How this conversion works
Absorbed dose is the energy deposited by ionizing radiation per kilogram of material. The SI unit is the gray (1 J/kg); the older unit is the rad.
One rad is exactly 0.01 Gy, so 1 Gy = 100 rad and 1 cGy = 1 rad.
Formula: result = value × (base units per source unit) ÷ (base units per target unit)
Where this conversion is used
- Reading radiotherapy and dosimetry records that use Gy, cGy or rad.
- Converting older literature in rads to SI grays.
- Material radiation-hardness ratings in krad.
Worked example: 2 Gy to rads
- 1 Gy = 100 rad.
- 2 × 100 = 200 rad.
Unit definitions
| Unit | In grays | Note |
|---|---|---|
| Gray (Gy) | 1 | 1 joule of energy absorbed per kilogram. |
| Milligray (mGy) | 0.001 | |
| Microgray (µGy) | 0.000001 | |
| Centigray (cGy) | 0.01 | |
| Rad (rad) | 0.01 | 100 erg/g = 0.01 Gy exactly. |
| Millirad (mrad) | 0.00001 | |
| Kilorad (krad) | 10 |
Grays to rads
| Gray (Gy) | Rad (rad) |
|---|---|
| 0.001 | 0.1 |
| 0.01 | 1 |
| 0.1 | 10 |
| 1 | 100 |
| 2 | 200 |
| 10 | 1,000 |
Rads to grays
| Rad (rad) | Gray (Gy) |
|---|---|
| 1 | 0.01 |
| 10 | 0.1 |
| 100 | 1 |
| 500 | 5 |
| 1,000 | 10 |
Things to watch for
- Grays and sieverts are not interchangeable. The sievert (and rem) measure equivalent or effective dose, which applies weighting factors for radiation type and tissue. This converter does not convert grays to sieverts.
Precision and sources
Results are calculated from the definitions above and rounded to 12 significant digits; the converter shows up to eight decimal places and the tables four. Reference: NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B.
Frequently asked questions
Why are centigrays used in radiotherapy?
1 cGy equals 1 rad, so centigrays let clinicians keep familiar numbers after the switch to SI units.