How this instrument works
Absorbed dose, measured in gray, is purely physical: joules of energy deposited per kilogram of tissue, regardless of what kind of radiation deposited it. Effective dose, measured in sievert, adjusts that physical number for biological impact by multiplying it by a radiation weighting factor, W. X-rays, gamma rays, and electrons carry W = 1, so their gray and sievert figures are numerically identical. Protons carry W = 2. Alpha particles carry W = 20 — twenty times as damaging per unit of absorbed energy as an X-ray delivering the identical physical dose, because their large, slow-moving, densely charged track ionizes tissue far more heavily along its path than a photon or electron does.
That weighting scheme comes from the International Commission on Radiological Protection's Publication 103, its 2007 recommendations, which set the current radiation weighting factors used across medical, occupational, and environmental dosimetry. Effective dose also converts cleanly into two other common units here: multiply sievert by 1000 for millisievert, or by 100 for rem, the older unit still seen on some equipment and in some regulatory tables.
Alpha particles can't penetrate skin, so an external alpha source poses little external risk — but the same particles are intensely damaging if they get inside the body through inhalation or ingestion, where they deposit that weighted dose directly into living tissue at close range. That's a genuine limitation of this calculator worth stating plainly: it computes a single whole-body effective dose from one radiation type and one absorbed-dose figure, but real dosimetry for internal exposure or for a specific organ often requires summing several exposure pathways and tissue-specific weighting factors that this simple converter doesn't attempt.
- Enter Absorbed dose in gray (Gy), from a dosimeter reading or a physics calculation.
- Select the Radiation type — sets the weighting factor: X-ray/gamma/electrons at 1, protons at 2, alpha particles at 20.
- Read Effective dose in sievert, then the same result converted to millisievert and rem below it.
Worked example — 0.01 Gy of X-rays vs. 0.005 Gy of alpha particles
0.01 Gy absorbed from X-rays, weighting factor 1: effective dose = 0.01 × 1 = 0.01 Sv, which is 0.01 × 1000 = 10 mSv, or 0.01 × 100 = 1 rem. Because W = 1 for X-rays, the sievert and gray figures come out numerically identical here — the weighting factor only pulls them apart for other radiation types.
Take half that absorbed dose, 0.005 Gy, but from alpha particles at weighting factor 20: effective dose = 0.005 × 20 = 0.1 Sv, equal to 100 mSv — ten times higher than the X-ray case above, despite starting from half the physical energy deposited. The weighting factor, not the raw absorbed number, decides the biological cost.
0.02 Gy of protons at weighting factor 2 gives 0.02 × 2 = 0.04 Sv, or 40 mSv, or 4 rem — a middle case, twice the raw gray figure once weighted, sitting between the X-ray and alpha results on a per-gray basis.
Questions
What's the real difference between absorbed dose and effective dose?
Absorbed dose (gray) is a purely physical measurement — how much energy landed in a kilogram of tissue, full stop. Effective dose (sievert) folds in biology: it weights that physical number by how damaging the specific radiation type tends to be, so two very different physical doses can carry the same expected harm, or two identical physical doses can carry very different harm, depending on what kind of radiation delivered them.
Why does alpha radiation get a weighting factor as high as 20?
Because alpha particles are heavy, slow, and doubly charged, so they deposit their energy in a short, extremely dense track rather than spreading it thinly the way an X-ray photon does. That dense ionization causes disproportionately more biological damage per unit of absorbed energy — which is why the same 0.005 Gy of alpha radiation produces ten times the effective dose that an identical absorbed dose of X-rays would in this calculator's own worked example.
How does this compare to a chest X-ray or a CT scan?
A standard adult chest X-ray delivers roughly 0.1 mSv of effective dose, and a standard chest CT roughly 6.1 mSv, per RadiologyInfo.org's patient safety figures — both from X-rays, where W = 1, so those numbers are simply the absorbed dose in gray multiplied by a thousand. A dental X-ray runs lower still; a full-body CT runs considerably higher. These reference points only apply to photon radiation — the same millisievert total from alpha particles would reflect a much smaller physical dose.
Is rem the same thing as rad?
No, and the two get confused often. Rad measures absorbed dose, the older equivalent of gray (1 rad = 0.01 Gy); rem measures effective dose, the older equivalent of sievert (1 rem = 0.01 Sv). The rem figure this calculator reports is the weighted, biological quantity — the direct counterpart to sievert and millisievert, not a raw physical measurement.
Does this calculator account for which organ was exposed?
No — it computes a single whole-body effective dose from one absorbed-dose figure and one radiation weighting factor. Full ICRP dosimetry also applies separate tissue weighting factors per organ and sums exposures across the whole body when they differ by location; this converter is the simpler radiation-type step of that calculation, not a substitute for organ-specific dosimetry.
Why do sievert, millisievert, and rem all show up as separate results?
Because different fields default to different units by habit and by regulation — millisievert is standard in most medical-imaging literature, sievert is the base SI unit used in physics and larger occupational doses, and rem persists in some older US regulatory and equipment contexts. All three describe the identical effective dose; they're conversions, not independent measurements.
References
- ICRP Publication 103 — The 2007 Recommendations of the ICRP, Annals of the ICRP
- RadiologyInfo.org — Radiation Dose in X-Ray and CT Exams
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.