Verification: factors follow standard unit definitions; round for display only.

Gray to rad

Convert Gy to rad when SI absorbed-dose readings from dosimeters, treatment-planning systems, or international reports must be expressed in the legacy rad unit still common in US regulations, legacy equipment, and industrial sterilization documentation.

The gray (Gy) is the SI unit of absorbed dose—the mean energy imparted by ionizing radiation per unit mass of material. By definition 1 Gy = 1 joule per kilogram (J/kg). The rad (radiation absorbed dose) is the traditional CGS-derived unit still cited in many US health-physics and industrial contexts. The conversion between gray and rad is a fixed decimal factor, not a biological weighting:

rad = Gy × 100  ·  equivalently   1 Gy = 100 rad exactly

This relationship is exact by definition (NRC 10 CFR §20.1004, ICRU/SI). It applies only to absorbed dose. Do not use this factor to convert sieverts to grays or rems to rads without applying the appropriate radiation weighting or quality factor for dose equivalent.

Step-by-step conversion (worked example)

Convert 1.8 Gy to rad—a typical fraction size in external-beam radiotherapy when expressed in SI:

1. Confirm the quantity is absorbed dose (Gy), not dose equivalent (Sv).
2. Multiply by 100: 1.8 × 100 = 180 rad
3. Optional check in therapy notation: 1.8 Gy = 180 cGy, and 1 cGy = 1 rad numerically.

Second worked example (diagnostic radiology)

Convert 0.012 Gy (12 mGy)—order of magnitude for a single CT series entry on a dose report:

1. 0.012 Gy × 100 = 1.2 rad
2. Sanity check: 12 mGy × 100 = 1,200 mrad = 1.2 rad

Quick mental estimate

Move the decimal two places right when going from Gy to rad (multiply by 100). Example: 0.5 Gy → 50 rad. For milligray, the same factor holds: 3 mGy → 300 mrad. Use the calculator above for audit trails; use mental math for field checks only after confirming you are in absorbed dose, not equivalent dose.

Gray to rad conversion chart

Where gray → rad comes up

• Radiology & nuclear medicine: Modern DICOM dose reports and patient dose indices are often in mGy; legacy US charts, ALARA worksheets, or vendor manuals may still list rad or mrad.
• Radiation therapy: Prescriptions may be written in Gy or cGy while older QA records, film dosimetry archives, or regulatory filings use rad. Remember 1 cGy = 1 rad for absorbed dose in tissue-equivalent media.
• Industrial gauging & sterilization: Process specs frequently use krad or Mrad (1 Mrad = 10 kGy = 10,000 Gy).
• Compliance & reporting: Translating instrument calibrations, shielding calculations, or incident logs between SI (Gy) and US customary (rad) documentation without mixing in rem or Sv limits.

Rad to gray

Convert rad to Gy when legacy survey meters, sterilization certificates, or NRC-style tables quote absorbed dose in rad but your dosimetry software, treatment plan, or international standard expects gray.

Invert the gray-to-rad relationship by dividing by 100. Each rad represents exactly 0.01 Gy of absorbed dose.

Gy = rad ÷ 100  ·  equivalently   1 rad = 0.01 Gy exactly

Step-by-step conversion (worked example)

Convert 250 rad to gray:

1. Confirm the reading is absorbed dose (rad), not exposure (R) or dose equivalent (rem).
2. Divide by 100: 250 ÷ 100 = 2.5 Gy
3. Cross-check: 2.5 Gy × 100 = 250 rad (round-trip)

Second worked example (industrial)

Convert 5,000 rad (5 krad) on a cobalt sterilization certificate:

1. 5,000 rad ÷ 100 = 50 Gy
2. Equivalent SI notation: 50 kGy would be 5,000,000 rad—watch k/M prefixes carefully

Quick reference (rad → Gy)

Reverse conversion is essential when importing US equipment datasheets into SI-based treatment-planning systems or when normalizing historical incident reports for modern risk assessment tools that expect gray.

Absorbed dose vs dose equivalent, common mistakes, and related radiation tools

Gray and rad describe energy deposition only. Sievert and rem add radiation-weighting for biological effect—never interchange these families with the Gy↔rad factor alone.

Absorbed dose vs dose equivalent

For photons and electrons (X-rays, gamma, most beta), the radiation weighting factor is 1, so numerically 1 Gy ≈ 1 Sv and 1 rad ≈ 1 rem in the same tissue—but that is a special case of dose equivalent, not permission to treat Gy and Sv as interchangeable units in every report. Neutrons, alpha particles, and many mixed fields require explicit weighting (NRC quality factors up to 20 for alpha).

Prefix relationships (exact)

Common mistakes to avoid

• Converting Gy to rem with ×100—rem is dose equivalent. Use Gy → rad (×100) for absorbed dose; for equivalent dose under photon/electron radiation, Gy → Sv (×1 in SI) or rad → rem (×1), not Gy → rem directly unless WR = 1 is documented.
• Assuming 1 Gy = 100 rem—100 applies between Gy and rad, and separately between Sv and rem, not across quantity families.
• Confusing gray with sievert on patient records—CT dose index is absorbed dose (Gy/mGy); occupational limits are often equivalent dose (mSv). Convert each with its own unit pair.
• Mixing roentgen (R) with rad—exposure in air is not absorbed dose in tissue; use the correct quantity for the measurement type.
• Rounding before unit conversion—carry full precision through multiply/divide by 100; round once for display, especially in therapy cumulative dose sums.

Exactness and round-trip verification

The factor 100 between gray and rad is exact by definition, not an empirical approximation. Converting 2.45 Gy → 245 rad → 2.45 Gy should recover the original within floating-point limits. Anchor checks: 1 Gy = 100 rad, 1 rad = 0.01 Gy, 1 Gy = 1 J/kg.

Related radiation converters

For the inverse of this page, use rad to gray. For dose equivalent (not absorbed dose), see sievert to rem, rem to sievert, and millisievert to sievert. When radiation type and weighting are explicitly defined, see gray to sievert, sievert to gray, and sievert to millisievert.