How many grays is 1 rem?

For x-rays and gamma rays (weighting factor 1), 1 rem = 0.01 Gy. For alpha particles (weighting factor 20), 1 rem = 0.0005 Gy. The exact value depends on the radiation type and its weighting factor.

What is the difference between rem and gray?

The gray measures physical absorbed dose (joules per kilogram of tissue). The rem weights that dose by the biological effectiveness of the radiation type — it is the older equivalent of the sievert. Gray is a physical quantity; rem is a 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Is rem still used or has it been replaced by sievert?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 and international contexts. The rem remains 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. Use the value specified in your applicable regulatory standard when in doubt.

Why does the formula divide 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, reversing the conversion gives Gy = rem / (W_R × 100).

Rem to Grays Converter

Convert radiation equivalent dose in rem to absorbed dose in grays (Gy); select the radiation type for the correct weighting factor.

Input

Dose in rem Radiation type / weighting factor (W_R)

Result

Enter a value and click Convert

Common Radiation Weighting Factors (ICRP)

Radiation type	W_R	100 rem equals
X-rays, gamma rays	1	1 Gy
Beta particles, electrons	1	1 Gy
Protons	2	0.5 Gy
Neutrons, thermal	5	0.2 Gy
Neutrons, fast	10–20	0.05–0.1 Gy
Alpha particles	20	0.05 Gy

Summary

The rem (roentgen equivalent man) is the older unit of equivalent dose that weights absorbed dose by the biological effectiveness of the radiation type. The gray (Gy) is the SI unit of absorbed dose, measuring energy deposited per kilogram of tissue. For x-rays and gamma rays (weighting factor 1), the conversion is straightforward: 100 rem = 1 Gy. For alpha particles (weighting factor 20), 100 rem equals only 0.05 Gy of absorbed dose.

How it works

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

Use cases

Convert US regulatory dose limits (stated in rem) to SI grays for scientific reporting.
Translate occupational exposure records between unit systems.
Cross-check patient radiation dose reports when equipment uses different units.
Academic problems converting equivalent dose back to absorbed dose.
Compare dose values across dosimetry systems that report in rem versus Gy.

Frequently Asked Questions

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