Conversion calculator

Gray to Sievert Converter

Three quantities wearing similar-looking units. Two of them have identical dimensions and are still different things, and the third does not convert to either.

Value

Try:

1 mSv — Equivalent dose — weighted for harm

0.001000 Sv

Comparable to Head CT at about 2.0000 mSv.

Equivalent dose — weighted for harm

Units of the same quantity, which do convert
Unit	Value	What it is
sievert	0.001000	Absorbed dose weighted by how harmful that radiation is. Same dimensions as the gray, different quantity.
millisievert	1.0000	The everyday unit — background radiation, scans and exposure limits are all in these.
microsievert	1,000.00	For small exposures: a dental X-ray, a flight, a day of background.
rem	0.100000	The older equivalent-dose unit. Exactly a hundredth of a sievert, and it pairs with the sievert.
millirem	100.00	Still the working unit in parts of US practice.

Units of the other quantities, which do not
Unit	Measures	Converts?
gray (Gy)	Absorbed dose	Only through a radiation weighting factor, below
milligray (mGy)	Absorbed dose	Only through a radiation weighting factor, below
rad (rad)	Absorbed dose	Only through a radiation weighting factor, below
becquerel (Bq)	Activity	No — a different physical thing entirely
kilobecquerel (kBq)	Activity	No — a different physical thing entirely
megabecquerel (MBq)	Activity	No — a different physical thing entirely
curie (Ci)	Activity	No — a different physical thing entirely
millicurie (mCi)	Activity	No — a different physical thing entirely
microcurie (µCi)	Activity	No — a different physical thing entirely

The gray and the sievert are both joules per kilogram and are different quantities — so nothing about the arithmetic stops a converter from bridging them silently. This one will not. And becquerels count decays in a source while sieverts measure energy absorbed by a body; turning one into the other is not an approximation but a category error.

The one bridge that does exist

Absorbed dose (Gy)

Radiation type

1 Gy of x-rays and gamma rays1.0000 Sv100.00 rem · w_R = 1

The same 1 Gy, as each kind of radiation
Radiation	w_R	Equivalent dose	Note
X-rays and gamma rays	1	1.0000 Sv	The reference case, and why gray and sievert look interchangeable in medical imaging.
Electrons and beta particles	1	1.0000 Sv	Also weighted at one.
Protons	2	2.0000 Sv	Twice the harm for the same energy deposited.
Neutrons, thermal	2.5	2.5000 Sv	Neutron weighting is a continuous function of energy; this is the low end.
Neutrons, around 1 MeV	20	20.0000 Sv	The peak of the neutron curve — as damaging as alpha, for the same deposited energy.
Alpha particles and fission fragments	20	20.0000 Sv	Twenty times the harm of gamma for the same energy, because it is deposited over a very short track.

The same energy, twenty times the harm. Alpha particles deposit their energy over a very short track, so the damage is concentrated rather than spread — which is why an alpha emitter is comparatively harmless outside the body and serious once inhaled or swallowed. In medical imaging the factor is 1, which is exactly why grays and sieverts look interchangeable there and are not in general.

Where 1.0000 mSv sits

Typical effective doses, for scale
Exposure	Typical dose	Yours is
Eating a banana	0.000100 mSv	10,000.00×
Dental X-ray	0.005000 mSv	200.00×
Chest X-ray	0.020000 mSv	50.0000×
Transatlantic flight	0.080000 mSv	12.5000×
Mammogram	0.400000 mSv	2.5000×
Head CT	2.0000 mSv	0.500000×
Natural background, one year	2.4000 mSv	0.416667×
Abdominal CT	8.0000 mSv	0.125000×
Annual limit for a radiation worker	20.0000 mSv	0.050000×

Five decades from a banana to an occupational limit. These are effective doses — spread across the whole body using tissue weighting factors — so the organs actually in a scanner’s beam received a good deal more than the figure suggests. The values are representative published figures; a real procedure varies with the equipment, the protocol and the patient.

What this converter covers

Units grouped by what they measure, with every cross-quantity conversion refused — and the one bridge that does exist, through the weighting factor.

Gray, milligray and rad — absorbed dose
Sievert, millisievert, microsievert, rem and millirem — equivalent dose
Becquerel, kilo- and megabecquerel, curie, milli- and microcurie — activity
Gray to sievert through the ICRP radiation weighting factor
Typical effective doses from a banana to an occupational limit

Quantities kept apart Weighting required SI and old units Real dose scale

Convert a value Why 1 Gy can be 20 Sv

Free, no signup — exact by definition, not an estimate.

Updated 8 September 2026

At a glance

Formula shown: Sv = Gy × w_R · 1 rad = 0.01 Gy · 1 rem = 0.01 Sv · 1 Ci = 3.7 × 10¹⁰ Bq
Scenario support: 1 Gy of gamma is 1 Sv and 1 Gy of alpha is 20 · a chest X-ray is 0.02 mSv
Educational estimate: Planning support from the values you enter — not professional advice.

Same units, different quantities

Radiation units cause more confusion than almost any other family, and the reason is structural: there are three different quantities in play, two of them share a dimension, and the names give no hint which is which.

What each quantity actually measures
Quantity	SI unit	Old unit	Measures
Absorbed dose	gray (Gy)	rad	Energy deposited per kilogram
Equivalent dose	sievert (Sv)	rem	That energy, weighted for biological harm
Activity	becquerel (Bq)	curie (Ci)	Decays per second in a source

The gray and the sievert are both joules per kilogram. They have identical dimensions, so nothing about the arithmetic would stop a converter from treating them as the same unit — and a good many do. The SI keeps them separate deliberately, precisely so that a figure carries which question it answers.

