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Radiation units explained: Sv vs rem, Gy vs rad, Bq vs Ci

What activity, absorbed dose and equivalent dose each measure, the exact SI and traditional unit conversions, and where the roentgen and weighting factors fit.

Practical guide

By Steven Michael Barber · Published October 10, 2026 · Updated November 3, 2026 · 6 min read

Radiation work in the United States runs on two sets of units at once. Federal regulations, older instruments and many procedures use rem, rad and curie. International standards, newer instruments and most scientific papers use sievert, gray and becquerel. The conversions are exact and simple, but the quantities behind them are different things, and mixing them up is a common source of errors. This guide sorts out what each unit measures and how to convert between them.

Three different questions

Radiation units answer three separate questions:

1. How radioactive is the source? That is activity, the number of atoms decaying per second. Units: becquerel (Bq) and curie (Ci).
2. How much energy did the radiation deposit? That is absorbed dose, energy per kilogram of material. Units: gray (Gy) and rad.
3. How much does that dose count for radiation protection? That is equivalent dose (and, summed over organs, effective dose). These are protection quantities built from reference weighting factors, not a measurement of one person's harm or risk. Units: sievert (Sv) and rem.

A becquerel never converts into a sievert by a fixed factor. Getting from activity to dose takes a model of the source, the distance, the shielding and the exposure time, such as the point-source formula in our dose rate guide.

Activity: becquerel and curie

One becquerel is one decay per second. It is a tiny amount, which is why everyday sources are quoted in kilobecquerels or megabecquerels. One curie was originally based on the activity of a gram of radium-226 and is now defined as exactly 3.7 × 10¹⁰ Bq.

• 1 Ci = 37 GBq
• 1 mCi = 37 MBq
• 1 µCi = 37 kBq
• 1 nCi = 37 Bq

Going the other way, divide by 37 at the matching prefix step: 370 MBq is 10 mCi.

Absorbed dose: gray and rad

One gray is one joule of energy absorbed per kilogram. One rad is exactly 0.01 Gy, so 1 Gy = 100 rad, 1 mGy = 100 mrad, and 1 cGy = 1 rad. The "centigray equals rad" shortcut is common in radiation therapy.

Absorbed dose is a physical quantity. It does not care what kind of radiation delivered the energy, which is exactly why it is not enough on its own to describe risk.

Equivalent dose: sievert and rem

Different radiations do different amounts of biological damage for the same energy deposited. An alpha particle leaves a dense track of ionization along a short path; a gamma ray spreads its energy thinly. To account for this, radiation protection multiplies absorbed dose by a radiation weighting factor, w_R:

HT = ΣR wR × DT,R

Here D_T,R is the absorbed dose from radiation R averaged over tissue or organ T. For one kind of radiation the sum has one term, H_T = w_R × D_T,R, which is the form used below. With D in gray, H comes out in sievert. With D in rad, H comes out in rem. One rem is exactly 0.01 Sv, so 1 Sv = 100 rem and 1 mSv = 100 mrem.

The calculator uses the weighting factors from ICRP Publication 103 (2007):

For neutrons, ICRP 103 gives three formulas by energy band. Between 1 and 50 MeV, for example, w_R = 5.0 + 17.0 × e^{−[ln(2E)]²/6}, with E in MeV. The factor peaks near 1 MeV; the worked example below shows that calculation. Our radiation unit converter carries the full energy table and formulas.

U.S. compliance under 10 CFR 20 uses the quality factors in 10 CFR 20.1004 instead. Its table assigns Q = 10 to high-energy protons, compared with ICRP 103's w_R = 2 for protons, and Q = 10 to neutrons of unknown energy. Where sufficient neutron-energy information exists, the rule permits the appropriate energy-dependent values in its own table. Those are different conventions from ICRP 103, so use the factors required by the radiation protection program rather than substituting one set for the other.

Worked example: converting common values

Dose equivalent. This is the NRC's name for the same product of absorbed dose and a radiation factor that ICRP calls equivalent dose; a badge report uses the NRC name. A badge report shows 500 mrem. In SI: 500 mrem × 0.01 = 5 mSv. A paper quotes 2.5 mSv; in traditional units that is 2.5 × 100 = 250 mrem. A 5 rem figure is 50 mSv.

Absorbed dose. 100 mrad is 100 × 0.01 = 1 mGy.

Activity. A vial labeled 370 MBq holds 370 ÷ 37 = 10 mCi. A 1 µCi check source is 37 kBq, or 37,000 decays per second.

Absorbed to equivalent dose. Suppose a tissue receives 0.5 mGy:

• From gamma rays: H = 1 × 0.5 mGy = 0.5 mSv.
• From alpha particles: H = 20 × 0.5 mGy = 10 mSv, twenty times as much for the same energy.

And 2 mGy from protons gives 2 × 2 = 4 mSv.

Neutrons. For 1 mGy from 1 MeV neutrons, the factor is 5.0 + 17.0 × e^{−[ln 2]²/6}. Here ln 2 ≈ 0.6931, its square is 0.4805, divided by 6 gives 0.0801, and e^−0.0801 ≈ 0.9230. So w_R ≈ 5.0 + 17.0 × 0.9230 ≈ 20.7, and H ≈ 20.7 mSv.

Where the roentgen fits

The roentgen (R) is older than all of these. It measures exposure: the charge that x-rays or gamma rays produce by ionizing air, defined as 2.58 × 10⁻⁴ coulomb per kilogram of air. Many survey meters still read in mR/h, and the gamma dose rate formula gives exposure rate in R/h.

Two conversions apply:

• To air kerma (gray): multiply by the measured mean energy spent per unit charge of ions formed in dry air, 33.97 J/C (ICRU Report 85). 1 R ≈ 2.58 × 10⁻⁴ × 33.97 ≈ 8.764 × 10⁻³ Gy of air kerma, or about 8.76 mGy, assuming electronic equilibrium. It is close, not exact, and it is a quantity in air, not the dose to tissue.
• To dose equivalent (rem or sievert): for photons, radiation protection practice treats 1 R as approximately 1 rem (10 mSv). This is a working convention, close enough for most survey work with gamma rays, but not an exact conversion.

Effective dose is a further step

Equivalent dose applies to one organ or tissue. Effective dose weights the equivalent doses to each organ by tissue weighting factors and adds them up, E = Σ_T w_T × H_T, to give a single protection quantity for the whole body, defined for a reference person rather than for any one individual. Both use the sievert, which is why two numbers in mSv can mean different things. Effective dose needs organ dose estimates that a unit converter cannot supply, and our calculators do not compute it.

Practical tips

• Write the unit next to every number in a calculation, including the prefix. Most unit errors are a lost "m" or "µ."
• When a record says "dose," find out whether it means absorbed dose, equivalent dose or effective dose before converting.
• Remember the two exact factors: 1 rem = 0.01 Sv and 1 rad = 0.01 Gy. Everything else in this guide follows from them, from the curie definition, or from a weighting factor.