Does Steel Thickness Affect Magnet Strength?
Yes. Steel that’s too thin can’t carry all of a magnet’s field, so it saturates and the magnet holds less. Listed pull force is measured on thick steel. On sheet steel under 1 mm (0.04 in) thick, such as a fridge door or filing cabinet, small magnets keep most of their pull, but larger magnets can lose half or more. As a rule of thumb, steel at least 1/8 of the magnet’s diameter thick gives about 90% or more of the rated pull force.
- Rated pull force is measured on
- Steel thick enough not to saturate
- On 24-gauge sheet (0.6 mm)
- About 20 to 80% of the rating, by size
- For 90% or more of the rating
- Steel about 1/8 of the magnet's diameter
- Our steel discs
- 1 mm (0.04 in) thick
Why Does Steel Thickness Affect Magnet Strength?
A magnet holds to steel by sending its field, or magnetic flux, through the steel and back to its other pole. Steel carries flux easily, but only up to a limit. Low-carbon steel saturates at about 2 tesla, and once it’s full it can’t carry any more, much like a pipe that’s too narrow for the flow.
In thick steel, the field turns inside the steel and comes back out beside the magnet, so the magnet reaches its full rating. In thin steel, the steel next to the magnet’s edges fills up first. The rest of the field leaks out the back of the sheet instead of adding pull, so the magnet holds less.
Bigger and thicker magnets put out more flux, so they need thicker steel. A small magnet can reach its full rating on a thin sheet that cuts a large magnet’s pull by more than half.
How Much Pull Force Is Lost on Thin Steel?
We calculated pull force on low-carbon steel of different thicknesses for four of our N52 discs. Each value is a percentage of the disc’s pull on thick steel:
| Steel | 1/4 × 1/8 Inch | 1/2 × 1/8 Inch | 1 × 1/4 Inch | 1 × 1/2 Inch |
|---|---|---|---|---|
| 24 gauge (0.024 in) | 78% | 52% | 26% | 21% |
| 22 gauge (0.030 in) | 92% | 64% | 32% | 26% |
| 20 gauge (0.036 in) | 98% | 76% | 39% | 30% |
| 18 gauge (0.048 in) | 100% | 95% | 51% | 40% |
| 16 gauge (0.060 in) | 100% | 99% | 64% | 50% |
| 14 gauge (0.075 in) | 100% | 100% | 78% | 61% |
| 11 gauge (0.120 in) | 100% | 100% | 99% | 91% |
| 1/4 in plate | 100% | 100% | 100% | 100% |
For example, on a 24-gauge steel panel our 1 × 1/2 Inch N52 disc, rated 54.5 lb, gives about 11 lb, while a 1/4 × 1/8 Inch disc keeps about 78% of its 2.9 lb rating.
About These Numbers
Real-world results can vary widely due to factors such as steel grade, paint or coatings, air gaps, and plate size. Use this as a general guide, and always test with your own materials for your specific application.
How Thick Does Steel Need to Be for Full Pull Force?
| N52 disc | For 90% of full pull | For 98% of full pull |
|---|---|---|
| 1/4 × 1/8 Inch | 0.7 mm (0.03 in) | 0.9 mm (0.04 in) |
| 1/2 × 1/8 Inch | 1.1 mm (0.04 in) | 1.4 mm (0.05 in) |
| 3/4 × 1/8 Inch | 1.4 mm (0.05 in) | 1.7 mm (0.07 in) |
| 1/2 × 1/4 Inch | 1.5 mm (0.06 in) | 1.8 mm (0.07 in) |
| 1 × 1/4 Inch | 2.3 mm (0.09 in) | 2.8 mm (0.11 in) |
| 1 × 1/2 Inch | 3.0 mm (0.12 in) | 3.7 mm (0.15 in) |
Rule of thumb: steel about 1/8 of the magnet’s diameter thick gives about 90% or more of the rated pull force, and about 1/6 of the diameter gets thick magnets close to 100%. Wider steel doesn’t help once it’s well past the magnet’s edge; thickness is what counts.
