What is the difference between gray and sievert?

The gray (Gy) is a physical unit: 1 Gy = 1 J of energy absorbed per kilogram of tissue, regardless of radiation type. The sievert (Sv) is a biological unit: it is the gray multiplied by a radiation weighting factor that reflects how damaging that radiation type is to living tissue. For X-rays and gamma rays the factor is 1, so Gy and Sv are numerically equal; for alpha particles the factor is 20, meaning 1 Gy of alpha delivers the same biological effect as 20 Gy of gamma.

Where do the weighting factors come from?

Radiation weighting factors (W_R) are defined by the International Commission on Radiological Protection (ICRP) in Publication 103 (2007). They represent the relative biological effectiveness of each radiation type compared with reference low-LET radiation, averaged over tissues and dose rates relevant to radiation protection.

Why does neutron W_R vary?

Neutron biological effectiveness depends strongly on neutron energy. ICRP 103 gives W_R values ranging from 2.5 (very low energy, < 1 MeV) up to a peak of around 20 near 1–2 MeV, then decreasing to about 5 at high energies. This tool uses W_R = 10 as a broad mid-energy representative value. For precise work, use the ICRP 103 formula with the actual neutron energy spectrum.

Is sievert the same as effective dose?

Not exactly. Equivalent dose (Sv) weights absorbed dose by radiation type and applies to a specific tissue or organ. Effective dose (also in Sv) takes a further step: it weights each organ's equivalent dose by a tissue weighting factor (W_T) to produce a whole-body risk estimate. This tool calculates equivalent dose only; computing effective dose requires knowing which organ received the dose.

How does rem relate to sievert?

1 rem = 0.01 Sv (or equivalently, 1 Sv = 100 rem). The rem is an older non-SI unit still common in US regulatory and occupational contexts. The tool displays rem in the results for reference.

Can I use this for medical or regulatory dose assessments?

This tool is for educational and quick reference use. Regulatory, occupational, and clinical dose assessments require authoritative dosimetry software, precise radiation energy spectra, and review by a qualified health physicist or medical physicist.

Grays to Sieverts Converter

Enter absorbed dose in grays, pick the radiation type, and get equivalent dose in sieverts.

Inputs

Equivalent dose (Sv) = Absorbed dose (Gy) × W_R

Absorbed dose

Radiation type

Weighting factors per ICRP Publication 103 (2007).

Quick examples

Results

Enter values and press Convert

ICRP 103 radiation weighting factors

Radiation type	W_R
Photons (X-rays, gamma rays)	1
Electrons and muons (beta particles)	1
Protons and charged pions	2
Neutrons (energy-dependent; peak ~20 at 1–2 MeV)	2.5 – 20
Alpha particles, fission fragments, heavy ions	20

Source: ICRP Publication 103 (2007), Table 2. Neutron W_R is a continuous function of energy; the representative value of 10 used in this tool is midrange. For precise neutron dosimetry use the ICRP 103 energy formula.

Summary

The gray (Gy) measures absorbed dose — the energy deposited in tissue per unit mass (1 J/kg). The sievert (Sv) measures equivalent dose, which weights absorbed dose by the biological effectiveness of the radiation type. The conversion is: Equivalent dose (Sv) = Absorbed dose (Gy) × Radiation weighting factor (W_R). For X-rays, gamma rays, and beta particles W_R = 1, so Gy and Sv are numerically equal. For protons W_R = 2, for alpha particles W_R = 20, and for neutrons W_R varies from 2.5 to 20 depending on energy. This tool uses ICRP Publication 103 (2007) weighting factors.

How it works

Enter the absorbed dose value and select the unit (Gy, mGy, or µGy).
Choose the radiation type from the dropdown — each type has an ICRP-103 weighting factor.
The tool multiplies absorbed dose in Gy by the weighting factor W_R to compute equivalent dose.
Results are shown in sieverts (Sv), millisieverts (mSv), microsieverts (µSv), and rem.
For neutrons, a representative mid-range W_R of 10 is used; consult ICRP-103 Figure 1 for energy-specific values.

Use cases

Converting radiotherapy absorbed dose in Gy to equivalent dose for comparative assessments.
Health physics calculations when switching between absorbed and equivalent dose quantities.
Radiation protection coursework and exam preparation.
Cross-checking hand calculations for mixed radiation fields.
Understanding how alpha and neutron radiation produce much higher biological impact per gray than X-rays.
Converting historical dose records from rem to sieverts (1 rem = 0.01 Sv).

Frequently Asked Questions

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