Radiation Dose Converter (Gy, rad, Sv, rem, R)

Gray, rad, sievert, rem, roentgen and C/kg — with the exact conversions separated from the two that are not conversions at all: gray to sievert needs a radiation weighting factor, and roentgen to gray needs an f-factor that depends on the material and the photon energy.

Last updated 24 Sep 2026

Gray, rad, sievert, rem and roentgen

One dose unit → all of them, with the factor named

What have you got?The first three are absorbed dose, the next four equivalent dose and the last two exposure. Three different quantities.

Its valueIn the unit chosen above.

Which radiation? (this sets the weighting factor)Absorbed dose becomes equivalent dose only when multiplied by this. For X-rays and gamma rays it is 1, which is why people think the gray and the sievert are the same unit.

For exposure: absorbed in what, at what energy?The f-factor, in gray per roentgen. Used only when you give or ask for an exposure. It depends on the material AND the photon energy.

Three boxes and two arrows, not a circuit, and the structure is the argument. Exposure at the top is ionisation in air, and turning it into an absorbed dose needs the f-factor of whatever is absorbing it. Absorbed dose, in gray and rad, is energy per kilogram, and converting between those two units is exact. Equivalent dose, in sievert and rem, is absorbed dose multiplied by a radiation weighting factor, so the first horizontal arrow is a MULTIPLICATION and not a unit conversion — it happens to be a multiplication by one for photons and electrons, which is why the two boxes are so often confused. The third box is crossed out on purpose: effective dose is a sum over organs of tissue-weighted equivalent doses, and there is no factor that takes you there from a single number. This page will not invent one.

0.005000SvExample

5 mGy of absorbed dose from an X-ray beam, tissue at 60 keV

Three quantities, and which arrows are real conversions

1 Gy = 100 rad · 1 Sv = 100 rem · 1 R = 2.58 × 10⁻⁴ C/kg | H = D × w_R · D = X × f · E = Σ w_T H_T — not on this page

D: absorbed dose, in gray. Energy per unit mass, 1 Gy = 1 J/kg. A physical quantity
H: equivalent dose, in sievert. Absorbed dose weighted for the kind of radiation
X: exposure, in roentgens. Ionisation produced in AIR, which is why converting it needs a material
w_R: radiation weighting factor (ICRP 103) or quality factor Q (10 CFR 20.1004). 1 for photons and electrons, 20 for alphas, and for neutrons no single number
f: f-factor, gray per roentgen. 0.008 764 in air exactly; 0.009 21 to 0.009 66 in soft tissue depending on photon energy
E: effective dose. Needs the dose to every organ and its tissue weighting factor, so this page REFUSES to compute it from a single number

Worked example

5 mGy of absorbed dose from an X-ray beam, tissue at 60 keV
5 mGy is 0.005 Gy, which is 0.5 rad — exactly, because a gray is exactly 100 rad
For X-rays the weighting factor is 1, so the equivalent dose is 0.005 Sv = 5 mSv = 0.5 rem = 500 mrem. The numbers coincide ONLY because w_R = 1
Had the same 5 mGy been alpha dose, the weighting factor would be 20 and the equivalent dose 0.1 Sv — twenty times as much from the same energy
Working back to exposure: at 60 keV the soft-tissue f-factor is 0.009407 Gy/R, so 0.005 Gy needs 0.532 R, which is 0.1371 mC/kg
In context, 5 mSv is 1.61 times the average annual natural background of 3.1 mSv, 50 chest radiographs, and 10% of the NRC's annual occupational limit
And the energy involved is tiny: 0.005 Gy over a 70 kg body is 0.350 J, which would warm it by about a millionth of a degree. Ionising radiation does its damage chemically, not thermally

Gray to sievert: the factor, and the two systems that disagree

Two things to take from this table. First, the top row is why the confusion exists at all: for X-rays, gamma rays and betas the factor is 1, so the number of grays and the number of sieverts are identical, and almost every dose a member of the public or a radiographer meets is of that kind. That is a coincidence of arithmetic, not a fact about units. Second, the bottom two rows are a live disagreement between the regulation and the science. 10 CFR 20 is US law and is a work of the United States Government, so its quality factors are quoted here directly; ICRP Publication 103 is copyrighted, so it is cited and not reproduced, but the structural point is reproducible: ICRP 103 replaced the single neutron factor with a continuous function of energy and cut the proton factor from 5 to 2. If a neutron dose matters to you, 10 is a regulatory convention, not a physical answer, and you need the spectrum.
Radiation	Quality factor Q, 10 CFR 20.1004	Radiation weighting factor w_R, ICRP 103	Do they agree?
X-rays, gamma rays, beta particles, electrons	1	1 (photons; electrons and muons)	They agree. This is why the gray and the sievert look interchangeable
Alpha particles, multiple-charged particles, fission fragments, heavy particles of unknown charge	20	20 (alpha particles, fission fragments, heavy ions)	They agree
Neutrons of unknown energy	10	a CONTINUOUS FUNCTION of neutron energy, peaking near 20 around 1 MeV	They disagree, and by up to a factor of two. There is no single modern number for neutrons
High-energy protons	10	2 (protons and charged pions)	They disagree by a factor of five. ICRP dropped the proton factor from 5 to 2 in the 2007 recommendations

