Cable Sag Calculator
Work out the sag (droop) of a cable or wire strung between two level supports — for cable railings, guy wires, string lights, clotheslines, and overhead spans. Enter the span, cable weight, and either the tension or your target sag, and get the sag, the tension at the supports, the cable length, and a safety factor — with by-tension and by-span tables and a downloadable PDF.
| Tension | Sag | Sag ratio | Support |
|---|---|---|---|
| Enter a span to compare | |||
| Span | Sag | Sag ratio | Cable |
|---|---|---|---|
| Enter a span to compare | |||
How to use the cable sag calculator
Enter the span and the cable weight, choose whether you know the tension or the sag you want, and get the sag, the sag ratio, the tension at the supports, the cable length, and a safety factor against the cable’s breaking strength — plus reference tables for tension and span.
Enter span & cable
The distance between supports in feet, then pick a cable preset or type the weight per foot from the spec.
Choose what you know
Solve sag from a tension you’ll apply, or solve the tension needed to hold a target sag.
Add breaking strength
Optional — enter the cable’s rated strength to see the safety factor against the support tension.
Read the result & save a PDF
See sag, support tension, cable length and safety factor, compare tensions and spans, and download it.
How cable sag is calculated
For a level span the cable hangs in a shallow curve that the parabolic model captures well: the sag depends on just three things — how heavy the cable is, how far it spans, and how hard it’s pulled.
Worked example — 30 ft span, 0.10 lb/ft string lights, 40 lb tension
Sag: 0.10 × 30² ÷ (8 × 40) = 90 ÷ 320 = 0.281 ft
In inches: 0.281 × 12 = 3.4 in
Sag ratio: 0.281 ÷ 30 = 0.9% of span
Support tension: 40 + 0.10 × 0.281 ≈ 40 lb
Cable length: 30 + 8 × 0.281² ÷ (3 × 30) = 30.01 ft
A 30-foot light string at 40 lb tension droops about 3.4 inches — just under 1% of the span, with the cable only about ⅛ inch longer than the straight run.
Cable sag reference charts
The relationships that come up most — how sag reacts to tension and span, common cable weights, and the sag-ratio feel for a span.
| Tension | Sag | Sag ratio | Effect |
|---|---|---|---|
| 20 lb | 6.75 in | 1.9% | Loose, visible drape |
| 30 lb | 4.50 in | 1.3% | Gentle sag |
| 40 lb | 3.38 in | 0.9% | Snug |
| 60 lb | 2.25 in | 0.6% | Tight |
| 80 lb | 1.69 in | 0.5% | Very tight, high anchor load |
Sag is inversely proportional to tension — double the tension and the sag halves. Chasing the last bit of sag out of a line costs a lot of added pull on the end anchors for little visible gain.
| Span | Sag | Sag ratio | Cable length |
|---|---|---|---|
| 15 ft | 0.84 in | 0.5% | 15.00 ft |
| 22.5 ft | 1.90 in | 0.7% | 22.50 ft |
| 30 ft | 3.38 in | 0.9% | 30.01 ft |
| 45 ft | 7.59 in | 1.4% | 45.03 ft |
| 60 ft | 13.50 in | 1.9% | 60.08 ft |
Sag grows with the square of the span — double the distance and the sag quadruples at the same tension. Long spans need much more tension to stay snug, which is why wide runs load their anchors hard.
| Cable | Weight | Typical use |
|---|---|---|
| 1/8″ steel cable | 0.03 lb/ft | Light cable railing infill, small spans |
| Vinyl clothesline | 0.05 lb/ft | Clotheslines, light utility |
| 3/16″ steel cable | 0.06 lb/ft | Cable railing, light rigging |
| String lights | 0.10 lb/ft | Patio and festoon lighting (bare wire) |
| 1/4″ steel cable | 0.11 lb/ft | Heavier railing, catenary support lines |
| 1/4″ EHS guy wire | 0.12 lb/ft | Guy wires, mast and pole support |
Weights are for the bare cable. For a loaded line — string lights with heavy bulbs, a line carrying ice or wind — use the effective loaded weight per foot, since the extra load sags the span like heavier cable.
| Sag ratio (sag ÷ span) | Feel | Where it fits |
|---|---|---|
| Under 1% | Taut, nearly straight | Railings, structural spans, high tension |
| 1–3% | Gentle, controlled sag | Most working spans and guy lines |
| 3–5% | Visible, relaxed drape | String lights, decorative runs |
| Over 5% | Loose, deep swag | Deliberate festoon look; check clearance |
Sag ratio is the quick way to picture a span without doing the math each time. A looser ratio means lower tension and easier anchors; a tighter one means a straighter line but much higher anchor loads.
Sag is a tug-of-war: the cable’s weight over the span pulls the line down, and the tension pulls it taut. The sag is where they balance — more weight or span deepens it, more tension flattens it.
- Span matters most — sag grows with the square of the span, so a wider run sags dramatically more.
- Tension is the lever — double it to halve the sag, but watch the load it puts on the anchors.
- Some sag is unavoidable — a weighted cable can never be pulled perfectly straight, so plan for a little droop.
