Gray to Rad Converter
Quickly convert from Gray to Rad.
How to convert
Formula:
β
Absorbed dose measures the energy deposited by ionising radiation in matter per unit mass.
Where is it used?
β’ Radiation Oncology β Cancer radiotherapy prescribes absorbed dose to the tumour in gray (Gy); typical curative courses deliver 45β80 Gy in fractions of 1.8β2 Gy per session, precisely planned with treatment-planning...
Examples:
β’ 1 gray (Gy) = 100 rad
β’ 1 rad = 0.01 Gy (10 mGy)
Absorbed dose measures the energy deposited by ionising radiation in matter per unit mass. The SI unit is the gray (Gy), equal to one joule of energy absorbed per kilogram of material. The older rad (radiation absorbed dose) is still used in the United States; 1 Gy = 100 rad. Absorbed dose is the foundation for assessing radiation effects on materials, electronics, and biological tissue β though biological harm also depends on radiation type, addressed by the equivalent dose (sievert).
When ionising radiation (X-rays, gamma rays, neutrons, alpha or beta particles) passes through matter, it transfers energy to the atoms and molecules of the medium. Absorbed dose quantifies this deposited energy: 1 gray = 1 J/kg. The gray applies to any material β tissue, silicon, steel β not just humans. The rad (0.01 Gy) predates the SI and remains common in US medical and industrial radiation protection literature. The erg/g unit appears in older scientific texts (1 erg/g = 10β»β΄ Gy).
Where is it used?
- Radiation Oncology β Cancer radiotherapy prescribes absorbed dose to the tumour in gray (Gy); typical curative courses deliver 45β80 Gy in fractions of 1.8β2 Gy per session, precisely planned with treatment-planning software.
- Radiation Hardening of Electronics β Space and military electronics are tested for total ionising dose (TID) in gray (Si) β dose absorbed by silicon β to ensure components survive satellite lifetimes or nuclear environments.
- Food Irradiation β International standards (Codex Alimentarius) specify absorbed doses for pathogen reduction: 0.5β1 kGy for sprout inhibition in potatoes, up to 10 kGy for spice decontamination.
- Material Research β Radiation damage in structural materials for nuclear reactors is expressed in absorbed dose (kGy or MGy) alongside displacements per atom (dpa) to characterise microstructural changes.
Common Conversion Mistakes
Confusing gray (absorbed dose) with sievert (equivalent dose)
The gray measures physical energy deposition regardless of radiation type. The sievert weights the gray by the radiation weighting factor (wR) to reflect biological effectiveness: H(Sv) = D(Gy) Γ wR. For X-rays and gamma rays wR = 1, so 1 Gy = 1 Sv numerically, but for alpha particles wR = 20, so 1 Gy β 20 Sv. Using gray when sievert is required (or vice versa) leads to incorrect dose assessment.
Ignoring the medium when quoting absorbed dose
Absorbed dose depends on the material being irradiated because different materials have different energy absorption coefficients. '1 Gy in tissue' is not the same as '1 Gy in silicon' if the radiation field is identical but the media differ. In radiation hardness testing, doses are explicitly stated as Gy(Si) or Gy(tissue) to avoid ambiguity.
Treating rad and rem as interchangeable
The rad is an absorbed dose unit (older CGS equivalent of gray), while the rem is an equivalent dose unit (older equivalent of sievert). They differ by the quality factor Q. For X-rays Q = 1, so 1 rad = 1 rem numerically, but for neutrons or alpha particles this equality breaks down. Mixing rad and rem errors produce factor-of-20 mistakes for alpha radiation.
Quick Reference Table
| From | To |
|---|---|
| 1 gray (Gy) | 100 rad |
| 1 rad | 0.01 Gy (10 mGy) |
| 1 milligray (mGy) | 0.1 rad |
| 1 centigray (cGy) | 1 rad |
| 1 erg/g | 0.0001 Gy (100 ΞΌGy) |
| 1 Gy | 10,000 erg/g |
| Typical chest X-ray dose | ~0.1 mGy (0.01 rad) |
Frequently Asked Questions
What is the difference between a gray and a rad?
Both measure absorbed dose β energy deposited per unit mass of material. The gray (Gy) is the SI unit: 1 Gy = 1 J/kg. The rad is the older CGS unit: 1 rad = 100 erg/g = 0.01 J/kg = 0.01 Gy. The centigray (cGy) is numerically equal to the rad (1 cGy = 1 rad), so radiation oncology in the US often prescribes in cGy to avoid confusion when staff are accustomed to rads.
How much absorbed dose is dangerous to humans?
Whole-body absorbed dose thresholds (acute exposure): below 0.25 Gy β no clinically detectable effects; 1 Gy β mild acute radiation syndrome possible; 4β5 Gy β LD50 (lethal dose for 50% of the population without medical treatment); above 6 Gy β lethal without intensive medical intervention; above 10 Gy β fatal within days. These thresholds apply to whole-body X-ray or gamma irradiation; partial body exposure or different radiation types alter the outcome.
Why is centigray (cGy) used in radiotherapy instead of gray?
Radiotherapy fractions are typically 1.8β2 Gy, and cumulative doses are 45β80 Gy. Historically, prescriptions were written in rad (e.g., 180 rad per fraction, 5,000 rad total). When SI units were adopted, the centigray β equal to 1 rad β allowed practices to convert without changing the numbers on prescriptions, reducing transcription errors. Many oncology centres worldwide still use cGy for this reason.
What absorbed dose does a CT scan deliver?
CT scans deliver significantly more dose than plain X-rays. Typical effective doses: head CT 1β2 mGy; chest CT 5β10 mGy; abdominal/pelvic CT 8β15 mGy. These are effective dose estimates (in mSv, numerically similar to mGy for X-rays) averaged over the body, per ICRP methodology. The actual absorbed dose to specific organs within the scan field is higher β up to 30β50 mGy for abdominal CT to organs in the primary beam.
Sources & Standards
- ICRP Publication 103 β Recommendations of the ICRP (2007)
- ICRU Report 85a β Fundamental Quantities and Units for Ionizing Radiation (2011)
- NCRP Report 160 β Ionizing Radiation Exposure of the Population of the United States (2009)
- IAEA Safety Reports Series No. 17 β Absorbed Dose Determination in External Beam Radiotherapy
Reviewed by The Unit Hub Editorial Team Β· March 2026