How it works
A weight hung from two symmetric ropes — a sign between two posts, a lamp on a cable, a load on a sling — is a static equilibrium problem. The two rope tensions must together support the full weight vertically while their horizontal components cancel, because the mass is not going anywhere.
For two ropes making the same angle θ above the horizontal, vertical balance gives 2·T·sin θ = m·g, so the tension in each rope is T = m·g / (2·sin θ). The shallower the ropes (small θ), the larger the tension: pulling a washing line or a sign nearly horizontal forces the tension far above the actual weight, which is why a taut horizontal cable can snap under a light load.
This calculator assumes two identical ropes at equal angles, an ideal massless line, and g ≈ 9.81 m/s². It also reports the horizontal pull each anchor must resist — the sideways force trying to rip the fixings out of the wall. For a single vertical rope use the tension calculator; for a rope pulling a block up a slope use the incline tension calculator.
Use it in real life
Hanging signs and banners: a 10 kg sign on two cables at 30° above horizontal puts about 98 N in each cable — but drop the angle to 10° and each cable now carries roughly 290 N, nearly three times the weight, from the same sign.
Zip lines and clotheslines: the flatter you tension the line, the more force builds at the anchors — the physics reason a 'tight' horizontal cable needs surprisingly heavy fixings.
Slings and rigging: lifting a load with a two-leg sling, wider sling angles multiply the tension in each leg, which is exactly why rigging charts de-rate slings as the included angle opens up.
Frequently asked questions
How do you find the tension in two ropes holding a weight?
For two symmetric ropes each at angle θ above the horizontal, the tension in each is T = m·g / (2·sin θ). The two vertical components add up to support the weight; the horizontal components cancel.
Why does the tension rise as the ropes get more horizontal?
Only the vertical component of each rope's tension holds the weight. As the ropes flatten toward horizontal, sin θ shrinks, so the tension must grow to keep 2·T·sin θ equal to the weight. Near-horizontal ropes can carry many times the actual load.
What if the two ropes are vertical?
At θ = 90° the ropes are vertical, sin θ = 1, and each rope simply carries half the weight: T = m·g / 2. That is the lowest tension possible for two ropes sharing the load.