Everything the calculator works out
One set of inputs gives you the sag, the sag ratio, the tension at the supports, the cable length and a safety factor — plus a comparison across tensions and spans, and a report for the job.
The figures behind every span
Built for anyone stringing a cable between two points
Patio lights, a cable railing, a guy wire, a clothesline — the weight-span-tension balance is the same, and knowing the sag and the anchor load before you pull keeps a span both good-looking and safe.
Hanging patio string lights or a clothesline and wanting the right drape without over-tensioning the wire or the posts.
- Work from a target sag
- Use a loaded weight for heavy bulbs
- Aim for a 3–5% drape look
Setting cable railing infill to the tension the system calls for, and checking the sag stays within the look and the gap spec.
- Tension to the system spec
- Keep sag ratio under ~1%
- Size posts to the support load
Checking guy-wire or support-line tension and the load it puts on anchors, with a safety factor against breaking strength.
- Check the safety factor
- Use the rated breaking strength
- Confirm against an engineer
7 tips for getting cable sag right
The things that quietly overload an anchor or leave a line too slack — and the checks that keep a span safe and looking right.
Cable sag FAQ
The geometry, tension and safety questions people ask most when stringing a cable between two supports.
For a level span, sag = weight × span² ÷ (8 × tension), with weight in lb/ft, span in feet and tension in pounds. The answer is in feet, so multiply by 12 for inches.
For a 30-foot light string at 0.10 lb/ft and 40 lb tension: 0.10 × 30² ÷ (8 × 40) = 0.28 ft, about 3.4 inches. Rearranged, tension = weight × span² ÷ (8 × sag), so you can start from a target sag and find the tension to hold it.
There’s no single right answer — sag is a trade-off — but roughly 1 to 5 percent of the span is a common working range. Too little sag means very high tension and heavy loads on the supports; too much looks slack and may hang too low for clearance.
String lights are usually hung with a deliberate, generous drape. Structural cable like guy wires and railings is tensioned to a spec, where the allowable sag follows from the required tension. On anything load-bearing, let the required tension and support strength drive the sag.
Yes. Sag is inversely proportional to tension, so doubling the tension halves the sag and halving it doubles the sag. That’s why a cable can be pulled tighter to lift a sagging line, within the limits of the cable and supports.
The catch is that reducing sag this way drives up the force on the end anchors fast, and you can never pull a cable perfectly straight. Chasing zero sag with extreme tension is how anchors get overloaded and cables snap — lower tension with a bit more sag is usually safer.
Because the cable has weight, gravity pulls every point of it down, and only the tension along the line resists that. A perfectly horizontal cable would need a vertical component of tension to hold the weight up, but a truly straight horizontal line has no vertical component to give — so some sag is unavoidable.
As tension rises the sag shrinks toward zero but never reaches it, while the tension climbs toward infinity. That’s why every real span has at least a little droop, and the honest question is how much sag to allow, not how to remove it.
Slightly more than the horizontal tension at midspan, because at the ends the cable also carries the vertical weight it’s holding up. For the shallow sags typical of most spans the two are very close, so the horizontal tension is a good first approximation of the pull on the anchors.
What matters is that this support tension can be large: a long or heavy span held at low sag puts hundreds of pounds on each end fitting. Size the posts, eye bolts and anchors to that force with a healthy safety factor, not to the cable’s weight.
The safety factor is the rated breaking strength divided by the working tension, and overhead or load-bearing spans commonly target 5:1 or more — the cable stressed to no more than about a fifth of what would break it.
Life-safety uses such as guy wires, rigging and anything above people often call for higher factors, set by the governing code or manufacturer. Use the rated breaking strength from the spec, not a guess, and apply the factor to the support tension, which is the highest in the span.
Only a little, for normal sags. The curved cable exceeds the straight-line span by an amount that grows with the square of the sag, so a shallow span adds a fraction of a percent while a deeply draped one adds more.
For the 30-foot example with a few inches of sag, the cable is longer than the span by well under an inch. Still add that arc length plus extra for wrapping the ends, tensioning hardware and trimming — cut the cable longer than the raw span and leave slack to terminate both ends.
For a bare cable it’s just the weight of the cable per linear foot, from the spec sheet. Heavier cable sags more at the same tension, which is why weight is one of the three inputs that set the sag.
For string lights or anything carrying load, the effective weight per foot should include the bulbs, fixtures, ice or wind the line will carry — not just the bare wire. A light string with heavy bulbs sags far more than the wire alone, so use a realistic loaded weight if the span has to carry anything.
Estimate Only — Not an Engineered Design: This calculator uses the parabolic model for a level span, sag = weight × span² ÷ (8 × tension), and assumes supports at equal height, a uniform cable weight, and shallow sag. Real spans are affected by supports at different heights, temperature, stretch and creep, wind and ice loads, dynamic loads, and connection details the model does not capture. The weight per foot must reflect the actual loaded weight of the line, not just bare wire. Support tension, cable length and safety factor are approximations for planning. For guy wires, cable railings, overhead rigging, or any load-bearing or life-safety span, follow the cable and hardware manufacturer’s tension and safety-factor specifications and consult a qualified engineer where required by code. For reference and estimating only; verify every load-bearing span against the manufacturer’s specs and an engineer.