Geiger Counters and Radiation Detectors: What to Know Before You Buy

A radiation detector can be interesting and useful, but it is optional. We have not tested any detector. This guide explains how to read specifications, based on federal guidance and general knowledge of these instruments, and it recommends no particular model.

Do you need one?

Federal instructions do not depend on owning one. Ready.gov says to get inside, stay inside and stay tuned, and its supply advice focuses on a radio, food, water and medicine. Buy those first. A phone cannot measure radiation on its own, so an app cannot replace an instrument.

What a reading tells you

• A detector reports radiation at that spot and moment. It cannot tell you what is happening a street away, and fallout can be uneven.
• Counts per minute are not a dose. EPA says of its own monitors, for a different reason, that count rate has no direct relation to exposure rate. On a home detector, a conversion to µSv/h assumes a type of radiation and depends on the instrument.
• Readings move around at normal levels. Learn what is typical in your home before relying on any change.
• A low reading does not make it safe to go outside. CDC says to stay inside until officials say otherwise.

Units

EPA gives 1 sievert = 100 rem. That makes 1 µSv/h equal to 0.1 mrem/h. Federal planners also use roentgens per hour (R/h). Their guidance treats 1 R of gamma exposure as about 1 rem. That is an approximation, and it is the one we use below.

Specifications worth checking

• What it detects. As a general rule, hobby units detect gamma and some beta. Check whether alpha is covered, and how. Fallout dose is mainly from penetrating radiation from particles on the ground.
• Maximum range. Hobby meters are generally built for low levels, so read the maker's stated range. Federal planners use 0.01 R/h (10 mR/h) for a "hot zone" and 10 R/hr for a dangerous radiation zone. By the approximation above, those are about 100 µSv/h and 100,000 µSv/h. Compare the maker's stated range with those. If a meter cannot reach them, it may saturate or read wrongly at those levels.
• Calibration. Check whether the maker documents how it was calibrated, and against what.
• Power. Battery life and how you charge it matter in an outage.

What a detector will not do

• It will not tell you which radioactive material is present, unless it is built for that.
• It will not give you a health verdict. Ask a healthcare provider or follow officials.
• It will not replace an instruction from officials to stay or to leave.

See how to read radiation maps and the fallout decay calculator.