Radiation dose calculator

What do sieverts, millisieverts and microsieverts measure?

The sievert (Sv) is the unit of effective dose, which weights the energy absorbed in each organ by how sensitive that organ is, so that different exposures can be added up as a single number. A millisievert (mSv) is a thousandth of a sievert and a microsievert (µSv) a thousandth of a millisievert. The older US unit is the rem: 1 mSv = 100 millirem. A chest X-ray at 0.1 mSv is 100 µSv or 10 millirem. Effective dose is a protection quantity, a way to compare exposures, not a prediction of harm for one person.

How much radiation does the average American receive in a year?

About 6.2 mSv (620 millirem) according to NCRP Report 160, quoted by the EPA, the NRC and the Health Physics Society. Natural background is about half: HPS gives 3.1 mSv a year, of which radon and thoron are about 2.3 mSv, and the FAA's table of the same NCRP figures lists cosmic rays 0.33, radon and thoron 2.28, the ground 0.21 and the body's own radioactivity 0.29 mSv, a total of 3.11. The other half is mostly medical imaging. These figures describe 2006: the 2019 NCRP Report 184 estimated medical exposure alone and found about 2.3 mSv per person in 2016, down from about 2.9 mSv recalculated for 2006, so a total for today would probably be lower than 6.2.

What is a banana equivalent dose?

It is a playful unit: the dose from eating one banana, about 0.1 µSv according to the Health Physics Society, because bananas contain potassium-40, a natural radioactive form of potassium. One banana equals 0.0001 mSv, so 10,000 bananas equal 1 mSv, and a chest X-ray is 1,000 bananas. Treat it as an illustration for scale, not as a measure of risk: HPS lists potassium-40 among the radionuclides that are normally present in the body anyway, and the comparison says little about X-rays, which deliver a dose to specific organs in a fraction of a second.

How much radiation do I get on a flight?

The FAA's advisory circular 120-61B lists doses calculated with its CARI-7 program for 27 nonstop US and international flights. They run from 6.6 µSv for New York to Chicago (1.8 hours) to 27 µSv for London to New York (6.8 hours) and 55 µSv for New York to Tokyo (13 hours), roughly 3.7 to 4.2 µSv per flight hour on these routes. Dose depends on altitude, latitude and solar activity; the table assumes quiet solar conditions. Aircrew receive more because of repeated exposure: UNSCEAR put the average annual dose for aircrews at 3 mSv.

What are the dose limits?

For radiation workers, the NRC limit in 10 CFR 20.1201 is 5 rem (50 mSv) a year of total effective dose equivalent. For members of the public, 10 CFR 20.1301 limits the dose from a licensed facility's operations to 0.1 rem (1 mSv) a year, and explicitly excludes background radiation and any medical exposure. So a CT scan is not covered by the 1 mSv figure. The FAA recommends that aircrew keep exposure to an average of 20 mSv a year over five years, with no more than 50 mSv in any one year, and for a pregnant crewmember no more than 0.5 mSv a month to the conceptus. The calculator shows your total as a share of each, as a yardstick.

What does ALARA mean?

As low as reasonably achievable: the basic principle of radiation protection, in which every reasonable effort is made to keep exposure down, taking into account the benefit of the activity. For patients it means that the test should be medically justified and done with the lowest dose that answers the clinical question, not that a needed scan should be skipped. Ask your clinician why a scan is needed and whether it can be answered another way, and keep a record of your imaging if you wish.

Is the dose from a scan dangerous?

This calculator cannot say. Effective dose is a way to compare exposures, and risk depends on age, sex, the organ exposed and the reason for the exam; for most medically indicated tests the benefit is much larger than the risk. Do not postpone or refuse a test your doctor ordered because of a number on a web page; ask the doctor or the imaging department about the dose and alternatives. For occupational exposure, your employer's radiation safety officer sets the rules.

How to read the result

Pick each source of exposure and how many times you had it: the calculator multiplies, adds and expresses the total in microsieverts, millisieverts and millirem, then compares it with 1 mSv (the public figure in 10 CFR 20.1301), with 6.2 mSv (the average American's year) and with 50 mSv (the worker limit). It also converts the total into days of ordinary natural background. At 3.1 mSv a year, a day of background is about 8.5 µSv, so 0.15 mSv is 18.2 days of it. You can add a dose of your own in millisieverts with the "Other" choice.

Worked example

A chest X-ray (0.1 mSv) and a round trip between London and New York (2 × 0.0273 mSv) total 0.15 mSv, which is 155 µSv, 15.5 millirem, 2.5% of an average American's 6.2 mSv year. A chest CT is 7 mSv by itself, 113% of that average, as much as 70 chest X-rays. It would take 10 chest X-rays to reach 1 mSv, and a dental X-ray is 1/7 of a chest X-ray. All of the medical numbers are typical values from the NRC page, not your own: the dose of a real scan depends on the machine, the protocol and the size of the patient.

Common mistakes

Treating the public limit of 1 mSv as a safe-or-dangerous line for a scan: it is a limit on a licensee's operations that excludes medical exposure and background. Comparing effective doses of different kinds without the context of the organ and the age. Assuming that a flight dose is the same on every route; polar routes, high altitudes and solar storms raise it. Using the banana figure as a measure of the safety of anything but bananas. And forgetting that the 6.2 mSv average is an average: HPS notes that the medical share is not evenly spread, since people with health problems, especially older people, receive most of it.

Limits of this method

The numbers are typical values from government and professional sources, not measurements of you. Effective dose is a protection quantity with built-in assumptions that make it unreliable as an individual risk estimate. The average annual dose of 6.2 mSv comes from NCRP Report 160 and describes the United States around 2006; later medical figures are lower. Natural background varies with altitude, geology and housing: the EPA lists 0.8 mSv a year of cosmic radiation in Denver against 0.3 at sea level. The tool does not account for radiation therapy, radiopharmaceutical details or occupational monitoring badges.

Doses used by the calculator

Flight doses are the CARI-7 values (ICRP 103 weighting, quiet solar conditions) from the FAA table; the NRC lists medical doses in millirem, which are divided by 100 here. These are typical values for illustration.

General information, not medical advice. For questions about a particular scan or your own exposure talk with your doctor or the imaging facility, and for work exposure with your radiation safety officer. Sources, read October 27, 2026: U.S. Nuclear Regulatory Commission, “Doses in Our Daily Lives” (updated September 14, 2026); U.S. EPA, “Radiation Sources and Doses”; Health Physics Society, “Common Sources of Radiation” fact sheet (revised September 2020) and Ask the Experts q12162; FAA Advisory Circular 120-61B, “In-Flight Radiation Exposure” (2014); 10 CFR 20.1201 and 20.1301 (eCFR); NCRP Reports 160 and 184 as summarized in Radiology (2020). Related: half-life calculator, weight converter, average calculator, percentage calculator. How we check our calculators.