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All Containers›Shop by use case›Model parts and paint›Magnets›The flux indexShop all model parts and paint →
Magnets

The flux index

By Dwight Davis, Founder

On this page11
Less than 50 kG squared millimetres squared. A strength limit, not a size one.

The requirement

"Each loose or separable magnet in a subject magnet product that fits entirely within the cylinder described in 16 CFR 1501.4 must have a flux index of less than 50 kG(2) mm(2)", tested by the method at 16 CFR 1262.4.

Note what it borrows

The cylinder described in 1501.4. The magnet standard does not define its own size gauge, it uses the small parts one, which is why the two rules belong on the same page.

And note what it adds

A flux index, which is a measure of magnetic strength and geometry together rather than of size. Two magnets the same size can sit on opposite sides of the limit.

Loose or separable

Which is the scope. A magnet embedded in a product and not separable is a different situation from one that can come out, and the abuse testing is what decides which it is.

So a strong magnet has to be too big to swallow

That is the practical effect of reading the two conditions together: if it fits in the cylinder it must be weak, and if it is strong it must not fit.

Neodymium is the material this is about

Rare earth magnets are small and very strong, which is precisely the combination the rule addresses, and it is one of the twelve leaves in this group.

Modellers use them for good reasons

Removable hatches, swappable loads, a canopy that lifts off cleanly. A small strong magnet does a job in a model that nothing else does as neatly.

Which is why the storage question is real

A modeller buys them by the hundred and keeps them loose in a dish or a jar on the bench, because that is how they are used.

They also find each other

A spilled jar of neodymium magnets does not scatter and sit still. They cluster, they cling to the underside of things, and they end up in places nobody looked.

The flux index is not something you can check at home

It is a measured value by a specified method. What you can check at home is the cylinder, and what you can control is the container.

References

16 CFR 1262.3, safety standard for magnets: "Each loose or separable magnet in a subject magnet product that fits entirely within the cylinder described in 16 CFR 1501.4 must have a flux index of less than 50 kG(2) mm(2)", tested in accordance with the method described in 16 CFR 1262.4. law.cornell.edu
16 CFR 1501.4, test method: "Place the article, without compressing it, into the cylinder. If the article fits entirely within the cylinder, in any orientation, it fails to comply with the test procedure." Articles are then subjected to the use and abuse testing at 16 CFR 1500.51 and 1500.52, and any liberated component other than paper, fabric, yarn, fuzz, elastic and string is tested in the cylinder individually. The cylinder's dimensions are given in Figure 1 of the section rather than in its text. law.cornell.edu
Written from 16 CFR 1501.4 and 16 CFR 1262, and from the catalog as it stood on 19 October 2026. No cylinder dimensions appear anywhere in this property, because the rule gives them in a figure rather than in its text. Whether a particular part or magnet passes either test is a determination about that article; we sell the container it is kept in. Whether a kit is a children's product is the four-factor test covered in the resin jewelry property.