Sag = (w x L²) / (8 x T) where w is the cable weight per linear foot, L is the span between supports, and T is the horizontal component of tension. This approximation is accurate when the sag ratio (sag / span) is below 0.1, which covers most practical applications.

The sag ratio (sag divided by span) is the key design parameter. For power transmission lines, typical sag ratios are 2-5% depending on temperature and ice loading. For catenary lighting or decorative cables, 3-5% gives a pleasing drape. For structural cables (bridges, guy wires), sag ratios are usually kept under 2%.

Temperature affects sag significantly. Cables expand in heat and contract in cold, which changes the sag. A power line designed for 2% sag at 60°F might sag 3-4% at 120°F. This is why power line clearances are calculated at the maximum expected conductor temperature, not at installation temperature.

## Frequently Asked Questions

**Q: What is a typical sag ratio for power lines?**

A: Power transmission lines typically have sag ratios of 2-5% at the ruling span. Distribution lines may sag 3-6%. The exact value depends on conductor type, span length, temperature, ice and wind loading, and required ground clearance. Higher tension means less sag but more stress on towers and hardware.

**Q: How does temperature affect cable sag?**

A: Cables expand when heated and contract when cooled. The coefficient of thermal expansion for aluminum conductor is about 12.8 x 10^-6 per degree F. A 100-degree F temperature increase on a 500-foot span can add 2-3 feet of sag. Power engineers calculate sag at the maximum expected conductor temperature to ensure ground clearance.

**Q: What is the difference between catenary and parabolic sag?**

A: A cable hanging under its own weight forms a catenary curve. The parabolic formula is a simplified approximation that is very accurate when the sag ratio is below 10%. For most engineering applications (power lines, rigging, zip lines), the parabolic formula is used because it is much simpler and the error is less than 1%.

---

Source: https://vastcalc.com/calculators/construction/sag-ratio
Category: Construction
Last updated: 2026-04-08