Tree Work
Spar Height Calculator
When you top a tree to create a spar pole for rigging, you need enough remaining strength to handle the loads. This estimates how tall a spar you can leave based on the trunk diameter at the cut and expected loads.
Safe spar height above ground (ft)
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- Working radius (ft)
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- Trunk section modulus (in³)
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- Notes
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FAQ
What is section modulus? +
For a circular section, S = π·d³/32. It captures how a beam resists bending — bigger diameter means dramatically more strength because of the cubic relationship.
Why a 4:1 safety factor? +
Wood is variable. Knots, decay, grain runout, and weather all reduce real strength below textbook numbers. ANSI Z133 generally implies 4:1 or better for arborist structural use.
Can I trust the result? +
Use it as a starting point. Real spar strength depends on hidden defects, lean, and load direction. When in doubt, top lower — you can always rig from a shorter spar.