The f-factor: turning an exposure into an absorbed dose

The roentgen measures ionisation in air, and turning it into energy absorbed by something needs the f-factor. The air value is not a measurement at all: 2.58 × 10⁻⁴ C/kg is the roentgen’s definition, exactly, and the ICRU’s mean energy expended per ion pair in dry air is 33.97 J/C, so the product is 0.008 764 Gy/R — 0.876 rad per roentgen. The soft-tissue rows are computed from that by the ratio of NIST’s mass energy-absorption coefficients for ICRU-44 soft tissue and for dry air, energy by energy. Note what the third column does across the diagnostic range: 0.009 21 Gy/R at 30 keV rising to 0.009 66 at 150 keV and above, a spread of 4.8%. The single figure “about 0.0096” that charts print is the high-energy end, and a mammography or dental beam is not there. Bone is higher still, up to about four times air at low energies, because of its calcium; it is not on this page because the energy dependence is far stronger and a single number would mislead.
Material and energy	Photon energy	Gy per R	rad per R	μ_en/ρ tissue ÷ air	Where it comes from
Dry air — the definition itself	—	0.008764	0.8764	1.0000	2.58 × 10⁻⁴ C/kg × 33.97 J/C, from the definition of the roentgen
Soft tissue at 30 keV (mammography, dental)	30 keV	0.009215	0.9215	1.0514	air f-factor × the NIST μ_en/ρ ratio at that energy
Soft tissue at 50 keV	50 keV	0.009325	0.9325	1.0639	air f-factor × the NIST μ_en/ρ ratio at that energy
Soft tissue at 60 keV (a chest radiograph’s effective energy)	60 keV	0.009407	0.9407	1.0733	air f-factor × the NIST μ_en/ρ ratio at that energy
Soft tissue at 100 keV	100 keV	0.009594	0.9594	1.0946	air f-factor × the NIST μ_en/ρ ratio at that energy
Soft tissue at 150 keV and above, including Co-60 and Cs-137	1,000 keV	0.009660	0.9660	1.1022	air f-factor × the NIST μ_en/ρ ratio at that energy

What these numbers look like next to real ones

Reference context, from public authorities, and nothing else: this is not medical advice and it says nothing about any individual. The figures for the United States come from the NRC and the EPA, both drawing on NCRP Report No. 160, and are works of the United States Government; the limits are quoted from 10 CFR 20.1201 and 20.1301 and from Articles 9 and 12 of Council Directive 2013/59/Euratom. Two things are worth noticing. The largest single contributor to an average person’s dose is radon in their own house, at 2.28 mSv a year, which is 23 chest radiographs. And the two occupational limits differ by a factor of two and a half on the same quantity: the NRC allows 50 mSv in a year, while the Euratom standard allows 20 mSv in a year, or up to 50 provided the five-year average stays at 20.
	mSv	mrem	Multiples of annual natural background	Source
Average annual dose, a person in the United States, all sources	6.200	620.0	2.000	NRC / NCRP 160
Of that, natural background	3.100	310.0	1.000	NRC / NCRP 160
Of that, radon in an average US home	2.280	228.0	0.735	EPA
One chest radiograph	0.100	10.0	0.032	NRC
One dental radiograph	0.015	1.5	0.005	NRC
One whole-body CT examination	10.000	1,000.0	3.226	NRC / EPA
NRC annual limit, member of the public (10 CFR 20.1301)	1.000	100.0	0.323	NRC
Euratom annual limit, member of the public (2013/59 Art. 12)	1.000	100.0	0.323	EU
Euratom annual limit, occupational (2013/59 Art. 9)	20.000	2,000.0	6.452	EU
NRC annual occupational limit, total effective dose equivalent (10 CFR 20.1201)	50.000	5,000.0	16.129	NRC

Three quantities wearing six units

Six units, three quantities, and only some of the arrows between them are conversions. That is the whole of this page. Absorbed dose is energy deposited per kilogram and its units are the gray and the rad. Equivalent dose is absorbed dose multiplied by a factor for how damaging that radiation is, and its units are the sievert and the rem. Exposure is ionisation produced in air, measured in roentgens or coulombs per kilogram. Within each of those three, converting is exact arithmetic. Between them it is not arithmetic at all.

