Fell Zone & Safe Distance Calculator
Estimate the fell zone radius and safe retreat distance when felling a tree, based on tree height and species lean characteristics.
Formulas Used
1. Lean Horizontal Offset
lean_offset = H × sin(θlean)
Horizontal displacement of the tree tip caused by natural lean.
2. Fell Zone Radius
Rfell = (H × cos(θlean) + lean_offset + Cr) × Sf × (1 / cos(θslope))
Where H = tree height, Cr = crown spread radius, Sf = species factor,
θslope = ground slope angle. The term 1/cos(θslope) amplifies
downhill reach on sloped terrain.
3. Safe Retreat Distance
Rsafe = 2 × H × Sf
Per ANSI Z133 and OSHA 1910.266, the minimum safe retreat distance is twice the tree height,
adjusted for species density.
4. Hinge Width
Whinge = 0.80 × DBH
The hinge should span 80% of the trunk diameter to maintain directional control.
5. Notch Depth
Dnotch = 0.25 – 0.333 × DBH
Open-face or conventional notch depth of one-quarter to one-third of DBH.
Assumptions & References
- The fell zone is modelled as a full circle (worst-case) to account for unexpected barber-chair splits or kickback.
- Species factor (Sf) approximates the effect of wood density on momentum and bounce distance (1.0 – 1.15).
- Slope amplification uses the secant of the slope angle; downhill reach is always greater than on flat ground.
- Crown spread radius is the horizontal distance from trunk centre to the outermost branch tip.
- Escape route should be pre-cleared, 45° to the rear of the intended felling direction, on the uphill side where possible.
- These calculations are estimates only. Always conduct a full site hazard assessment before felling.
- References: ANSI Z133-2017 (Arboricultural Operations Safety); OSHA 29 CFR 1910.266 (Logging Operations); HSE Guidance Note PM81 (Safe Use of Chainsaws); FAO Forestry Paper 100 (Chainsaw Safety).