What causes elephant foot in 3D printing?

Two things together. The first layer stays soft because it sits on the heated bed, and the weight of the layers above presses it outwards. It's more noticeable on large, heavy parts with wide bases.

Where is elephant foot compensation in Bambu Studio?

Process, then Quality, then Precision, then Elephant foot compensation. It shrinks the outline of the first layer so it ends up the right size once it spreads.

Where is elephant foot compensation in PrusaSlicer?

Print Settings, then Advanced, then Elephant foot compensation. You need Advanced or Expert mode to see it. It's already on in the official Prusa profiles, and about 0.2mm suits a 0.4mm nozzle.

How do I remove elephant foot from a finished print?

A deburring tool is the quickest way. Drag the swivel blade around the bottom edge and it shaves off the flared lip in a thin strip. A flat needle file works for straight edges, but go slowly so you don't round the corner off.

Does elephant foot matter?

For display pieces, mostly it just looks untidy. For anything that has to fit something else, like snap fits, rails, shaft holes and assemblies, the extra width at the bottom can stop parts going together.

How to fix elephant foot on 3D prints

Elephant foot is the lip that sticks out round the bottom of a print, like the base spread out a bit before it set. On a figure it just looks slightly off. On anything that has to fit into something else, it’s the reason it doesn’t.

Why it happens

Bambu’s wiki puts it down to two things happening at once. The first layer sits on the heated bed, so it stays soft for longer than the rest of the print. Meanwhile every layer printed on top adds weight and pushes down on it, so it spreads outwards. That’s why big, heavy parts with wide flat bases get it worst.

Prusa adds that the first layer is deliberately squished into the bed so it sticks, which makes it a little wider to begin with.

Fixing it in the slicer

Turn on elephant foot compensation. This shrinks the outline of the first layer so it ends up the right size after it spreads.

PrusaSlicer: Print Settings, then Advanced, then Elephant foot compensation (switch to Advanced or Expert mode to see it). Prusa says about 0.2mm suits a 0.4mm nozzle, and it’s already on in the official profiles.
Bambu Studio: Process, then Quality, then Precision, then Elephant foot compensation.

To find the right value properly, Bambu suggests printing a test part with compensation set to 0 and no brim. Then measure the bottom and the top with calipers, and the difference is your starting number. Prusa warns that if you see gaps appearing in the actual first layer, you’ve set it too high.

If compensation isn’t enough:

Lower the bed temperature about 5°C at a time, so the first layer firms up sooner.
Raise the first layer slightly. If your nozzle is too close to the bed, the first layer gets squashed wider than it should. Nudging the Z offset up by 0.05–0.1mm helps.
Change the model if you’re designing it yourself. A small 45° chamfer on the bottom edge hides elephant foot completely, because there’s no sharp corner to bulge past.

Cleaning it off a print that already has it

You don’t need to reprint. Once it’s cooled:

Deburring tool. This is the quick one. The swivel blade follows the edge as you drag it round, so it shaves the lip off in a thin curl without digging in. Hackaday’s writeup on cheap deburring tools has commenters using them for exactly this, and for brims and hole edges.
Needle file. A flat file works well on straight edges. Hold it flat against the side wall, not the bottom, and file along the edge so you don’t round the corner.
Hobby knife. Fine for small bits, but it’s easy to take off too much, and knives slip. Cut away from yourself and keep your other hand out of the way.

Check the fit as you go. It’s much easier to take a bit more off than to put it back.

Full disclosure: I sell a finishing kit that includes a deburring tool. Any swivel deburring tool does this job.