Radiation-Exposure Converter
Convert radiation exposure, the ionization produced in air by X-rays or gamma rays, between roentgen and coulomb per kilogram and their submultiples. Pick two units, enter a value and read the result instantly.
Select two units to see the conversion factor.
Exposure is an ionization-in-air quantity for X-rays and gamma rays. It is not absorbed dose and cannot be turned into gray, rad or sievert without the radiation energy and the material. This is a unit converter, not a safety or medical assessment.
Radiation Exposure Formula
Every exposure unit is a fixed multiple of one reference unit, the coulomb per kilogram (C/kg). To convert, multiply the value by the size of the source unit in C/kg, then divide by the size of the target unit in C/kg. The roentgen is defined as exactly 2.58 × 10⁻⁴ C/kg of dry air. The reference treats the historical tissue roentgen, parker and rep as equal to one roentgen, and so does this tool.
What Does Radiation Exposure Measure?
Exposure measures the amount of ionization that X-rays or gamma rays produce in air. It is defined as the electric charge of one sign liberated in a small mass of dry air, divided by that mass: X = dQ/dm. Its SI unit is the coulomb per kilogram (C/kg). The traditional unit is the roentgen (R), defined as exactly 2.58 × 10⁻⁴ C/kg of air, so one coulomb per kilogram is about 3,876 roentgen.
Exposure is a property of the radiation field measured in air, which made it convenient for early ionization chambers. It applies only to X-rays and gamma rays (photons), not to alpha or beta particles or neutrons, and it says nothing about how much energy is absorbed by a person or object. It was largely replaced in practice by absorbed dose and by equivalent and effective dose.
Exposure is not absorbed dose. Absorbed dose (gray, rad) is the energy deposited per kilogram of any material. As a rough illustration, one roentgen of photons corresponds to an absorbed dose in air of about 8.8 mGy (0.88 rad), but the figure changes with the photon energy and the material, so a single factor does not exist for people or tissue. For that reason this converter never changes exposure into dose. It also does not convert to sievert, which additionally accounts for radiation type and tissue sensitivity.
The list includes three historical units, the tissue roentgen, the parker and the rep (roentgen equivalent physical). They were introduced as tissue-related variations of the roentgen, and the reference page treats each as numerically equal to one roentgen. That is a convention for converting old documents rather than a physical identity, so treat results for these three as approximate.
Supported Units
This converter supports the following 7 exposure units. The last column shows how many C/kg one unit equals. The roentgen value is exact by definition. The tissue roentgen, parker and rep are shown with the same value as the roentgen, following the reference, and are labelled as historical.
| Unit | Symbol | Value in C/kg |
|---|---|---|
| Coulomb per kilogram | C/kg | 1 |
| Millicoulomb per kilogram | mC/kg | 0.001 |
| Microcoulomb per kilogram | µC/kg | 1e-6 |
| Roentgen | R | 0.000258 |
| Tissue roentgen (historical, taken as 1 R) | tissue R | 0.000258 |
| Parker (historical, taken as 1 R) | parker | 0.000258 |
| Rep (historical, taken as 1 R) | rep | 0.000258 |
How to Use the Radiation-Exposure Converter
- Type the exposure in the value box. Decimals and scientific notation such as 0.0025 or 2.5e-3 are accepted.
- Choose the From unit and the To unit.
- The result appears instantly, or press Convert Now. Very large or very small results are shown in scientific notation.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, open Show this value in all units for a full list, and Clear to start again.
Step-by-step by hand
- Find the size of each unit in the common base unit (C/kg): 1 R = 0.000258 C/kg and 1 C/kg = 1 C/kg.
- Divide the source size by the target size to get the conversion factor: 0.000258 ÷ 1 = 0.000258, so 1 R = 0.000258 C/kg.
- Multiply by your value: 100 R × 0.000258 = 0.0258 C/kg.
Exposure cannot be negative, so only zero and positive numbers are accepted.
Common Exposure Conversions
- Roentgen to SI: 1 R equals 0.000258 C/kg, which is 0.258 mC/kg.
- Instrument range: 100 R equals 0.0258 C/kg.
- SI to roentgen: 0.01 C/kg equals 38.75968992 R.
- Small exposures: 5 µC/kg equals 0.01937984496 R.
- Milli-units: 2.58 mC/kg equals 10 R.
- Historical unit: 10 rep is shown as 0.00258 C/kg, following the reference convention of treating it as equal to a roentgen.
Roentgen to Coulomb per Kilogram – Common Values
Quick reference for frequently used values
| Roentgen (R) | Coulomb/kg | Millicoulomb/kg | Microcoulomb/kg |
|---|---|---|---|
| 0.001 | 2.58e-7 | 0.000258 | 0.258 |
| 0.01 | 2.58e-6 | 0.00258 | 2.58 |
| 0.1 | 2.58e-5 | 0.0258 | 25.8 |
| 1 | 0.000258 | 0.258 | 258 |
| 5 | 0.00129 | 1.29 | 1,290 |
| 10 | 0.00258 | 2.58 | 2,580 |
| 50 | 0.0129 | 12.9 | 12,900 |
| 100 | 0.0258 | 25.8 | 25,800 |
| 1,000 | 0.258 | 258 | 258,000 |
Where and Why This Conversion Is Used
- Historical documents and old equipment: older X-ray and gamma instruments, manuals and safety records use roentgen.
- Ionization-chamber calibration: calibration laboratories report air-kerma and exposure standards, and exposure is given in C/kg in SI.
- Radiography and nondestructive testing: legacy exposure data are matched to SI-based specifications.
- Teaching: physics and radiation-protection courses cover roentgen and C/kg alongside gray and sievert.
Important Differences Between Related Radiation Measurements
- Exposure (R, C/kg): ionization produced in air by X-rays or gamma rays.
- Absorbed dose (Gy, rad): energy absorbed per kilogram of any material.
- Equivalent and effective dose (Sv, rem): absorbed dose weighted for radiation type and, for effective dose, tissue sensitivity.
- Activity (Bq, Ci): decays per second in a source.
Exposure and absorbed dose cannot be exchanged without knowing the radiation energy and the absorbing material. See the Radiation-Absorbed Dose Converter, the Radiation-Activity Converter and the Radiation Converter.
Educational disclaimer: this page converts units of measurement only. The results are not a radiation-safety assessment, a dose estimate for a person or a medical opinion, and they should not replace advice from a qualified radiation-safety officer, medical physicist or physician.