Field calculator
Ladder angle and setback calculator (4 to 1 rule)
A ladder set at the wrong angle is one of the most common causes of a fall: too steep and it tips back, too shallow and the base kicks out. The 4-to-1 rule sets a portable extension or straight ladder at the right angle, about 75.5 degrees, by placing the base one foot away from the wall for every four feet of height up to the contact point. Enter the height where the ladder rests against the wall or the upper landing, and the tool returns the base setback, the approximate ladder span to that point, and the reach you need to access a roof or landing. Two field rules go with the math. When a ladder is used to climb onto an upper level, the side rails have to extend about three feet above that landing so there is something to hold while stepping off, which is why a roof-access ladder is longer than the wall height alone. And the ladder must be secured: tied off at the top and footed or tied at the base, with three points of contact at all times and no standing on the top two rungs. Treat this as a setup guide and follow the ladder duty rating, the manufacturer instructions, OSHA, and the site fall-protection plan.
Result
Ladder angle and setback: the 4-to-1 rule sets a portable extension or straight ladder at about a 75.5 degree angle by placing the base one foot out from the wall for every four feet of height to the contact point. Enter the height where the ladder rests against the wall or the upper landing, and the tool returns the base setback, the approximate ladder span to that point, and the reach needed to access a roof or landing. The two field rules that matter: when a ladder is used to get onto an upper level, the side rails must extend about three feet above that landing so there is something to hold while stepping off, and the ladder must be secured at the top and footed or tied at the base. Keep three points of contact, do not stand on the top two rungs, and follow OSHA and the ladder duty rating. This is a setup guide, not a substitute for the manufacturer instructions and the site fall-protection plan.
anvilfield.com/calculators/ladder-angle-setback-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Ladder setback FAQ
What is the 4 to 1 ladder rule?
The 4 to 1 rule sets a leaning extension ladder so the base sits out 1 foot for every 4 feet of working height, about a 75 degree angle. A ladder reaching 16 feet up has its feet roughly 4 feet out. The arm test confirms it; verify the angle against OSHA and the AHJ.
How far should a ladder extend above the roof?
A ladder used to reach a roof should extend about 3 feet above the eave so there is a rail to hold while stepping on and off, and the top should be tied off. Where the ladder cannot extend that far, secure the top and add a grab rail. Confirm the rule against OSHA and the AHJ.
What are three points of contact?
Three points of contact means keeping three limbs on the ladder while climbing: two hands and one foot, or two feet and one hand. It keeps you anchored if a foot slips or a rung fails. Face the ladder and hoist tools on a line instead of carrying them, so both hands stay free for the rails.
Can you use a metal ladder near electrical?
No. A metal ladder near electrical gives fault current a path to ground through the worker, so use non-conductive fiberglass anywhere there is electrical exposure, including overhead service drops at the eave. Keep the ladder well clear of overhead lines. Confirm required clearances against OSHA and the AHJ, since they depend on the voltage.
What ladder duty rating do I need?
Match the duty rating to your weight plus all tools and material on the ladder, never just your body weight. ANSI A14 rates ladders Type IAA at 375 lb, IA at 300, I at 250, II at 225, and III at 200. For most roofing and service work, a Type IA or IAA is the practical choice.