How long does your ladder need to be

A leaning ladder that is too short leads people to climb the top rungs or set it at the wrong angle, two of the most common causes of falls. This calculator starts from the height of the point where the ladder rests and its angle with the ground, and tells you the minimum ladder length, how far from the wall to place the base and whether the angle is the recommended one.

How the calculation works

• Length to the support point: the height divided by the sine of the angle with the ground.
• Extension: if you use the ladder to step onto a roof or a platform, it should go about 1 metre beyond the landing point; this is added along the ladder.
• Base distance: the height divided by the tangent of the angle.
• Ratio: how many times the height exceeds the base distance.

The 1-in-4 rule and 75 degrees

The most widespread safety guidance, such as that of the UK's HSE and the US agency OSHA, calls for an angle of about 75° with the ground: for every 4 metres of height, the base goes about 1 metre out from the wall. The exact 1-in-4 ratio corresponds to about 76°, very close.

At an angle of 75° with 1 metre of extension:

The 1-in-4 rule (height divided by 4) is a quick way to measure the base on site: it gives values very close to those at 75°.

What changes with the angle

Support point at 4 metres, 1 metre of extension:

The angle changes the ladder length little but the base distance a lot. A ladder that is too shallow tends to slide at the base; one that is too steep can tip backwards when you lean.

The reading bands

• Below 70°: too shallow, the base may slide.
• 70° to 74°: slightly shallow, move the base closer.
• 74° to 77°: in line with the recommended 75° or so.
• Above 77° up to 80°: slightly steep, move the base out.
• Above 80°: too steep, risk of tipping.

Safety tips

• Stand the base on a flat, firm, non-slip surface.
• Always keep three points of contact (two hands and a foot, or two feet and a hand).
• Do not climb the top rungs and do not lean sideways beyond the stiles.
• Respect the maximum load and the manufacturer's instructions.

Limits of the calculator

The calculator only does geometry: it does not assess the ladder's condition, the ground, wind or the type of work. For extension ladders, the usable length is shorter than the total because the sections overlap: check the manufacturer's data. For professional work at height, your country's safety rules apply.

Frequently asked questions

What is the right angle for a leaning ladder?

About 75° with the ground, according to the most widespread safety guidance: for every 4 metres of height, the base goes about 1 metre from the wall.

What is the 1-in-4 rule?

For every 4 units of height to the support point, the base should be 1 unit out from the wall. It corresponds to about 76°.

What length ladder do I need to reach 4 metres?

At 75° you need about 4.14 metres to the support point; if you must step off at the top, add about 1 metre: about 5.14 metres.

Why should a ladder extend 1 metre above the landing?

To have something to hold when stepping from the ladder onto the roof or platform, and back.

What happens if the ladder is too shallow?

The base tends to slide away from the wall, especially on smooth surfaces.

And if it is too steep?

The weight shifts backwards and the ladder can tip over when you lean or climb high.

Is an extension ladder's length its usable length?

No: the sections overlap, so the usable length is shorter than the total. Check the manufacturer's data.

Does the calculator apply to stepladders?

No, only to ladders leaning against a wall or support. Stepladders have a fixed angle set by their opening.

Does the calculator guarantee safety?

No: it only does geometry. Always check the ladder's condition, the ground and the manufacturer's instructions.