Sieverts, Microsieverts and Nanosieverts, Explained

Open any radiation reading and you'll hit a wall of unfamiliar units: sieverts, millisieverts, microsieverts, nanosieverts — and sometimes a completely different-looking number labelled CPM. It looks like physics homework. It isn't. Once you see how the units relate, a radiation reading becomes as readable as a temperature.

Open the live radiation map and every monitor shows a dose rate in two of these units side by side. Here's how to read them.

The sievert, and its ladder of smaller units

The base unit is the sievert (Sv). It measures dose — not just the energy of the radiation, but how much biological effect that energy has. A whole sievert is a big amount, far more than anything in daily life, so we almost never use it on its own. Instead we step down a ladder, and every rung is a factor of a thousand:

• 1 sievert (Sv) = 1,000 millisieverts
• 1 millisievert (mSv) = 1,000 microsieverts
• 1 microsievert (µSv) = 1,000 nanosieverts (nSv)

That's the whole trick. Each step left-to-right divides by a thousand; each step the other way multiplies by a thousand. Natural background radiation is tiny, so it lives down at the microsievert and nanosievert end of the ladder.

Why background is quoted in µSv/h and nSv/h

A single dose figure isn't quite enough — you also want to know how fast the dose is arriving. So background readings add "per hour." A reading of 0.1 µSv/h means you'd pick up one-tenth of a microsievert of dose for each hour spent there.

Because one microsievert is a thousand nanosieverts, the very same reading can be written as 100 nSv/h. Same amount, tidier number. That's why the map shows both: 0.1 µSv/h (100 nSv/h) is one reading, not two.

For a sense of the scale, typical natural background sits around 0.05–0.20 µSv/h — the green band on the map.

CPM: the raw signal behind the dose

Some sensors report a stranger-looking number: CPM, counts per minute. This is the raw output of a Geiger tube — the literal number of radioactive events it detects each minute. It is not a dose. A tube might click along at a few dozen CPM in ordinary background.

To turn CPM into a dose rate, you divide by a conversion factor that's specific to that tube — a published figure that says how many counts per minute correspond to one microsievert per hour. Different tubes have different factors, so the same CPM on two different detectors can mean different doses.

On our map this matters because the two networks report differently:

• EPA RadNet (the US government monitors) reports a dose rate directly, already in nanosieverts per hour — no conversion needed.
• Safecast (the global citizen-science sensors) reports CPM, which we convert to nanosieverts per hour using each tube's published factor, and we show the raw count too, so nothing is hidden.

The one-line summary

If you remember nothing else: the sievert is the dose, "per hour" is the rate, each unit down the ladder is a thousand times smaller, and CPM is a raw count you convert into a dose. With that, every number on the radiation map turns into something you can actually read — and, almost always, it's telling you the reassuring story that the background is perfectly normal.