Convert Coulombs/Kilogram to Roentgens
Coulomb/Kilogram (C/kg) to Roentgen (R) radiation exposure 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 C/kg = 3875.969 R
1 Coulomb/Kilogram = 3875.969 Roentgens
1 Roentgen = 0.000258 Coulombs/Kilogram
1 C/kg in every supported unit
Conversion chart: Coulomb/Kilogram to Roentgens
Conversion table
| Coulomb/Kilogram (C/kg) | Roentgen (R) |
|---|---|
| 0.01 C/kg | 38.75969 R |
| 0.1 C/kg | 387.5969 R |
| 1 C/kg | 3875.969 R |
| 2 C/kg | 7751.938 R |
| 3 C/kg | 11627.907 R |
| 5 C/kg | 19379.845 R |
| 10 C/kg | 38759.69 R |
| 20 C/kg | 77519.38 R |
| 50 C/kg | 193798.45 R |
| 100 C/kg | 387596.9 R |
| 1000 C/kg | 3875969 R |
Coulomb/Kilogram (C/kg)
Definition: The SI derived unit of radiation exposure, defined as one coulomb of ionization charge produced in one kilogram of dry air by incident X-ray or gamma radiation. It is the base unit this entire converter category is defined relative to.
History: Adopted as part of the SI system once electrical charge (the coulomb) and mass (the kilogram) were combined to give exposure a coherent SI-derived unit, replacing the older CGS-based roentgen.
Current use: Used in modern radiation-physics literature and calibration standards, though the older roentgen remains far more common in practical instrument readouts.
Roentgen (R)
Definition: The older, still widely-used CGS unit of exposure, defined as the amount of X-ray or gamma radiation that produces one electrostatic unit of ionization charge per cubic centimeter of dry air at standard conditions.
History: Named after Wilhelm Röntgen, the discoverer of X-rays in 1895, the roentgen was adopted in the 1920s as the first internationally standardized radiation quantity — decades before the SI coulomb/kilogram unit existed — because early ionization-chamber instruments could measure it directly.
Current use: Still the unit most exposure meters, film-badge dosimeters, and older regulatory dose limits are calibrated in, particularly in the US, even though the SI coulomb/kilogram is the technically current standard.
Supported Units
| Unit | Symbol | In C/kg |
|---|---|---|
| Coulomb/Kilogram | C/kg | 1 C/kg |
| Millicoulomb/Kilogram | mC/kg | 0.001 C/kg |
| Microcoulomb/Kilogram | µC/kg | 1E-06 C/kg |
| Roentgen | R | 0.000258 C/kg |
| Tissue Roentgen | R (tissue) | 0.000258 C/kg |
| Parker | D | 0.000258 C/kg |
| Rep | rep | 0.000258 C/kg |
About These Parameters
- Value
- The exposure reading you want to convert, expressed in the "From" unit. Accepts decimals, and can represent anything from a diagnostic X-ray's microcoulomb/kilogram reading to an older instrument's roentgen scale.
- From Unit
- The unit your exposure figure is currently measured in — a legacy film-badge dosimeter's roentgen reading, or a modern ionization-chamber's coulomb/kilogram output.
- 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 roentgen-calibrated instrument's reading into the modern SI coulomb/kilogram.
How Radiation-Exposure Conversion Works
The Formula
Every unit here is defined by a fixed multiplier relative to coulomb per kilogram. To convert a value from one unit to another:
result = value × (factor of "From" unit ÷ factor of "To" unit)
For Coulomb/Kilogram → Roentgen: multiply by 3875.969. For example, 1 C/kg × 3875.969 = 3875.969 R.
Why Exposure Was the First Radiation Quantity Ever Measured
Before physicists could measure how much energy was absorbed inside an arbitrary material, they could measure something simpler: how much electric charge X-rays or gamma rays freed by ionizing the air inside a detector chamber. That's exactly what an ionization chamber does, and it's why the roentgen — defined via ionization in air — became the first internationally standardized radiation unit in the 1920s, well before the gray or sievert existed. Exposure's restriction to "ionization in air" specifically was a practical limitation of the earliest instruments, not a deliberate choice to measure air over anything else.
From Exposure in Air to Dose in Tissue
Because exposure only describes ionization in air, mid-20th-century physicists needed a way to translate that air reading into something meaningful for biological tissue — hence tissue roentgen, parker, and rep, three near-synonymous attempts at that translation, all numerically matching the roentgen in this table. None of them survived as distinct concepts once the gray and sievert were properly defined to describe absorbed dose and biological effect directly, without needing an intermediate "exposure in air" step at all — which is why exposure units are now mostly of historical interest outside of legacy instrument calibration.
Example
An exposure of 1 C/kg equals 3875.969 R. For scale, a single chest X-ray delivers roughly 0.01-0.02 roentgen of exposure, a dental X-ray is typically a few thousandths of a roentgen, and older occupational exposure limits were commonly expressed in roentgens per year rather than the SI coulomb/kilogram.
Frequently Asked Questions
How many Roentgens are in 1 Coulomb/Kilogram?
1 Coulomb/Kilogram (C/kg) equals exactly 3875.969 Roentgens (R).
Is exposure the same thing as absorbed dose?
No. Exposure specifically measures ionization produced in air by X-rays or gamma rays, while absorbed dose (covered by the separate Radiation-Absorbed-Dose Converter) measures energy actually absorbed by any material, including human tissue. The two are related but not identical — one roentgen of exposure corresponds to roughly 0.00877 gray of absorbed dose in soft tissue, not exactly 1-to-1.
Why do tissue roentgen, parker, and rep all have the same value?
All three were mid-20th-century attempts to bridge "exposure measured in air" (what instruments of the time actually detected) and "dose absorbed by tissue" (what mattered biologically), proposed by different researchers around the same era before the gray and sievert cleanly separated those two concepts. They ended up numerically converging because they were all approximating the same underlying air-to-tissue relationship.
Why is exposure only defined for X-rays and gamma rays, not all radiation?
Exposure's definition is tied to the specific physics of an ionization chamber, which measures how much charge X-ray or gamma photons free in air. Alpha and beta particles interact with matter differently and don't fit the same ionization-in-air measurement model, so exposure as a quantity was never extended to cover them — dose rate and absorbed dose, by contrast, apply to any type of ionizing radiation.
Why is the roentgen still used if coulomb/kilogram is the SI unit?
A huge amount of exposure-measuring equipment — especially older film-badge dosimeters and legacy ionization-chamber instruments still in service — was built and calibrated in roentgens decades before the SI coulomb/kilogram unit existed, and replacing that installed base wasn't practical. The roentgen also produces more convenient everyday numbers for typical medical exposures than the very small coulomb/kilogram figures do.