A Figure Eight Knot is a stopper knot formed by passing the working end over and under the standing line in an 8-shaped loop, then back through the first bight. Climbers and sailors rely on it to terminate a line so it cannot run through a belay device, block, or fairlead. It jams less than an Overhand and inspects easily — you can see at a glance whether the knot is dressed correctly. Tied on a bight or rewoven, it becomes the primary tie-in for rock climbing, holding 70–75% of rope tensile strength.
Figure Eight Knot Interactive Calculator
Vary rope MBS, diameter, knot efficiency, and rescue load to see knotted strength, minimum tail length, and safety factor.
Equation Used
The calculator applies the article relationship for a Figure Eight knot: the knotted minimum breaking strength equals the unknotted rope MBS multiplied by knot efficiency, and the minimum free tail is 10 rope diameters. The safety factor compares that knotted strength with the applied rescue load.
- Dressed Figure Eight knot efficiency is typically 0.70 to 0.75.
- Minimum tail length follows the 10-diameter rule.
- Safety factor is calculated directly from knotted MBS divided by applied rescue load.
- Dynamic effects, edge friction, shock loading, abrasion, and wet or icy rope effects are not included.
How the Figure Eight Knot Actually Works
The Figure Eight Knot works by forcing the rope to bend around itself in two opposing curves, creating internal friction between the working end and the standing part. When load comes onto the standing line, the rope tries to straighten, but the second bend captures the first and locks the geometry in place. The harder you pull, the tighter the curves nip together. That's why it holds without slipping yet stays inspectable — you can count three pairs of parallel strands when it's dressed correctly, and any deviation tells you the knot is wrong.
The knot is designed this way for one reason: it has to be readable by a second person under load. A climbing partner glances at your harness, sees the figure 8 profile with both strands parallel through every curve, and knows the tie-in is sound. If the strands cross over each other inside the knot — a common dressing error when you rush the rewoven version — the knot still holds, but strength drops by 5–10% and it cinches tighter under load, becoming hard to untie after a leader fall.
Failure modes are predictable. If the tail is too short, less than about 10 rope diameters past the knot, cyclic loading can work the tail back through the knot until it capsizes. If the rope diameter is at the low end of what the knot was tied for — say a 7.7 mm half rope used where a 10.5 mm single was expected — the knot dresses smaller and can slip a little before setting. And if you tie it on a wet, frozen, or ice-glazed rope, the cinch is unreliable on the first load cycle. Stopper knot strength reduction is real: expect 70–75% of the rope's rated tensile strength once the knot is set.
Key Components
- Standing Part: The loaded length of rope running back to the anchor, belay, or winch. This is the section that takes the working tension and pulls the knot tight against itself. It must run straight out of the knot — any twist here unwinds the knot's geometry under load.
- Working End (Tail): The free end of the rope that forms the knot. Industry rule for climbing tie-ins is a minimum tail of 10 rope diameters — for a 10 mm rope that's 100 mm of tail past the knot. Anything shorter risks the tail walking back through under cyclic load.
- First Bight: The initial loop that the working end passes through to start the 8 shape. It sets the size and orientation of the finished knot. A loose first bight produces a sloppy knot that needs a hard tug to set; a tight one jams before the knot is fully dressed.
- Crossing Turn: The over-and-under wrap that gives the knot its 8-shaped silhouette. This is the single feature that distinguishes a Figure Eight from a simple Overhand — the extra half-turn spreads the bend radius and reduces strength loss by roughly 5%.
- Dressed Profile: The final geometry with all three strand pairs lying parallel and no crossovers. A correctly dressed knot retains 70–75% of rope tensile strength; a poorly dressed one retains 60–65% and is far harder to untie after loading.
Industries That Rely on the Figure Eight Knot
The Figure Eight Knot earns its place because it does three jobs at once — it stops a rope from running, it inspects in a glance, and it unties after heavy loading. That combination is rare in knot work. You'll find it on climbing harnesses, sailboat sheets, rescue lines, arborist climbing systems, and theatrical rigging. The variant you choose depends on whether you need a stopper, a fixed loop, or a tie-in around a closed object.
- Rock & Alpine Climbing: The Rewoven Figure Eight (Figure Eight Follow Through) is the standard tie-in to a climbing harness — used by Petzl, Black Diamond, and Mammut in all their official tie-in instructions. Holds roughly 75% of rope strength on an 8.9–10.5 mm dynamic single rope.
- Sailing & Yachting: Tied as a stopper on jib and mainsail sheet ends to prevent the line running through fairleads and clutches on production yachts like the Beneteau First 36 or J/70. Sailors call this version the Flemish knot.
