Advanced Fall Distance Inputs
Enter all values in one selected unit system. Positive anchor height means the anchor is above the dorsal D-ring.
Formula Used
Free fall distance:
FFD = lanyard length + slack + worker movement drop - anchor height above D-ring
Required clearance:
Clearance = FFD + deceleration distance + harness stretch + D-ring to feet allowance + safety margin
Free fall physics:
t = √(2d / g) and v = √(2gd)
Estimated arrest force:
Average force = m × (g + v² / (2 × arrest distance))
Peak force = average force × peak force multiplier
How to Use This Calculator
- Select metric or imperial units before entering values.
- Enter worker mass or weight, lanyard length, slack, and anchor position.
- Add deceleration travel, harness stretch, body allowance, and safety margin.
- Enter available clearance below the work level.
- Set your free fall, force, and anchor rating limits.
- Press the calculate button to view the result above the form.
- Use the CSV or PDF buttons to save the calculation.
Example Data Table
| Scenario | Lanyard | Anchor above D-ring | Slack | Deceleration | Safety margin |
|---|---|---|---|---|---|
| Low anchor review | 1.8 m / 6 ft | -0.3 m / -1 ft | 0.15 m / 0.5 ft | 1.1 m / 3.5 ft | 1.0 m / 3 ft |
| Level anchor setup | 1.8 m / 6 ft | 0 m / 0 ft | 0.1 m / 0.33 ft | 1.1 m / 3.5 ft | 1.0 m / 3 ft |
| Overhead anchor setup | 1.8 m / 6 ft | 1.2 m / 4 ft | 0.05 m / 0.16 ft | 1.0 m / 3.3 ft | 1.0 m / 3 ft |
Free Fall Distance in Fall Protection
Why the Distance Matters
Free fall distance is the vertical drop before an arrest system begins to slow a falling worker. It is not the same as total clearance. A short free fall can still need large clearance when the absorber opens, the harness shifts, and the worker body hangs below the D-ring.
Anchor Position Changes Everything
The anchor location is a major input. An overhead anchor can reduce free fall. A low anchor can increase it. Slack also adds distance. Movement before the line becomes tight can add more distance. These small details can change the final clearance need.
Physics Behind the Calculation
The calculator uses basic motion equations. Distance and gravity estimate fall time and speed. The speed is then used with arrest distance to estimate force. The force result is only a planning estimate. Real devices behave differently because webbing, absorbers, connectors, and body movement are not perfectly rigid.
Clearance Planning
Clearance is the practical safety number. It includes free fall, deceleration distance, harness stretch, body allowance, and a safety margin. If available clearance is lower than required clearance, the system needs review. Options may include a higher anchor, a shorter connector, a self-retracting lifeline, or a different work method.
Force and Anchor Review
Force estimates help compare setups. Higher fall distance raises velocity. Higher velocity raises arrest force. More arrest distance lowers force. The anchor load multiplier lets you add conservatism for geometry or special rigging. Always confirm equipment ratings and site rules with a qualified person.
Best Use
Use this tool during early planning, training, and comparison. It helps explain how each input affects the result. It does not replace manufacturer instructions, engineering judgment, rescue planning, inspection, or legal compliance checks.
FAQs
1. What is free fall distance?
Free fall distance is the vertical distance traveled before the fall arrest system starts slowing the worker. It depends on lanyard length, slack, worker movement, and anchor height.
2. Is free fall distance the same as required clearance?
No. Required clearance includes free fall distance, deceleration travel, harness stretch, body allowance, and a safety margin. It is the space needed below the work level.
3. Why does anchor height matter?
An anchor above the D-ring usually reduces free fall distance. An anchor below the D-ring can increase free fall and may increase the required clearance and arrest force.
4. What does deceleration distance mean?
Deceleration distance is the distance used by the shock absorber or arrest device while slowing the fall. More deceleration distance usually lowers arrest force.
5. Why is harness stretch included?
Harness webbing and D-ring position can shift during arrest. This adds extra distance. Including it gives a more conservative clearance estimate.
6. What is fall factor?
Fall factor compares free fall distance with lanyard length. A higher value means a more severe fall condition. It helps compare different setups quickly.
7. Can this calculator approve a work setup?
No. It is a planning aid only. A competent safety professional must review equipment, regulations, anchor strength, rescue needs, and site conditions.
8. Why are force results estimates?
Real arrest forces depend on equipment design, body motion, absorber behavior, and test conditions. This tool uses simplified physics to support early planning.