Absorbed Dose Converter (gray, rad)

Understanding the absorbed dose converter — concept and principle

Absorbed dose measures how much energy ionising radiation deposits in a given mass of material, such as tissue, water or a detector. The SI unit is the gray (Gy); older literature, some US documents and many radiotherapy records still use the rad, and clinicians often write doses in centigrays (cGy). This converter switches between grays, milligrays, centigrays, rads and millirads, which is useful when reading treatment plans, imaging reports, dosimetry papers and older safety data side by side.

How it is calculated

The converter first changes your value into grays and then into the target unit: result = value × (size of "from" unit in Gy) ÷ (size of "to" unit in Gy).

• 1 Gy = 1 J/kg
• 1 mGy = 0.001 Gy
• 1 cGy = 0.01 Gy = 1 rad
• 1 rad = 0.01 Gy (originally defined as 100 erg per gram)
• 1 millirad = 0.00001 Gy = 0.01 mGy

The principle

Absorbed dose is defined as energy imparted divided by mass, D = dE ÷ dm. One gray means one joule of radiation energy absorbed per kilogram. The rad was the CGS-era unit, 100 erg per gram; since 1 erg = 10⁻⁷ J and 1 g = 10⁻³ kg, 100 erg/g = 0.01 J/kg, so exactly 100 rad make 1 Gy. That is also why a centigray and a rad are the same size, and why radiotherapy departments that moved to SI units chose cGy: the numbers on charts did not change.

Worked example

1. Input: 1 Gy, target unit mGy.
2. 1 Gy in grays = 1 × 1 = 1 Gy.
3. Divide by the size of a milligray: 1 ÷ 0.001 = 1,000 mGy.
4. The same 1 Gy is also 100 cGy, 100 rad and 100,000 millirad, as the full list shows.

How to read the result

A gray is a large dose. Diagnostic imaging delivers doses typically in the milligray range, while conventional radiotherapy often gives around 2 Gy per daily fraction to the tumour, with totals of tens of grays over a course. When you compare numbers from different sources, first check which unit is used: a value of "200" can mean 200 cGy (2 Gy) in one document and 200 mGy (0.2 Gy) in another.

Common mistakes

• Confusing gray with sievert. The sievert expresses equivalent or effective dose, which weights the absorbed dose by radiation type and tissue; for X-rays and gamma rays the weighting factor is 1, for alpha particles it is 20.
• Confusing rad (radiation absorbed dose) with rem (the old unit of equivalent dose) or with the roentgen (a unit of exposure in air).
• Mixing up the prefixes m (milli, 1/1,000) and c (centi, 1/100).

Frequently asked questions

How many rads are in a gray?

Exactly 100. One rad is 0.01 Gy, the same as one centigray.

Can I convert gray directly to sievert?

Only with a radiation weighting factor (and tissue weighting for effective dose). For photons and electrons 1 Gy corresponds to 1 Sv of equivalent dose.

What does the gray measure physically?

Energy absorbed per unit mass, independent of the type of radiation or its biological effect.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition · ICRP Publication 103, The 2007 Recommendations of the International Commission on Radiological Protection