The exact ones first. 1 Gy = 100 rad and 1 Sv = 100 rem, both exactly, both straight out of 10 CFR 20.1004. And 1 R = 2.58 × 10⁻⁴ C/kg exactly, because that is the definition. Those three you can do in your head.

Gray to sievert is not a conversion. It is a multiplication by a radiation weighting factor, and the factor is a property of the radiation. For X-rays, gamma rays, beta particles and electrons it is 1, so the two numbers coincide — and because that covers nearly every dose anybody ever meets, almost everybody concludes that the gray and the sievert are the same unit with different names. They are not. For alpha particles the factor is 20: one gray of alpha dose is twenty sieverts. For neutrons there is no single answer. 10 CFR 20.1004 gives 10 for neutrons of unknown energy, which is a regulatory convention adopted so that a number exists; ICRP Publication 103 gives a continuous function of neutron energy which peaks near 20 around 1 MeV. Those two disagree by up to a factor of two, and they disagree by a factor of five for protons, where the regulation says 10 and ICRP 103 says 2. The dropdown offers both so that a number can be traced to the system it came from.

Roentgen to gray needs the f-factor, which is not one number. A roentgen is ionisation in AIR, so converting it to energy absorbed in something else depends on that something else. In dry air the f-factor is exactly the roentgen’s definition times the ICRU’s mean energy per ion pair, 33.97 J/C: 0.008 764 Gy per roentgen. In soft tissue it is that multiplied by the ratio of mass energy-absorption coefficients, which NIST tabulates, and it runs from 0.009 21 Gy/R at 30 keV to 0.009 66 at 150 keV and above. Nearly 5% across the diagnostic range, and the single value charts print is the figure for the top of it. Bone reaches about four times air at low energies and is deliberately left off this page, because a single number for bone would be more misleading than useful.

And what this page refuses. Effective dose. Effective dose is the sum over organs of each organ’s equivalent dose weighted by that organ’s own tissue weighting factor, and there is no factor that turns a partial-body exposure into a whole-body equivalent — you need to know the dose organ by organ, which means knowing the geometry of the exposure and running it through a phantom. Any page offering you a single multiplier for that is selling you something. If what you have is already an effective dose, this page will happily move it between sievert, millisievert, rem and millirem, because those are real conversions.

The other occupational-exposure page in this section, the noise exposure dose calculator, has the same shape of problem: two regulatory systems, disagreeing arithmetic, and a number that means nothing until you say whose rules you are under.

Frequently asked questions

Is 1 gray the same as 1 sievert?

Only for some kinds of radiation, and it is a coincidence when it is. The gray measures energy absorbed per kilogram and the sievert measures the same energy multiplied by a factor that accounts for how damaging that particular radiation is. For X-rays, gamma rays, beta particles and electrons that factor is 1, so the numbers match — which covers nearly every dose anybody meets and is why the two units get used interchangeably. For alpha particles the factor is 20, so 1 Gy is 20 Sv. For neutrons there is no single number at all: the regulation says 10 for neutrons of unknown energy and ICRP 103 gives a continuous function of energy peaking near 20.

How many rad in a gray?

Exactly 100, and this one really is a unit conversion: 10 CFR 20.1004 defines the gray as an absorbed dose of 1 J/kg, which is 100 rad, and the rad as 100 erg/g, which is 0.01 J/kg. Likewise 1 Sv = 100 rem exactly. Those two are conversions. Gray to sievert is not.

How do I convert roentgen to rad or gray?

Multiply by the f-factor, and the f-factor depends on what is absorbing the radiation and at what photon energy. In dry air it is exactly 0.008 764 Gy/R, being the roentgen’s definition of 2.58 × 10⁻⁴ C/kg times the ICRU’s 33.97 joules per coulomb of ionisation. In soft tissue it runs from about 0.009 21 Gy/R at 30 keV to 0.009 66 at 150 keV and above. The common shorthand “1 R is about 1 rad” is right to within 5% for tissue and wrong by 12% for air.

Can this page give me an effective dose?

No, and it will not pretend to. Effective dose weights the equivalent dose to each organ by that organ’s own tissue weighting factor and adds the results, so it needs the dose to each organ separately — which requires knowing the geometry of the exposure, not just its size. Turning a partial-body exposure into a whole-body equivalent is the job of a dosimetry calculation with an anthropomorphic phantom, and there is no factor that does it. If a number you have is already an effective dose, this page will convert it between sievert, millisievert and rem, because those are real unit conversions.

