My protocol says × g but my centrifuge is set in rpm — what speed do I use?
Spin down a sample
Converting between relative centrifugal field and rotor speed, and which radius to use.
A protocol that says "12,000 × g" is specifying a force, and a centrifuge dial is set in revolutions per minute. The two are related only through the radius of your rotor, so the same rpm is a different force on every machine. Converting is the whole job, and it needs a number from the rotor, not from the centrifuge.
Which radius matters more than people expect. A fixed-angle rotor might have its tube bottom at 9 cm and its top at 4 cm, so the pellet experiences more than twice the field the liquid surface does. Manufacturers quote the maximum, and protocols almost always mean it, so use the maximum unless something says otherwise — but be aware the number is a range, not a point.
When you move a protocol between machines, match the force and keep the time, not the other way round. Matching rpm on a different rotor is how a gentle spin becomes a hard one.
Swinging-bucket and fixed-angle rotors pellet differently even at the same force, because the path a particle travels to the wall is not the same shape. A protocol developed on one may need its time adjusted on the other.