How this instrument works
Absorbed dose answers one question: how much energy did ionising radiation deposit in each kilogram of the material it passed through? The SI unit is the gray, named for British physicist Louis Harold Gray and adopted by the International Commission on Radiation Units and Measurements in 1975, defined as exactly one joule of deposited energy per kilogram. Its predecessor, the rad — short for radiation absorbed dose — was introduced in 1953 as one hundred ergs per gram, which happens to equal exactly one hundredth of a joule per kilogram, so 1 Gy equals exactly 100 rad with no rounding anywhere.
Because both units describe the same physical quantity through a fixed, definitional factor of one hundred, converting between them is arithmetic, not physics — nothing about the type of radiation, the tissue being irradiated, or the exposure duration changes that ratio. That is what makes gray-to-rad conversion different from most radiation unit questions: it is purely a unit-system translation, the same way converting metres to feet never needs to know what you're measuring.
The rad persists mainly in older American clinical and regulatory literature, radiation-therapy archives predating SI adoption, and some US Nuclear Regulatory Commission documentation that still runs in parallel with gray. Modern dosimetry, international radiotherapy planning and essentially all research published after the 1980s reports absorbed dose in gray, milligray or centigray — a centigray, incidentally, equals exactly one rad, which is why radiotherapy prescriptions sometimes still read in cGy as a quiet nod to the older unit.
- Type your absorbed dose into the Grays (Gy) field, which opens at 1 — a dose roughly in range of a single fractionated radiotherapy treatment.
- Read the equivalent dose in rad on the Rads (rad) line, recalculated with every keystroke.
- Working backwards from rad? Divide your figure by 100 to return to gray.
- This calculator only rescales absorbed dose — it does not estimate biological risk, which needs the different quantity described in the FAQ below.
Worked example — a 1 Gy radiotherapy fraction
A radiotherapy plan delivers a 1 Gy fraction to a treatment site, a dose within the range of a single external-beam session. Enter 1 into Grays (Gy) and Rads (rad) reads 100.0 — the identical dose, just expressed in the unit an older US chart or a 1970s dosimetry record would have used.
The factor scales cleanly in either direction: a 25 Gy total course, spread across several fractions, converts to 2500.0 rad, while a smaller 0.5 Gy diagnostic imaging dose converts to 50.0 rad. Every one of these pairs is the same energy-per-kilogram quantity, multiplied by exactly 100 to change its unit label, never its physical meaning.
Questions
Is 1 Gy always exactly 100 rad, regardless of radiation type?
Yes. Both gray and rad measure absorbed dose — energy deposited per kilogram — and the relationship between them, 1 Gy = 100 rad, is a fixed definitional conversion between two unit systems, unrelated to whether the radiation is alpha, beta, gamma, X-ray or neutron. Every one of those radiation types deposits energy that can be measured in gray or rad using the same factor of 100; what differs by radiation type is a completely separate quantity, described below.
Why doesn't this calculator also convert gray to sievert?
Because gray and sievert measure two different physical quantities, not the same one at a different scale. Gray is absorbed dose, pure deposited energy per kilogram; sievert is equivalent dose, which weights that energy by a radiation-dependent quality factor — 1 for X-rays, gamma rays and beta particles, but as high as 20 for alpha particles. Converting gray to sievert needs that quality factor, and it isn't a single fixed number the way 100 is for gray to rad, so a general Gy-to-Sv converter would need extra radiation-type input or risk quietly assuming a factor of 1. This page sticks to the one conversion that is genuinely fixed.
What does ‘absorbed dose’ actually measure?
It measures energy transferred from radiation into matter, expressed per unit mass — one gray means one joule of energy deposited in each kilogram of the material, whether that material is human tissue, a dosimeter crystal, or a slab of concrete. It says nothing on its own about biological harm, only about the raw physical quantity of energy absorbed, which is why dosimetry, radiotherapy planning and radiation-shielding calculations all start from gray or rad before any risk assessment is layered on top.
Where does the rad unit still appear today?
Mostly in older American clinical records, historical radiotherapy archives from before SI adoption in medicine, and some legacy US Nuclear Regulatory Commission and Department of Energy documentation that continues to run rad alongside gray for continuity with decades-old data. International and most modern research literature has used gray, milligray or centigray since the International Commission on Radiation Units and Measurements formally adopted SI dose units.
How does a centigray relate to a rad?
They're identical — one centigray equals exactly one rad, since centi- means one hundredth and 1 Gy equals 100 rad, making 1 cGy equal 1 rad by the same arithmetic. Radiotherapy prescriptions sometimes still print doses in cGy rather than Gy specifically because the numbers then match older rad-based charts digit for digit, easing the transition for clinicians trained before the SI changeover.
Who defined the gray, and when?
The gray is named for British physicist Louis Harold Gray, a pioneer of radiation dosimetry, and it was adopted as the SI unit of absorbed dose by the International Commission on Radiation Units and Measurements in 1975, formally replacing the rad in international scientific use. The rad itself dates to 1953, defined as 100 erg of energy per gram of absorbing material — a cgs-system unit that translates exactly, if inconveniently, into the SI figure used today.