Is 1 mSv dangerous?

This page does not answer that, because it is a medical and regulatory question rather than an arithmetic one. What it can give you is context from public authorities: the NRC puts the average annual dose to a person in the United States at 6.2 mSv, about half of it natural background and most of that radon; a chest radiograph at about 0.1 mSv; a whole-body CT at about 10 mSv. The annual limit for a member of the public is 1 mSv in both the US and EU systems, and for a radiation worker 50 mSv in the US and 20 mSv a year in the EU. Take any actual decision to the authority that governs your situation.

Why is there a quality factor Q and also a weighting factor w_R?

Because they come from different documents at different dates and the regulation has not followed the science. Q is the regulatory quantity in 10 CFR 20.1004, where it takes four values. w_R is ICRP’s modern quantity, set out in Publication 103 of 2007, where neutrons get a continuous function of energy and protons were cut from 5 to 2. For photons, electrons and alphas the two agree, so most of the time the distinction does not bite. For neutrons and protons it does, and this page offers both so that you can see which system a number came from.

What is 1 R in C/kg?

2.58 × 10⁻⁴ C/kg, exactly — that is the definition the ICRU adopted in 1971, so the other direction is 1 C/kg = 3875.97 R. The roentgen is a unit of EXPOSURE, which is ionisation produced in air, and it is the oldest of the quantities here. It is not a dose to anything until an f-factor has been applied.

Related calculators

References

Code of Federal Regulations, 10 CFR 20.1004, Units of radiation dose, with Table 1004(b).1. A work of the United States Government: quoted directly. One gray is an absorbed dose of 1 J/kg, which is 100 rad; one sievert is 100 rem. The quality factors are 1 for X-rays, gamma rays and beta particles, 20 for alpha particles and heavy particles of unknown charge, 10 for neutrons of unknown energy and 10 for high-energy protons.
Code of Federal Regulations, 10 CFR 20.1201 and 20.1301. The annual occupational limit is a total effective dose equivalent of 5 rem (0.05 Sv); the annual limit for a member of the public is 0.1 rem (1 mSv). A work of the United States Government.
International Commission on Radiological Protection. The 2007 Recommendations of the ICRP, ICRP Publication 103, Annals of the ICRP 37(2–4), Table 2 (radiation weighting factors) and Table 3 (tissue weighting factors). ICRP publications are copyrighted and the tables are NOT reproduced here; they are cited so you can look them up. What matters on this page is structural: ICRP 103 gives w_R = 1 for photons and electrons, 2 for protons and charged pions, 20 for alpha particles, fission fragments and heavy ions, and for neutrons a CONTINUOUS FUNCTION of energy rather than a single number.
International Commission on Radiation Units and Measurements. The mean energy expended in dry air per ion pair formed, W/e = 33.97 J/C. With the roentgen’s definition of 2.58 × 10^-4 C/kg exactly (ICRU, 1971) this gives the f-factor in air as 0.008 764 Gy/R — 0.876 rad per roentgen — computed on this page rather than quoted.
Hubbell JH, Seltzer SM. Tables of X-Ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients, NIST Standard Reference Database 126, physics.nist.gov. The soft-tissue f-factors on this page are the air f-factor multiplied by the ratio of μ_en/ρ for ICRU-44 soft tissue to that for dry air at the same photon energy — computed here, energy by energy, which is why the table shows the f-factor rising from 0.009 21 Gy/R at 30 keV to 0.009 66 at 150 keV and above rather than quoting one number.
U.S. Nuclear Regulatory Commission. Doses in Our Daily Lives, nrc.gov, drawing on NCRP Report No. 160. The average person in the United States receives about 620 mrem (6.2 mSv) a year from all sources, about half of it natural background; a chest radiograph is about 10 mrem (0.1 mSv), a dental radiograph 1.5 mrem and a whole-body CT about 1000 mrem (10 mSv). A work of the United States Government.
U.S. Environmental Protection Agency. Radiation Sources and Doses, epa.gov. Gives the same 6.2 mSv annual average and puts radon in an average US home at 2.28 mSv a year, the largest single contributor. A work of the United States Government.
Council Directive 2013/59/Euratom laying down basic safety standards, Articles 9 and 12. The occupational effective dose limit is 20 mSv in any single year, or up to 50 mSv in one year provided the average over any five consecutive years does not exceed 20 mSv. The limit for a member of the public is 1 mSv a year. Worth knowing next to the NRC’s 50 mSv: the two regimes differ by a factor of two and a half on the same quantity.