H = w·L² / (8·d)
V = w·L / 2
T = √(H² + V²)
V = P / 2
T = √(H² + V²)
- Measure the anchor-to-anchor span and pick units.
- Choose your target sag after tightening the line.
- Enter the expected clothes load, ideally when wet.
- Select uniform or point load to match usage.
- Add wind, wetness, and dynamic percentages if needed.
- Pick a safety factor; increase it for public areas.
- Press Calculate and compare results to your line rating.
| Scenario | Span | Sag | Load model | Clothes load | Extras (wind/wet/dynamic) | Safety factor | Typical tension (approx.) |
|---|---|---|---|---|---|---|---|
| Everyday family drying | 8 m | 15 cm | Uniform | 18 kg | 10% / 15% / 10% | 3.0 | ~ 1,900 N |
| Short span, low sag | 5 m | 8 cm | Uniform | 12 kg | 10% / 10% / 10% | 3.0 | ~ 2,100 N |
| One heavy blanket centered | 7 m | 20 cm | Point | 10 kg | 15% / 20% / 15% | 3.5 | ~ 1,300 N |
Why sag controls tension more than load
Tension rises rapidly when you chase a straight, “guitar‑string” line. With the parabolic approximation, horizontal pull scales with L² and inversely with sag d. Halving sag can nearly double horizontal force, even if the laundry weight stays the same. In practice, adding a few centimeters of sag often reduces anchor stress more than upgrading to a thicker rope.
Choosing a realistic load model for your setup
Uniform loading fits typical mixed garments spread across the span, so the calculator converts total force W into w = W/L. A center point model is better for a heavy blanket or rug that concentrates mass near midspan. If you are unsure, run both. Use the larger tension value for hardware sizing and for deciding if the line rating is adequate.
Accounting for wetness, wind, and handling shocks
Wet fabric can weigh far more than dry fabric, and gusts create extra drag and flapping forces. The extra‑load fields apply a multiplier to clothing and line weight to represent these effects. Dynamic allowance covers clipping, tightening, or someone bumping the line. Conservative percentages help prevent surprise overloads during real use, not just calm‑day conditions.
Interpreting strength targets and safety factors
The required minimum strength is computed as support tension multiplied by your safety factor. A factor of 2 suits controlled backyards with robust anchors; 3–4 is prudent near walkways, children, or brittle mounting surfaces. If you enter a rated line strength, utilization indicates how close you are to the stated capacity. Lower utilization means more margin for aging and knots.
Good installation habits that protect anchors
Anchors fail before rope in many gardens. Use eye bolts, pulleys, and turnbuckles with ratings above the calculated tension, and inspect threads and corrosion regularly. Posts should be braced and set deep enough to resist sustained horizontal pull. Trees move and grow, so recheck tension after rain and seasonal changes. Replace lines that show glazing, fraying, or permanent stretch for long-term safety.