Typical Steel Surfaces and What to Expect
| Surface | Typical steel thickness | What to expect |
|---|---|---|
| Fridge doors, filing cabinets, steel whiteboards, appliance panels | Often 1 mm (0.04 in) or less | Small magnets hold well; large magnets lose much of their rating |
| Steel doors, car body panels | About 0.6 to 1.2 mm | Similar: plan for less pull from large magnets |
| Steel shelving, tool cabinets, electrical boxes | About 0.8 to 1.5 mm | Most small and medium magnets reach near full pull |
| Steel plate, beams, workbench tops, machine bases | 3 mm (1/8 in) and up | Full rated pull for almost any magnet |
These are typical ranges, not specifications, so measure your steel if the hold matters.
What About Steel Strike Plates?
Our Steel Discs are 1 mm (0.04 in) thick, which suits small and medium magnets. Calculated pull for a few pairings:
| Magnet (N52) | Steel disc, 1 mm thick | Pull force, about |
|---|---|---|
| 1/4 × 1/8 Inch (2.9 lb) | 3/8 Inch | 2.6 lb (91%) |
| 1/2 × 1/8 Inch (8.15 lb) | 1/2 Inch, same size | 2.5 lb (30%) |
| 1/2 × 1/8 Inch (8.15 lb) | 5/8 Inch | 6.9 lb (85%) |
| 3/4 × 1/8 Inch (12.1 lb) | 1 Inch | 9.4 lb (78%) |
| 1 × 1/4 Inch (33.7 lb) | 1-1/4 Inch | 16.8 lb (50%) |
Two important notes :
- Choose a disc slightly wider than the magnet. The magnet’s strongest field is at its edge, and a disc that ends right at the edge misses it. In our calculations, a same-size disc held far less than one slightly wider, even when it was thick.
- For large magnets, use thicker steel or a second magnet. A 1 mm disc limits magnets of about 1 inch and up. A pair of magnets doesn’t saturate, because the second magnet brings its own field. See magnet-to-magnet vs. magnet-to-steel and using strike plates.
Does the Type of Steel Matter?
Yes. Low-carbon (mild) steel, like our A3 steel discs, carries the most flux. High-carbon and hardened steels and 400-series stainless steel are magnetic but saturate sooner, so they need to be thicker for the same hold. Common 300-series stainless steels, such as 304 and 316, barely attract a magnet at all; see magnets and stainless steel.
How to Get More Holding Power on Thin Steel
- Use several smaller magnets instead of one big one. Small magnets lose less on thin steel. On 24-gauge sheet, three 1/2 × 1/8 Inch discs spaced apart hold about 13 lb together, more than one 1 × 1/2 Inch disc (about 11 lb).
- Use a magnet pair where you control both sides, such as a closure.
- Add thicker steel behind or in place of the thin sheet, at least 1/8 of the magnet’s diameter thick.
- Allow for it when sizing. For loads that slide down a wall, use the high end of our 7 to 10× rule on thin steel. See shear force and our magnet calculator.
Takeaway
Steel thickness matters most for large, strong magnets. Small magnets reach their rating on about 1 to 1.5 mm of steel, while a 1 × 1/2 Inch disc needs about 3 to 4 mm. Use steel at least 1/8 of the magnet’s diameter thick, a strike plate slightly wider than the magnet, or several smaller magnets when the steel is thin.
- Steel Discs & Strike PlatesLow-Cost Target for a Magnet
- Disc MagnetsEvery Diameter and Thickness
- Grade N52 MagnetsOur Strongest Grade
More Questions About Magnets and Steel
How are steel strike plates used most effectively?
Use a plate slightly wider than the magnet and as thick as your design allows. See using strike plates.
Is magnet-to-magnet stronger than magnet-to-steel?
Touching, about the same; through a gap or on thin steel, a pair holds much more. See magnet-to-magnet vs. magnet-to-steel.
How is the pull force of a magnet measured?
Straight off a thick, flat, ground steel plate with a force gauge. See how pull force is measured.
Does a magnet with 20 lbs of pull force actually hold 20 lbs?
Only straight off thick steel; on a wall, thin steel or through a gap, it holds much less. See does a 20 lb magnet hold 20 lb?
Why don't magnets work with all types of stainless steel?
Common 304 and 316 stainless are mostly non-magnetic; 400-series stainless is magnetic. See magnets and stainless steel.