A fridge magnet and a ruler: the two checks that decide whether old pans will work on a new induction hob

When I first put one of my old pans on the new induction hob, nothing happened. The display flashed, the cooking zone switched off, and for a moment I thought there was something wrong with the hob. It turns out the problem was the pan, not the appliance.

Induction works differently from gas or a conventional electric hob. Coils underneath the glass create a magnetic field that heats a suitable metal in the base of the pan. That means an old pan needs the right kind of base and shape, before the hob can work properly with it.

Fortunately, you can easily find out if your utensils will work with the induction hob. A fridge magnet and an ordinary ruler can eliminate most unsuitable pans in about a minute.

Check 1: put a magnet on the bottom

Start with the easiest test. Take a fridge magnet and hold it against the bottom of the pan.

If it sticks firmly, the base contains magnetic material and has passed the first test for induction cooking. Experts recommend this check. If the magnet barely holds or simply falls away, that particular pan is unlikely to work properly.

Cast iron and enamelled steel usually pass easily, as do many stainless-steel pans. However, “stainless steel” alone isn't a guarantee: what matters is whether the base is magnetic.

Pure aluminium and copper normally fail. But don't reject a pan based purely on what the rest of it is made from. Some aluminium, copper and non-stick pans have a magnetic layer built into the bottom specifically so they can be used on induction.

Newer induction-compatible cookware may also carry the familiar coil or spiral symbol on its base or packaging. If you're buying a pan and the packaging isn't clear, the same fridge-magnet trick works. If the magnet sticks firmly to the base of the pan, it is a quick indication that you're looking in the right direction.

Check 2: turn the pan over and get a ruler

A magnetic pan can still be a poor match if years of cooking have changed its shape.

Turn the cold pan upside down and place a straight ruler across the base. Most induction hob brands advise against using a pan with a base that bulges outward. A badly distorted pan may sit unevenly, perform poorly or be harder for some hobs to detect.

This is particularly worth checking with pans that have spent years over gas flames or have been exposed to sudden temperature changes.

Don't immediately reject every pan with a tiny hollow in the middle, though. Some cookware is deliberately slightly concave when cold and becomes flatter as it heats.

Irregular or warped cookware can also vibrate against the glass on some induction hobs, especially at high power. A little humming or buzzing is not automatically a fault, however; some noise is normal with induction cookware.

The size of the pan matters too

There is one more measurement worth making before declaring a pan suitable: the diameter of its base.

Choose a cooking zone that is roughly the same size. A pan more than about 2 cm smaller or larger than the zone can give poorer results. A pan that is too small may not be detected at all, while an oversized pan can heat better in the centre than around the edges.

Remember that the size printed on a pan often refers to its rim. The flat magnetic area underneath can be considerably smaller.

Don't slide a good pan across the glass

Once you've found the keepers, treat the new surface with care. Lift pans when moving them rather than dragging them across the glass, particularly if the bottom is rough.

Heavy cast-iron cookware works very well on induction, but it is should be lowered gently rather than dropped onto the surface.

What about the pans that fail?

A pan that doesn't pass the magnet test does not necessarily have to go straight in the bin. Depending on its materials and handles, it may still be useful in the oven or on another type of hob.

Otherwise, check your local rules for metal recycling or ask cookware retailers whether they have a take-back scheme. The important point is to test first. Switching to induction doesn't automatically mean replacing every pan you already own.