Convert Grays to Femtograys
Gray (Gy) to Femtogray (fGy) absorbed radiation dose conversion — enter any value below to get an instant result, or use the table for common values.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Result
1 Gy = 1E+13 fGy
1 Gray = 1E+13 Femtograys
1 Femtogray = 1E-13 Grays
1 Gy in every supported unit
Conversion chart: Gray to Femtograys
Conversion table
| Gray (Gy) | Femtogray (fGy) |
|---|---|
| 0.01 Gy | 1E+11 fGy |
| 0.1 Gy | 1E+12 fGy |
| 1 Gy | 1E+13 fGy |
| 2 Gy | 2E+13 fGy |
| 3 Gy | 3E+13 fGy |
| 5 Gy | 5E+13 fGy |
| 10 Gy | 1E+14 fGy |
| 20 Gy | 2E+14 fGy |
| 50 Gy | 5E+14 fGy |
| 100 Gy | 1E+15 fGy |
| 1000 Gy | 1E+16 fGy |
Gray (Gy)
Definition: The SI derived unit of absorbed dose, equal to one joule of energy absorbed per kilogram of matter, full stop — no time component and no biological weighting factor. It is the base unit this entire converter category is defined relative to.
History: Named after British physicist and radiobiology pioneer Louis Harold Gray, whose research establishing the quantitative relationship between absorbed radiation energy and biological effect led to the unit's adoption by the SI in 1975, replacing the older rad.
Current use: The standard modern unit for reporting absorbed radiation dose in medical physics, radiotherapy planning, and radiation-safety regulation worldwide.
Femtogray (fGy)
Definition: An extremely small submultiple of the gray, used only in the most sensitive radiation-detection research contexts.
History: Included for completeness at the small end of the decimal-prefix system rather than reflecting any distinct historical development.
Current use: Appears only in specialized low-background physics and dark-matter-detector research literature discussing residual absorbed dose at the outer limit of detectability.
Supported Units
| Unit | Symbol | In Gy |
|---|---|---|
| Rad | rd | 0.01 Gy |
| Millirad | mrd | 1E-05 Gy |
| Joule/Kilogram | J/kg | 1 Gy |
| Joule/Gram | J/g | 1000 Gy |
| Joule/Centigram | J/cg | 100000 Gy |
| Joule/Milligram | J/mg | 1000000 Gy |
| Gray | Gy | 1 Gy |
| Exagray | EGy | 1E+18 Gy |
| Petagray | PGy | 1E+15 Gy |
| Teragray | TGy | 1E+12 Gy |
| Gigagray | GGy | 1E+09 Gy |
| Megagray | MGy | 1000000 Gy |
| Kilogray | kGy | 1000 Gy |
| Hectogray | hGy | 100 Gy |
| Dekagray | daGy | 10 Gy |
| Decigray | dGy | 0.1 Gy |
| Centigray | cGy | 0.01 Gy |
| Milligray | mGy | 0.001 Gy |
| Microgray | µGy | 1E-06 Gy |
| Nanogray | nGy | 1E-07 Gy |
| Picogray | pGy | 1E-10 Gy |
| Femtogray | fGy | 1E-13 Gy |
| Attogray | aGy | 1E-16 Gy |
About These Parameters
- Value
- The absorbed dose amount you want to convert, expressed in the "From" unit. Accepts decimals, and can represent anything from a diagnostic imaging patient dose in milligray to an industrial sterilization total dose in kilogray.
- From Unit
- The unit your absorbed-dose figure is currently measured in — a radiotherapy plan's centigray prescription, an older medical record's rad figure, or a physics calculation's raw joule/kilogram.
- To Unit
- The unit you want the result converted into. Use the swap button to flip From and To instantly, which is handy when translating an older US rad-based medical record into the modern SI gray or vice versa.
How Absorbed-Dose Conversion Works
The Formula
Every unit here is defined by a fixed multiplier relative to the gray. To convert a value from one unit to another:
result = value × (factor of "From" unit ÷ factor of "To" unit)
For Gray → Femtogray: multiply by 1E+13. For example, 1 Gy × 1E+13 = 1E+13 fGy.
From the Rad to the Gray
The rad was the standard US unit of absorbed dose for most of the 20th century, defined as 100 erg of absorbed energy per gram of matter. When the SI adopted the gray in 1975 — defined instead as one joule per kilogram, a cleaner fit with the rest of the SI system — the conversion worked out to exactly 100 rad per gray, a nicer relationship than many CGS-to-SI conversions manage. Older US medical records, some legacy radiotherapy equipment, and a handful of regulatory documents still use rad, which is why this converter keeps it alongside the modern gray.
Joule-per-Mass Units Are All the Same Formula
Joule/kilogram, joule/gram, joule/centigram, and joule/milligram aren't four different physical concepts — they're the gray's own defining ratio (energy absorbed ÷ mass) simply re-expressed for whichever mass unit a calculation already has in hand. A physics or dosimetry calculation that starts with energy in joules and a sample mass in grams doesn't need to convert the mass to kilograms first; it can read the result straight off the joule/gram scale, which is exactly 1,000 times the joule/kilogram (gray) value for the same absorbed energy and mass.
Example
An absorbed dose of 1 Gy equals 1E+13 fGy. For scale, a single chest CT scan delivers roughly 7 milligray to the patient, a full course of external-beam radiotherapy for a solid tumor commonly totals 60-80 gray delivered over several weeks in small daily fractions, and industrial sterilization of medical devices typically requires a cumulative dose of 25-40 kilogray.
Frequently Asked Questions
How many Femtograys are in 1 Gray?
1 Gray (Gy) equals exactly 1E+13 Femtograys (fGy).
What's the difference between gray and gray/second?
Gray (this converter) is a cumulative total — the full amount of energy absorbed per kilogram, with no time component. Gray/second (covered by the separate Radiation Converter) is a rate — how fast that absorbed energy is arriving right now. Multiply a gray/second dose rate by the exposure duration in seconds to get the total absorbed dose in gray.
How many rad are in a gray?
Exactly 100 rad equal 1 gray, since the rad is defined as 0.01 gray. This clean 100-to-1 relationship is one of the tidier CGS-to-SI conversions in radiation physics.
Is absorbed dose in gray the same as biological harm?
Not directly — gray measures physical energy absorbed per kilogram, with no adjustment for how damaging a given radiation type is to living tissue. The sievert (covered alongside sievert/second in the Radiation Converter) applies a biological weighting factor on top of the gray to estimate actual biological risk, which is why radiation-protection dose limits are typically expressed in sievert rather than gray.
Why does radiotherapy use centigray instead of gray or rad?
Centigray happens to equal the older rad numerically (both are 0.01 gray), so switching radiotherapy prescriptions from rad to centigray let clinics adopt the SI-compatible unit without changing any of the actual numbers on existing treatment protocols — a typical daily fraction of "200 rad" simply became "200 centigray," with no recalculation needed.