What this calculator does
The four-to-one rule puts the ladder base one unit out for every four units of height. Reaching four metres means setting the base one metre from the wall, giving a 4.12 metre ladder at 76 degrees.
Both directions of error are dangerous. Too steep and the ladder can tip backwards as you climb, particularly when reaching. Too shallow and the base slides out, which is the more common failure. The 75 degree target sits at the point where both risks are lowest, which is why it is written into safety guidance rather than left to judgement.
The formula
The base distance is the working height divided by the ratio. The ladder length is the diagonal by Pythagoras, and the angle is the arctangent of height over base. The figures are for the reach height at the top support point, so a ladder used for roof access needs roughly another metre projecting above the edge.
| Term | Meaning |
|---|---|
| Four-to-one rule | Base out one unit for every four of height, giving about 75 degrees. |
| Working height | Height to the top support point, not the top of the ladder. |
| Roof access | Requires the ladder to project about 1 m above the landing point. |
| Three points of contact | Two hands and a foot, or two feet and a hand, maintained while climbing. |
The inputs explained
| Field | What to enter |
|---|---|
| Height to reach (m) | Height to reach in metres, measured to where the ladder touches the wall. |
| Out-to-height ratio | Out-to-height ratio. Four is the standard rule, giving about 75 degrees. |
When to use it
Setting a ladder safely
Knowing the base distance before climbing is the practical form of the rule.
Choosing a ladder length
A ladder must be long enough for the height plus the lean plus any projection above the landing.
Checking a setup
Measuring the base distance against the height verifies the angle without a spirit level.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the ratio change the angle?
The same working height at a range of out-to-height ratios.
| Out-to-height ratio | Base distance from the wall | Resulting angle from the ground | Ladder length needed |
|---|---|---|---|
| 1 : 3 | 1.33 m | 71.6° | 4.22 m |
| 1 : 3.5 | 1.14 m | 74.1° | 4.16 m |
| 1 : 4 | 1.00 m | 76.0° | 4.12 m |
| 1 : 4.5 | 0.89 m | 77.5° | 4.10 m |
Questions
What angle should a ladder be at?
About 75 degrees from the ground, which the four-to-one rule produces. Steeper risks tipping backwards, particularly when reaching sideways. Shallower risks the base sliding out, which is the more frequent cause of ladder falls.
How do I check the angle without measuring?
Stand with your toes at the ladder base and extend your arms straight ahead at shoulder height. At roughly 75 degrees your palms should just reach a rung. Many ladders also carry an angle indicator on the stile.
How far should a ladder extend above the roof?
About a metre, or three rungs, above the landing point. That gives something to hold while stepping on and off, which is when most ladder accidents happen. The reach height entered here is the wall contact point, not the ladder top.
Does the ladder need securing?
Yes for anything beyond brief work. Tying the stiles at the top is standard practice, and a base secured or footed by another person is the usual alternative. The correct angle reduces the risk of sliding but does not remove it.
For roof geometry, see the roof pitch calculator. For a permanent ramp, see the ramp slope calculator.