How many rem is 1 gray?

For x-rays and gamma rays (weighting factor 1), 1 Gy = 100 rem. For alpha particles (weighting factor 20), 1 Gy = 2,000 rem. The exact value depends on the radiation type.

What is the difference between gray and rem?

The gray measures physical absorbed dose (joules per kilogram of tissue). The rem (or sievert in SI) weights that dose by the biological effectiveness of the radiation type. Gray is the physical quantity; rem is the biological equivalent quantity.

Is rem still used or has it been replaced by sievert?

The sievert (Sv) is the SI unit and is preferred in scientific contexts. The rem is still widely used in the United States, especially in regulatory and medical settings. 1 Sv = 100 rem.

What radiation weighting factor should I use for neutrons?

Neutron weighting factors range from 5 to 20 depending on energy. For fast neutrons the ICRP recommends 20; for thermal neutrons values around 5 are typical. When in doubt, use the value specified in your applicable regulatory standard.

Why does the formula multiply by 100?

Because the SI system and the older CGS-based system differ by a factor of 100: 1 Gy = 1 J/kg, while 1 rad = 0.01 J/kg. Since 1 rem = 1 rad × W_R, the conversion is rem = Gy × W_R × 100.

Grays to Rem Converter

Convert absorbed radiation dose in grays (Gy) to rem; enter a value and get instant results for any radiation weighting factor.

Input

Dose in grays (Gy) Radiation type / weighting factor (W_R)

Result

Enter a value and click Convert

Common Radiation Weighting Factors (ICRP)

Radiation type	W_R	1 Gy equals
X-rays, gamma rays	1	100 rem
Beta particles, electrons	1	100 rem
Protons	2	200 rem
Neutrons, thermal	5	500 rem
Neutrons, fast	10–20	1,000–2,000 rem
Alpha particles	20	2,000 rem

Summary

The gray (Gy) measures absorbed dose — the energy deposited in tissue per kilogram. The rem (roentgen equivalent man) is the older unit that accounts for biological effectiveness by multiplying absorbed dose by a radiation weighting factor. For x-rays and gamma rays the factor is 1, making 1 Gy equal to 100 rem. For alpha particles the factor is 20, so 1 Gy = 2,000 rem.

How it works

Enter the dose value in grays (Gy) in the input field.
Select the radiation type or enter a custom weighting factor (W_R).
The tool calculates: rem = Gy × W_R × 100.
The result shows the equivalent dose in rem and millirem.
Use the swap button to convert rem back to gray.

Use cases

Convert a CT scan dose report from grays to rem for patient records.
Translate radiation worker dose limits between unit systems.
Cross-check occupational exposure thresholds against regulatory limits stated in rem.
Academic problems converting absorbed dose to equivalent dose.
Compare dose values across equipment that reports in different unit systems.

Frequently Asked Questions

Last updated: 2026-07-14 · Reviewed by Nham Vu