- Technical Rope Rescue: Figure Eight on a Bight is the go-to anchor knot in NFPA 1006 and NFPA 1670 rope rescue programs, used by fire-rescue teams to build single-point anchors on 12.5 mm static kernmantle rope.
- Arboriculture: Used as a stopper on the climber's end of a moving rope system (MRS) with kit like the Petzl ZigZag and Notch Rope Runner Pro, where a runaway rope tail through the friction device is a serious incident.
- Theatrical & Stage Rigging: Stopper knot on flying-system hand lines and counterweight arbor purchase lines in venues running JR Clancy or Tiffin Scenic counterweight systems, preventing the line escaping through head and loft blocks.
- Caving & Canyoning: Figure Eight on a Bight as a primary or backup anchor knot on 9–11 mm static rope, where wet conditions and grit demand a knot that still unties after repeated rappels.
The Formula Behind the Figure Eight Knot
There isn't a single closed-form equation for tying a Figure Eight, but there is a critical sizing relationship every rigger needs: the minimum tail length and the resulting effective rope strength. At the low end of typical climbing rope diameters (7.7 mm half ropes) you need a longer tail in absolute terms relative to the knot body, because the knot footprint is smaller and tail walk-back happens faster. At the high end (12.5 mm static rescue rope) the knot is bulky and stable, but you still don't drop below the 10-diameter rule. The sweet spot for general-purpose climbing on 9.8–10.2 mm single rope is a tail of 100–120 mm and an expected working strength of about 73% of rated rope MBS (minimum breaking strength).
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| MBSknot | Minimum breaking strength of the rope with the knot tied | kN | lbf |
| MBSrope | Rated minimum breaking strength of the unknotted rope | kN | lbf |
| ηknot | Knot efficiency factor — typically 0.70–0.75 for a dressed Figure Eight | dimensionless | dimensionless |
| Ltail,min | Minimum tail length past the knot | mm | in |
| drope | Nominal rope diameter | mm | in |
Worked Example: Figure Eight Knot in a high-angle wind-turbine technician rescue rig
Your rope-access team in Aberdeen is rigging a casualty lowering line from the nacelle of a Vestas V117 wind turbine — 91 m hub height — using 11 mm Teufelberger Patron PLUS static kernmantle with a rated MBS of 30 kN. You're tying a Figure Eight on a Bight as the master point at the anchor and need to verify working strength against the 2-person rescue load and confirm the tail length meets your team's SOP.
Given
- MBSrope = 30 kN
- drope = 11 mm
- ηknot,nominal = 0.73 dimensionless
- Rescue load (2 persons + kit, 10:1 SF target) = 2.7 kN
Solution
Step 1 — compute the nominal knotted strength using the typical dressed efficiency of 0.73:
Step 2 — at the low end of typical Figure Eight efficiency (poorly dressed, strands crossed inside the knot, η ≈ 0.66):
That's a 2.1 kN drop just from sloppy dressing — meaningful when you're sizing against a system safety factor. At the high end of efficiency (clean dress, fully set under pre-load, η ≈ 0.77):
Step 3 — verify the tail length minimum:
Step 4 — check static safety factor against the 2.7 kN rescue load at the nominal knotted strength:
That's below the 10:1 rope-rescue target, but only marginally. Pre-loading the knot (body-weight bounce-test before committing) pushes you toward the high-end efficiency and lifts SF to about 8.6. If you needed a true 10:1, you'd step up to 12.5 mm rope with MBS around 36 kN, which would give SF ≈ 9.7 at nominal η.
Result
Working strength of the dressed Figure Eight on a Bight is 21. 9 kN at nominal efficiency, with a required tail of 110 mm. In practice that means the knot will hold a 2-person rescue load with about 8:1 static margin — comfortable for a controlled lower, but tight against the 10:1 best-practice target for unattended life-safety anchors. The spread from low (19.8 kN) to high (23.1 kN) is entirely a dressing-quality issue: a clean, pre-loaded knot sits at the high end, while a rushed dress with crossed strands sits at the low end. If your load-test pull reads below 19 kN, suspect one of three things first: the rope is wet or icy and hasn't set on the first cycle, the bight legs are unequal length so load isn't distributed evenly through the knot, or the rope is near end-of-life with a glazed sheath that won't bite on itself.
