An SI prefix renames a quantity without changing it. Set a number and a power of ten below, then slide the exponent along a ladder running from quetta at 1030 to quecto at 10-30, and watch the same physical quantity pick up a new name at every named step — and lose one entirely at the exponents that have no prefix.
The exponent slider does one thing: it moves the quantity by whole factors of ten. Every step is a multiplication or division by exactly ten, which is why the marker lands on discrete notches rather than gliding between them. Its position along the ladder is the exponent — the logarithm of the power of ten it selects — and that is what makes the ladder evenly spaced across sixty orders of magnitude. On a linear scale, everything below a kilometre would be crushed into the leftmost pixel.
The value slider proves the complementary point, and it is the one most people get wrong. Drag it from 1 to 999 and every readout changes while the marker does not move a pixel. A prefix scales the unit, not the number written in front of it, so the mantissa is free to be anything without disturbing which prefix applies. That is why 1 km and 999 km sit on the same rung despite differing by a factor of a thousand.
What the sim solves on every frame is a single subtraction of exponents: take the number you set, and shift it by the difference between the exponent it is written on and the exponent of the prefix you want to read it in. Nothing else happens — no lookup, no rounding — which is why the conversion is exactly reversible. The SI prefix converter runs that same subtraction one pair at a time, and the guide to SI units in physics covers the seven base units the prefixes attach to.
The last thing it demonstrates is a misconception the prefix table encourages: that every exponent has a name. It does not. Between 103 and 106 there is nothing — kilo, hecto and deca crowd near the bottom and then the prefixes jump in threes — so a quantity at 104 has no prefix name at all and must be written in scientific notation. Park the slider there and the sim says so rather than inventing something. The gram button shows the other trap: prefixes attach to the gram, but the coherent SI unit of mass is the kilogram, so its marker sits at 103 instead of 100 and every mass reading shifts by a thousand.
It changes the power of ten the quantity is written on, one decade per step. Moving it from 3 to 4 multiplies the underlying value by exactly ten and slides the marker one notch to the right. It never touches the number in front, which is why the headline can jump from kilometres to a bare scientific-notation figure while the digits 250 stay put.
Because the SI prefixes are a fixed list of 24, not a rule that covers every exponent. Above 10^3 they step in threes, so 10^4, 10^5, 10^7 and 10^8 have no name at all. The lab refuses to invent one: park the slider on 4 and it tells you the nearest named steps are kilo and mega, and writes the value in scientific notation instead.
Because the kilogram is the only SI base unit whose name already contains a prefix. Prefixes attach to the gram, so you write milligram and megagram, never microkilogram. Press the gram button and the cream diamond marking the coherent SI unit slides from 10^0 to 10^3, and the coherent readout drops by a factor of a thousand relative to the metre ladder.
There are 24, running from quetta at 10^30 down to quecto at 10^-30. The four outermost ones, ronna and quetta at the top and ronto and quecto at the bottom, were approved by the General Conference on Weights and Measures in November 2022. That was the first expansion of the prefix set since 1991, driven largely by the scale of global data storage.
No. Compound prefixes were abolished with the SI itself, so a millimicrometre is not a legal unit; you write nanometre. The lab enforces this by giving every exponent at most one name. The kilogram looks like an exception but is not, because its prefix is part of the base unit's name rather than something you attached to it.