SSKC-021 | Simple Signman Knowledge Center

How to Calculate the Magnet Holding Force You Actually Need

Choosing a magnet often starts with a deceptively simple question:

“How many pounds of holding force do I need?”

The answer is rarely the same as the weight of the object being held.

A 20 lb load does not automatically require a magnet rated for 20 lb of pull force. Published pull-force ratings are generally measured under controlled conditions that may be very different from the actual application.

Steel thickness, air gaps, surface condition, load direction, friction, vibration, temperature, geometry, and mounting method can all change real-world magnetic performance.

Table of Contents

• 1. What Does “Holding Force” Actually Mean?
• 2. Start With the Actual Load
• 3. Determine the Load Direction
• 4. Compare Published Ratings With Real Conditions
• 5. Account for the Air Gap
• 6. Evaluate the Target Steel
• 7. Consider Friction in Shear Applications
• 8. Account for Dynamic Loads
• 9. Apply an Appropriate Safety Factor
• 10. Practical Example
• 11. Using Multiple Magnets
• 12. Validate the Complete Assembly
• 13. Magnet Selection Decision Tree
• 14. Frequently Asked Questions

1. What Does “Holding Force” Actually Mean?

Magnetic holding force is the force available to resist movement or separation in a particular magnetic system.

It should not automatically be treated as equivalent to a manufacturer's published pull-force rating.

Published pull force is commonly measured by pulling a magnet perpendicular to a suitable steel test surface under defined conditions.

Your application may involve:

• thinner steel;
• paint or powder coating;
• an adhesive layer;
• a plastic cover;
• a curved or irregular surface;
• vertical loading;
• vibration or impact;
• elevated or low temperatures;
• outdoor exposure.

Each of these can affect the performance of the complete magnetic system.

For a detailed explanation of pull-force ratings, see SSKC-001 — How Strong Are Neodymium Magnets? Pull Force Explained.

2. Start With the Actual Load

The first step is to define exactly what the magnetic system must support.

For a stationary object, begin with the total weight of the supported assembly — not just the weight of one component.

Also include components such as:

• brackets;
• frames;
• sign panels;
• hardware;
• cables;
• covers;
• accessories;
• anything else carried by the magnetic mounting system.

3. Determine the Load Direction

This is one of the most important steps.

Perpendicular Load — Pull

If the load attempts to pull the magnet directly away from the steel surface, the magnet is primarily resisting a pull or tensile load.

This configuration is usually the closest to the conditions represented by a published pull-force rating, although the actual steel, air gap, geometry, and other conditions still matter.

Parallel Load — Shear

If a magnet is mounted on a vertical wall and the load attempts to slide downward, the application is primarily a shear or sliding application.

In this case, friction between the contacting surfaces becomes extremely important.

A magnet advertised with 50 lb of pull force should therefore not automatically be assumed to support a 50 lb vertical load.

See SSKC-013 — Magnet Pull Force vs Shear Force for a detailed explanation.

4. Compare Published Ratings With Real Conditions

Before using a catalog pull-force value, ask how that value was established.

Relevant questions include:

• What type of steel was used?
• How thick was the steel?
• Was the magnet in direct contact with the steel?
• Was the surface flat and clean?
• What was the direction of the test?
• Was the value measured or calculated?
• Was the magnet tested individually or as part of an assembly?

5. Account for the Air Gap

One of the most common reasons for lower-than-expected holding force is an air gap.

In magnetic engineering, the air gap is not limited to visible air.

Anything separating the magnet from the target steel can effectively contribute to the magnetic gap, including:

• paint;
• powder coating;
• vinyl;
• plastic;
• adhesive;
• rubber;
• protective film;
• surface contamination;
• surface irregularities.

Even relatively small separations can substantially affect magnetic performance, depending on the magnet and magnetic circuit.

There is no universal percentage that can be applied to every magnet and every gap.

Learn more in SSKC-012 — The Air Gap Effect.

6. Evaluate the Target Steel

The steel is part of the magnetic circuit.

A magnet tested against thick laboratory steel may perform differently when installed on thin sheet metal.

If the steel cannot efficiently carry the magnetic flux, increasing magnet strength may produce diminishing improvements in holding force.

Important variables include:

• steel thickness;
• steel composition and magnetic properties;
• available contact area;
• geometry;
• surface condition;
• air gap.

There is no single minimum steel thickness that works for every magnet.

See SSKC-011 — How Steel Thickness Affects Magnet Holding Force.

7. Consider Friction in Shear Applications

For a vertical mounting application, the magnet creates a normal force toward the steel while friction helps resist sliding.

As a simplified engineering relationship:

Friction Force ≈ Coefficient of Friction × Normal Force

However, the coefficient of friction depends strongly on the actual surfaces and conditions.

Painted steel, polished metal, rubber, plastic, moisture, oil, dust, and surface wear can all produce very different results.

This is one reason rubber-coated magnetic systems can sometimes perform well in sliding applications: the rubber may increase friction and protect the surface even though the coating itself creates some separation between the magnet and the steel.

