Ladder Reach Calculator
| Reach target | Vertical top | Set-back | Ladder length |
|---|---|---|---|
| 10 ft | 6 ft | 1.5 ft | 8 ft |
| 15 ft | 11 ft | 2.75 ft | 16 ft |
| 20 ft | 16 ft | 4 ft | 20 ft |
| 25 ft | 21 ft | 5.25 ft | 24 ft |
| 30 ft | 26 ft | 6.5 ft | 28 ft |
Ladder sizing starts with an honest number: the height you actually need to touch, not the wall's total height. Your arms add roughly four feet of reach above where the rails end, so a 20-foot gutter line is a 16-foot vertical job before the lean angle adds a single inch.
Bottom line: the 4-to-1 rule sets the geometry - for every four feet of working height the base sits one foot out, which trades a little ladder length for a lot of stability. The calculator converts your reach target into vertical top, set-back and the ladder size to buy or rent, rounding up to the nearest standard length.
The honest part: the highest standing point is never the top - extension ladders want you on the fourth rung from the top at most, and never on the three that straddle the roof edge. If the math says the job needs the top section, the job needs a bigger ladder.
How to use
- Enter the height of the thing you must reach - gutter line, second-story window sill, roof edge - and read the ladder size, not the wall height.
- Set the base one foot out for every four feet up: the set-back row is the distance the feet belong from the wall before anyone climbs.
- Subtract four from the vertical top for your real working zone - that is where your hands reach, and where the safe rungs end.
Frequently asked questions
What size ladder for a two-story house?
A two-story wall runs about 18 to 20 feet to the eave, so your reach target lands near 20 feet: the vertical top is 16 feet, the 4-to-1 set-back is 4 feet, and the ladder length comes to about 16.5 - round up to a 20-foot extension ladder. Buying the wall height is the classic mistake: a 20-foot wall does not need a 24-foot ladder, it needs a 20-foot one set four feet out.
What is the 4 to 1 ladder rule?
One foot of set-back for every four feet of working height: a ladder topped out at 16 feet plants its feet 4 feet from the wall, at a 75-degree angle. It is the lean where the rails carry your weight instead of tipping backward when you climb - arm's length test: stand at the feet and touch the rails at shoulder height without stretching.
How high can you safely stand on a ladder?
Never on the top three rungs of an extension ladder, and never on the top two steps of a stepladder - the rails above your feet are what keep the whole column from pivoting. Practically, your feet stop about 3 feet below the vertical top, which the calculator builds into the working-zone number.
What do ladder duty ratings mean?
They are weight limits including everything you carry: Type III holds 200 pounds, Type II 225, Type I 250, Type IA 300 and Type IAA 375. A person plus a bundle of shingles plus a nail gun adds up faster than most people expect - a 190-pound homeowner with a bundle crosses Type III before the first rung.