Radioactive decay calculator
Activity is λN: how many atoms you have, times how likely each is to go this second. The units are where it gets strange. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
One becquerel is almost nothing
A becquerel is one disintegration per second. That is the entire definition, and it makes the unit absurdly small for anything you would meet: an ordinary adult human body runs at roughly four to five thousand becquerels all by itself, mostly from the potassium-40 in ordinary potassium.
Which means a headline about a few hundred becquerels being detected somewhere is a statement about the sensitivity of the instrument, not about danger. Detecting single atoms decaying is something we are extremely good at.
The curie is a round number, not a measurement
The older unit is the curie, and it is exactly 3.7×1010 Bq. Not approximately. Exactly, because that is how it was defined.
It started as the activity of one gram of radium-226, which is a perfectly sensible way to define a unit if you have a gram of radium and no better idea. But as the measurement of radium got better the unit would have had to keep moving, so it was cut loose and fixed at a round number near where it had been. The definition stopped being about radium at all.
That is the same move the metre made when it stopped being a bar in Paris, and the same one the mole made in 2019. A unit that is defined by an object has to change whenever the object is measured more carefully, which is the opposite of what a unit is for.
Activity is not dose
This page gives activity — how many atoms are decaying. It says nothing about how much energy those decays carry, what kind of radiation they emit, whether it can get through skin, or whether the material is inside you. Those are different quantities with different units, and the gap between them is where almost all confusion about radiation lives.