Millisieverts (mSv) = 1 sievert (Sv)

The idea

Radiation dose is measured in sieverts (Sv), but many everyday exposures are much smaller than one sievert, so the millisievert (mSv) is used. The prefix milli- means one thousandth, so 1 mSv is 0.001 Sv and 1000 mSv make 1 Sv. To convert millisieverts to sieverts, divide by 1000; to convert sieverts to millisieverts, multiply by 1000. For example, a dose of 2500 mSv is 2.5 Sv, and a dose of 0.04 Sv is 40 mSv. Being able to convert between these units lets you compare doses of ultraviolet, X-ray or gamma radiation fairly when judging the risk of harm to body tissue.

What earns the marks

• The sievert (Sv) is the unit of radiation dose.
• The millisievert (mSv) is a smaller unit of radiation dose, equal to one thousandth of a sievert.
• 1000 mSv = 1 Sv, so 1 mSv = 0.001 Sv.
• To convert mSv to Sv, divide the value in mSv by 1000.
• To convert Sv to mSv, multiply the value in Sv by 1000.
• Converting units allows doses to be compared on the same scale when assessing risk of harm.

Common mistakes and how to put them right

• Multiplying by 1000 when converting mSv to Sv.

  Put it right by dividing by 1000, because 1 mSv is smaller than 1 Sv.

• Writing 1000 Sv = 1 mSv.

  Put it right by remembering that the millisievert is the smaller unit, so 1000 mSv = 1 Sv.

• Ignoring the unit when comparing doses.

  Put it right by converting all values to the same unit, usually Sv or mSv, before comparing.

Exam tips

• Write the conversion factor 1000 mSv = 1 Sv at the start of a calculation so you do not invert it.
• Check whether the question gives a dose in mSv or Sv, and convert before comparing or calculating risk.
• Give the unit with your answer, and use mSv for small doses and Sv for large doses to keep numbers sensible.