Do not use a universal pull-to-shear conversion factor.

The actual assembly should be tested.

8. Account for Dynamic Loads

A static load sitting motionless is very different from a load exposed to movement.

Real applications may experience:

• vibration;
• impact;
• sudden acceleration or deceleration;
• doors opening and closing;
• vehicle movement;
• wind;
• equipment movement;
• repeated loading cycles.

These conditions can temporarily produce forces greater than the static weight of the object.

They may also encourage sliding, rocking, peeling, or progressive movement.

9. Apply an Appropriate Safety Factor

Engineering applications commonly use a safety factor to provide margin between expected service loads and the capacity of the system.

A simplified concept is:

Required System Capacity = Design Load × Selected Safety Factor

But the safety factor should not be used as a shortcut to compensate for unknown magnetic performance.

First evaluate the real magnetic system — including steel, air gap, load direction, friction, geometry, temperature, and dynamic effects — and then apply a safety factor appropriate to the application and consequences of failure.

Higher-risk applications may require larger safety margins, formal engineering review, redundant attachment, or secondary mechanical retention.

There is no single safety factor appropriate for every magnetic application.

10. Practical Example

Consider a 15 lb component mounted magnetically to a vertical painted steel panel.

It would be incorrect to simply select a magnet advertised with 15 lb of pull force.

The evaluation should instead consider:

Only after these conditions are understood should the magnet or magnetic assembly be selected and validated.

11. Using Multiple Magnets

Using several magnets can distribute the load and improve stability, but their rated forces should not automatically be added together and assumed to represent actual system capacity.

Performance can be influenced by:

• spacing between magnets;
• steel thickness;
• steel saturation;
• load distribution;
• assembly stiffness;
• surface flatness;
• air-gap consistency;
• whether all magnets contact the target equally.

If the steel or magnetic circuit becomes limiting, simply adding stronger magnets may provide less improvement than expected.

For more information, see SSKC-014 — Magnetic Saturation Explained.

12. Validate the Complete Assembly

The best final verification is testing the actual or representative assembly under realistic conditions.

Whenever practical, reproduce:

• the actual magnet;
• the actual steel thickness and material;
• the actual coating;
• the actual air gap;
• the actual mounting orientation;
• the expected temperature range;
• the expected vibration or movement;
• the expected service load.

Testing should evaluate the failure mode that matters to the application — pull-off, sliding, rotation, peeling, or another form of movement.

13. Magnet Selection Decision Tree

Step 1: What is the total load?

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Step 2: Is the primary load perpendicular, shear, rotational, peeling, or a combination?

↓

Step 3: What steel will the magnet contact?

↓

Step 4: Is there paint, coating, adhesive, rubber, plastic, or another air gap?

↓

Step 5: Are vibration, impact, movement, temperature, moisture, or outdoor exposure involved?

↓

Step 6: What are the consequences if the magnetic attachment fails?

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Step 7: Select an appropriate magnetic solution and engineering safety factor.

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Step 8: Test the complete assembly under representative conditions.

14. Frequently Asked Questions

If my object weighs 20 lb, do I need a 20 lb magnet?

Not necessarily. The magnet's published pull-force rating and the real working capacity of the assembly are not automatically equivalent. Load direction, steel, air gap, friction, geometry, and operating conditions must be considered.

Can I simply double the weight for safety?

Not as a universal rule. A safety factor should be selected according to the application and consequences of failure after realistic system performance has been evaluated.

Can I add the pull force of several magnets together?

Sometimes multiple magnets can provide approximately additive benefits under favorable conditions, but this should not be assumed. Steel saturation, spacing, unequal contact, assembly flexibility, and load distribution can change the result.

Why does my magnet feel much weaker through paint?

The paint creates additional separation between the magnet and steel. This effective air gap can reduce magnetic force.

Why does a strong magnet slide down a wall?

Because pull force and resistance to sliding are different. In a vertical application, friction and the available normal force play major roles.

Is the strongest magnet always the safest choice?

No. A stronger grade may provide limited improvement if the steel, air gap, geometry, or magnetic circuit is already limiting performance. Excessive force can also create handling, impact, pinch, or breakage risks.

Safety Reminder

Powerful permanent magnets can create significant pinch, impact, and projectile hazards. Brittle neodymium magnets can chip or fracture if allowed to collide.

Applications where failure could cause injury, property damage, or equipment damage should be evaluated with appropriate engineering safety factors and, where necessary, secondary mechanical retention.

For mounting considerations, see SSKC-010 — How to Mount Neodymium Magnets Safely.

From Holding Force to a Complete Magnet Specification

Determining the required holding force is only one part of magnet selection.

A complete specification may also need to define material, grade, geometry, dimensions, tolerances, magnetization direction, polarity, coating, temperature, target steel, air gap, environment, quantities, and inspection requirements.

Continue with SSKC-020 — How to Specify a Custom Magnet.

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