Set a number and a power of ten, then slide the exponent along the ladder. The quantity never changes — only the prefix that names it. Watch the marker cross kilo, mega, micro and the rest, and see what happens on the exponents that have no prefix at all.
As written with its prefix
250 km
kilo (k) · factor 103
103 has a name: kilo (k).
Coherent SI value Q = v × 10n
2.5 × 105 m
The metre is already the coherent SI unit of length, so nothing is shifted.
Marker position
103
Three decades above the unprefixed unit.
Exponent n103
One step is one decade. The marker slides; the quantity gets a new name.
Value v250
The number in front. It changes the size — but never moves the marker.
24 SI prefixes span quetta, 1030 down to quecto, 10-30. Ronna, quetta, ronto and quecto were added in November 2022.
Tip: park the exponent on 4 or 5. Nothing is named between kilo and mega, so the readout falls back to scientific notation instead of inventing a prefix.