When to Use a Figure Eight Knot and When Not To
The Figure Eight isn't the only stopper knot, and it isn't always the best loop knot either. The choice between Figure Eight, Bowline, and Overhand comes down to whether you need to untie under load, how often the knot will be cycled, and whether a second person needs to inspect it.
| Property | Figure Eight Knot | Bowline | Overhand Knot |
|---|---|---|---|
| Knot efficiency (% of rope MBS retained) | 70–75% | 70–75% | 60–65% |
| Inspectability (visual check by second person) | Excellent — distinctive 8 profile, parallel strands | Moderate — easy to mis-tie, 'around the tree' check needed | Poor — looks similar to half-hitch when loose |
| Untie after heavy load | Moderate — works free with effort after a fall | Easy — flips loose with thumb pressure on the bight | Very hard — jams permanently |
| Resistance to cyclic loosening | High — locks tighter under cyclic load | Low — known to shake loose without a stopper | High — but not used as a tie-in for this reason |
| Standard application fit | Climbing tie-in, rescue master point, sheet stopper | Mooring lines, rescue loop where untie speed matters | Quick stopper, end-of-rope marker |
| Typical rope diameter range | 7.7–13 mm | 8–24 mm | any |
| Minimum tail length | 10 × diameter | 12 × diameter (with stopper backup) | 5 × diameter |
Frequently Asked Questions About Figure Eight Knot
Two things compound under a fall: rope elongation pulls the knot through itself and the dynamic load over-cinches the curves beyond their static seated position. On a 9.8 mm single rope catching a Factor 1 fall, peak knot tension can briefly hit 8–9 kN, which sets the knot at roughly 4× the tightness of a body-weight hang.
The fix is mechanical — roll the knot away from you between thumb and forefinger, working each of the three strand pairs in turn. Start with the strand closest to the standing part, because that's the one carrying the most cinch force. If you tie a deliberately loose knot to make untying easier, you've defeated the inspectability rule and the tail can walk back. Better to tie it correctly and accept 30 seconds of work to untie.
Use the Rewoven (Follow Through) version. The reason is structural: the Rewoven passes around the closed loop of your harness tie-in points, so there's no carabiner in the load path between you and the rope. Every carabiner you add is another component that can be cross-loaded or fail.
Figure Eight on a Bight requires a locking carabiner to clip into the harness, which is acceptable for rescue master points where the anchor isn't a closed loop, but adds an unnecessary failure mode at the harness. Petzl, Black Diamond and the UIAA all specify the Rewoven version for personal tie-in.
New kernmantle rope has a factory-applied sheath treatment — typically a silicone or PTFE-based finish — that reduces friction during manufacturing and lowers wet-handling drag. That same finish reduces the strand-on-strand grip inside the knot on the first few load cycles. You're seeing the knot bed in.
It's not a safety issue if the tail is at full 10-diameter length, but it's why every rope-rescue and climbing SOP includes a pre-load step: weight the system before committing. After 5–10 load cycles the sheath finish wears at the knot contact zones and slip stops. If you're seeing slip on a rope that's already been used for weeks, that's different — check for sheath glazing or contamination with chalk dust, which polishes the sheath fibres.
The efficiency percentage holds reasonably well across diameters — you'll still see roughly 70–75% retention — but the absolute knotted strength is far lower because the rope MBS is lower. A 7.7 mm half rope might rate 9 kN MBS, giving a knotted strength near 6.6 kN. That's adequate for a half-rope system used in pairs, but it would be undersized as a single tie-in.
Half-rope systems are designed to share load across two strands clipped alternately. If you use a single 7.7 mm strand as a primary tie-in, you've stepped outside the manufacturer's certification. Tie on with the rope the system was designed for.
Three checks, in order. First, count three pairs of parallel strands on the face of the knot — every strand should have a partner running alongside it with no crossovers. Second, trace the rope from the standing part through the knot and out the tail; it should weave under-over-under without skipping. Third, confirm the tail is at least a fist-width past the knot (roughly 10 cm on a 10 mm rope).
If any of those three fail, retie. The most common error is a strand crossing over its partner inside the second loop, usually because the climber rushed the rewoven step. That single error costs about 5% of knot strength and makes untying after a fall significantly harder.
You can — it's called the Flemish Bend, and it's strong — but the climbing community moved away from it for rappel joins because the knot profile is bulky and prone to catching on edges and cracks during rope retrieval. The Offset Overhand Bend (often called the Euro Death Knot, despite the name being more dramatic than the failure record) presents a flat profile that pulls cleanly over an edge.
If you're joining two ropes for a non-retrievable application — a fixed line, a tandem rescue rig — the Flemish Bend is a fine choice. For pull-down rappels, use the Offset Overhand with long tails (minimum 30 cm) and pre-set the knot under body weight before committing.
References & Further Reading
- Wikipedia contributors. Figure-eight knot. Wikipedia
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