What is the difference between a sievert and a gray?

A gray (Gy) is one joule of energy absorbed per kilogram of tissue — a purely physical quantity. A sievert (Sv) is a gray multiplied by the radiation weighting factor (wR), which accounts for biological effectiveness. For X-rays and gamma rays (wR = 1), 1 Sv equals 1 Gy numerically, but they remain conceptually distinct.

Why do different radiation types have different weighting factors?

Different radiation types cause different amounts of biological damage per unit of absorbed energy. Alpha particles, for example, are densely ionizing and cause roughly 20 times more damage than the same absorbed dose of X-rays, so their wR is 20. Neutrons are intermediate, with wR values of 5–20 depending on energy.

What weighting factor does this tool use for neutrons?

The tool uses wR = 20, which is the ICRP-103 value for high-energy neutrons and the most conservative common choice. For thermal or intermediate-energy neutrons, ICRP-103 specifies lower values (as low as 5). If your neutron energy is known, consult the full ICRP-103 wR function for greater accuracy.

Is this tool appropriate for effective dose calculations?

No. Effective dose (also in Sv) involves an additional tissue weighting factor (wT) that accounts for the varying radiosensitivity of organs. This tool converts equivalent dose (Sv) to absorbed dose (Gy) using only the radiation weighting factor (wR), not the tissue weighting factor.

What is 1 mSv in mGy for an X-ray?

For X-rays (wR = 1), 1 mSv equals 1 mGy. The weighting factor is 1, so the numbers are identical. For alpha particles (wR = 20), 1 mSv would be 0.05 mGy.

Sieverts to Grays Converter

Name: Sieverts to Grays Converter
Availability: InStock
Author: Nham Vu

Select radiation type, enter dose in sieverts, and get the equivalent absorbed dose in grays using the correct radiation weighting factor.

Inputs

Absorbed dose (Gy) = Equivalent dose (Sv) ÷ Radiation weighting factor (w_R)

Radiation type

Weighting factors per ICRP Publication 103 (2007).

Equivalent dose

Quick examples

Results

Select radiation type and enter a dose to convert

Sv vs. Gy — what the units measure

Gray (Gy) — absorbed dose

One gray is one joule of energy absorbed per kilogram of tissue (1 J/kg). It is a physical quantity describing energy deposition, with no adjustment for biological effect. The older unit rad equals 0.01 Gy.

Sievert (Sv) — equivalent dose

One sievert equals the absorbed dose in grays multiplied by the radiation weighting factor (w_R). It normalizes different radiation types to the same biological risk scale. For X-rays and gamma rays, w_R = 1, so 1 Gy produces 1 Sv. For alpha particles, w_R = 20, so 1 Gy produces 20 Sv of equivalent dose.

Radiation type	w_R (ICRP-103)	1 Sv equals
Photons (X-rays, gamma)	1	1 Gy
Electrons & muons	1	1 Gy
Protons & charged pions	2	0.5 Gy
Neutrons (high energy)	20	0.05 Gy
Alpha particles	20	0.05 Gy
Heavy ions & fission fragments	20	0.05 Gy

Summary

Select radiation type, enter dose in sieverts, and get the equivalent absorbed dose in grays using the correct radiation weighting factor.

How it works

Select the type of radiation (photon, electron, proton, alpha, or neutron).
Enter the equivalent dose value in sieverts (Sv), millisieverts (mSv), or microsieverts (µSv).
The tool divides the dose by the radiation weighting factor (wR) for that radiation type.
Results are displayed in grays (Gy), milligrays (mGy), and micrograys (µGy).
For neutrons, the tool uses the ICRP-103 recommended wR of 20 (conservative value for high-energy neutrons).

Use cases

Converting medical imaging dose reports from Sv to Gy for physics calculations.
Radiation protection work requiring absorbed dose from measured equivalent dose.
Educational exercises on the relationship between equivalent and absorbed dose.
Cross-checking dosimetry calculations in nuclear safety reviews.
Comparing doses from different radiation types on a common absorbed-dose scale.
Teaching the role of radiation weighting factors in radiobiology.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu