Radiation Dose Converter — gray, sievert, rad, rem

The radiation dose converter above keeps the four radiological quantities apart instead of blending them into one list: absorbed dose, equivalent dose, activity and exposure. Each answers a different question, each has its own SI unit and its own pre-SI unit, and treating a gray as interchangeable with a sievert is the most common and most consequential error in radiation unit work.

Arb Digital publishes this converter as part of a free technical reference library. All factors come from the SI definitions maintained by the BIPM and the conversion tables in NIST Special Publication 811, and the historical units, the rad, rem, curie and roentgen, are all exactly defined relative to their SI replacements.

What This Radiation Dose Converter Does

Choose a quantity and the tool loads only the units that belong to it, which makes an invalid cross-quantity conversion structurally impossible. Absorbed dose mode covers gray and its submultiples, the rad, the millirad, joules per kilogram and ergs per gram. Equivalent dose mode covers sievert, millisievert, microsievert, rem, millirem and microrem. Activity mode covers becquerel through terabecquerel, the curie and its submultiples, disintegrations per second and per minute, and the rutherford. Exposure mode covers coulombs per kilogram and the roentgen.

The four supporting panels show the SI unit, a practical submultiple, the legacy unit, and one genuinely related quantity. In absorbed dose mode that fourth panel shows the equivalent dose in sieverts computed with the radiation weighting factor you supply, and in equivalent dose mode it works the reverse way. The weighting factor is asked for explicitly rather than assumed, because assuming it is what makes other tools wrong.

How to Use It

1. Select the quantity. Gray or rad means absorbed dose. Sievert or rem means equivalent dose. Becquerel or curie means activity. Roentgen means exposure.
2. Enter the value. Radiological figures span many orders of magnitude, so exponent input is supported.
3. Choose your from and to units from the filtered lists. The headline result and the full conversion sentence update immediately.
4. Set the radiation weighting factor if you want the gray-to-sievert link. Leave it at 1 for photons and electrons; set it to 20 for alpha particles.
5. Read the panels for the SI value, a submultiple, the legacy unit and the paired quantity, then use Swap to verify the inverse conversion.

The Formula and How It Is Calculated

Every unit is held as a multiplier to the SI unit of its quantity, so a conversion is result = value × factor(from) ÷ factor(to). The gray is defined as one joule of energy absorbed per kilogram of matter. The rad, the older unit, is exactly 0.01 Gy. The sievert has the same dimensions of joules per kilogram but a different meaning, and the rem is exactly 0.01 Sv. The becquerel is one nuclear transformation per second, and the curie is exactly 3.7 × 10¹⁰ Bq, a number that originated as the approximate activity of one gram of radium-226. The roentgen is exactly 2.58 × 10⁻⁴ coulombs per kilogram of air.

The link between absorbed and equivalent dose is H_T = Σ w_R × D_T,R: the equivalent dose in a tissue is the absorbed dose from each radiation type multiplied by that type's weighting factor, summed over all types present. These definitions are listed in the BIPM SI Brochure and NIST Special Publication 811, and the regulatory context for the units is summarised by the US Nuclear Regulatory Commission.

Gray Versus Sievert: Same Dimensions, Different Questions

Both the gray and the sievert reduce to joules per kilogram, which is exactly why they get confused. The SI keeps them as separate named units precisely to stop that confusion, because they measure different things.

The gray is a purely physical quantity: how much energy the radiation deposited per kilogram of material. It applies equally to tissue, concrete or a silicon detector, and it makes no claim about consequences. The sievert is a protection quantity: absorbed dose adjusted for how damaging that particular radiation type is to biological tissue at the microscopic scale. One gray of gamma radiation is one sievert. One gray of alpha radiation, which deposits its energy along a very short, very dense track, is twenty sieverts. The physics of energy deposited is identical; the biological weighting is not. Because of this, sieverts are only meaningful for tissue, and quoting a sievert dose to a concrete shield or an electronic component is a category error.

Where the Weighting Factors Come From

Radiation weighting factors are not derived from a formula. They are consensus values published by the International Commission on Radiological Protection, based on radiobiological evidence about the relative effectiveness of different radiation types at causing stochastic damage per unit of absorbed energy. The current set assigns 1 to photons, electrons and muons, 2 to protons and charged pions, 20 to alpha particles, fission fragments and heavy ions, and a continuous energy-dependent function to neutrons that peaks near 20 around 1 MeV and falls to roughly 2.5 at very low and very high energies.

Two things follow. First, a neutron field cannot be converted from gray to sievert without knowing the neutron energy spectrum, so a single-number answer for neutrons is always an approximation. Second, the factors are revised as evidence accumulates; earlier recommendations used a slightly different neutron function and a value of 5 rather than 2 for protons. If you are reconciling old records against current ones, check which recommendation set was in force, because the same measured gray figure can yield different sievert values under different revisions.

Equivalent Dose Versus Effective Dose

A further distinction hides inside the sievert, and it trips up even experienced readers. Equivalent dose applies the radiation weighting factor to a single tissue. Effective dose goes one step further and applies a second set of tissue weighting factors, reflecting that different organs carry different risk per unit dose, then sums across the whole body. Both are reported in sieverts.

That shared unit means you cannot tell from the number alone which quantity you are looking at. A dose to the thyroid from a targeted procedure is an equivalent dose to that organ. A whole-body figure quoted for a CT examination or an annual occupational record is almost always an effective dose. They are not interchangeable, and comparing an organ equivalent dose to a whole-body effective dose as though they were the same figure will overstate or understate the comparison substantially. Always check which one a document means before converting or comparing.

Activity Is Not Dose

Activity, measured in becquerels, counts nuclear transformations per second in a source. It says nothing on its own about the dose anyone receives. The dose from a source depends on activity, but also on the energy and type of the emissions, the distance, the shielding, the exposure time, and whether the material is external or has been taken into the body.

The becquerel is an inconveniently small unit; one becquerel is a single decay per second, and ordinary environmental samples contain many thousands. The curie, at 3.7 × 10¹⁰ Bq, is inconveniently large for the same reason, which is why practical work runs in kilobecquerels through gigabecquerels or in microcuries and millicuries. Seeing a headline number in becquerels and assuming it implies a hazard, or seeing one in curies and assuming it does not, both get the reasoning backwards. Rates over time can be handled alongside the time converter, and the underlying energy figures in the energy converter.

Full Radiological Unit Conversion Table

Common Mistakes to Avoid

• Treating gray and sievert as the same unit — they share dimensions but not meaning, and the ratio between them is the radiation weighting factor, which ranges from 1 to 20.
• Converting a neutron dose with a single weighting factor — the neutron factor is a function of energy, so an accurate figure needs the spectrum.
• Comparing an organ equivalent dose to a whole-body effective dose — both are quoted in sieverts but they answer different questions.
• Reading activity as dose — becquerels count decays in a source and imply nothing about received dose without distance, shielding, emission type and time.
• Dropping a factor of 100 between rad and gray or rem and sievert — both legacy units are exactly one hundredth of their SI counterpart, and the slip is easy to make in both directions.

Related Free Tools From Arb Digital

Radiological work touches several other unit families. Convert deposited energy in the energy converter, source and beam power in the power converter, half-lives and exposure times in the time converter, shielding thickness in the length converter, and shield or sample mass in the weight converter. Very large and very small magnitudes read more easily through the scientific notation converter, and the general unit converter covers everything else.

Frequently Asked Questions

This tool converts units only. It is not a dose assessment, a shielding calculation, or a substitute for a radiation safety evaluation by a qualified health physicist or radiation protection adviser, and it must not be used to judge whether any exposure is safe.