The becquerel is not even in the same family. It counts events in a source; the others describe what happens in a body. Those are related by an exposure situation, not by a factor.

So this converter groups the units by quantity and refuses every crossing. It is occasionally frustrating and it is the only honest arrangement.

Why one gray can be twenty sieverts

The same energy deposited in tissue does different amounts of harm depending on how it was delivered. Spread thinly along a long track it does less; concentrated into a short one it does far more, because the damage lands in a small volume and overwhelms repair.

One gray, as each kind of radiation
Radiation	w_R	1 Gy is
X-rays, gamma, electrons	1	1 Sv
Protons	2	2 Sv
Neutrons, thermal	2.5	2.5 Sv
Neutrons, around 1 MeV	20	20 Sv
Alpha particles	20	20 Sv

A factor of twenty from the same joules. This is why an alpha emitter is comparatively harmless in a sealed source — alpha particles are stopped by a sheet of paper or the dead outer layer of skin — and serious once inhaled or swallowed, where that same weighting applies to living tissue with nothing in between.

It also explains why the two units look interchangeable in a hospital. Medical imaging is X-rays and gamma, where the factor is 1, so a milligray and a millisievert come out numerically equal. That coincidence trains an intuition which then fails everywhere else.

One honest caveat: neutron weighting is a continuous function of energy rather than the two values in the table, running from about 2.5 at thermal energies up to 20 near 1 MeV and back down at high energy. The two rows above are points on a curve, not categories.

Becquerels are not sieverts

“How many sieverts is a megabecquerel?” has no answer, and unlike most such questions on this site it does not have one even in principle from a single extra number.

A becquerel is one decay per second in a source. A sievert is energy absorbed by a body, weighted. Getting from the first to the second needs the isotope — what it emits and at what energy — the geometry, the distance, any shielding, the exposure time, and whether the material stayed outside the body or was inhaled or swallowed. That is a dose assessment, and it is a professional exercise rather than an arithmetic one.

The same activity can therefore correspond to almost any dose. A megabecquerel of a pure alpha emitter in a sealed container delivers essentially nothing through the container wall, and the same megabecquerel inhaled delivers a great deal. A megabecquerel of a short-lived gamma emitter behaves differently again.

So the calculator returns nothing rather than a figure. A converter that produced a number here would be inventing every one of the inputs above, and the number would look exactly as authoritative as the ones that are real.

rad and rem, and which goes with which

The older units survive in US practice and in a lot of older literature, and the arithmetic is the easiest part of this page: a rad is exactly a hundredth of a gray, and a rem is exactly a hundredth of a sievert.

The trap is the pairing rather than the factor. rad goes with gray — both absorbed dose. rem goes with sievert — both equivalent dose. Converting a rad figure to sieverts by multiplying by 0.01 silently asserts a weighting factor of 1, which is right for X-rays and wrong by up to twenty for anything else.

The old and new units, correctly paired
Old	New	Factor	Quantity
rad	gray	0.01 exactly	Absorbed dose
rem	sievert	0.01 exactly	Equivalent dose
curie	becquerel	3.7 × 10¹⁰ exactly	Activity

The curie is the odd one, and its number is not arbitrary: it was originally the activity of one gram of radium-226, later fixed at exactly 3.7 × 10¹⁰ decays a second so that the unit stopped depending on a measurement. It is also inconveniently large — a curie is a serious quantity of material, which is why millicuries and microcuries are what actually appear on labels.

Effective dose is not organ dose

One more layer sits above the sievert, and it is the one the familiar numbers belong to. Equivalent dose says how much harm the radiation does to the tissue it hits. Effective dose spreads that across the whole body using tissue weighting factors, so that exposures to different parts can be compared on one scale.

Typical effective doses
Exposure	Effective dose	Chest X-rays
Eating a banana	0.0001 mSv	0.005
Dental X-ray	0.005 mSv	0.25
Chest X-ray	0.02 mSv	1
Transatlantic flight	0.08 mSv	4
Head CT	2.0 mSv	100
Natural background, one year	2.4 mSv	120
Abdominal CT	8.0 mSv	400
Annual limit, radiation worker	20 mSv	1000

Because it is a whole-body figure, an effective dose understates what the tissue actually in the beam received — sometimes by a large factor. A mammogram delivers a few millisieverts of effective dose and a much larger local dose to breast tissue; that the two numbers differ is the design of the quantity, not an error in it.

The figures are also representative rather than definitive. The same nominal scan can differ several-fold between hospitals depending on the equipment and the protocol, and a real dose for a real patient comes from the department that did it. This page converts units; it does not assess anyone’s exposure, and no conversion can.

Sources and methodology

The weighting factors here are the ICRP 103 values, and the distinction they encode — between energy deposited and harm done — is why the SI keeps the gray and the sievert as separate units despite their sharing a dimension. The dose figures used for scale are representative published values rather than measurements of any particular procedure.

ICRP Publication 103 — The 2007 Recommendations of the International Commission on Radiological ProtectionInternational Commission on Radiological Protection · verified 2026-09-08 · The radiation weighting factors that turn absorbed dose into equivalent dose — 1 for photons and electrons, 2 for protons, 20 for alpha particles and a continuous energy-dependent function for neutrons — and the distinction between equivalent and effective dose
SI Brochure, 9th edition — the gray, the sievert and the becquerelBureau International des Poids et Mesures · verified 2026-09-08 · That the gray and the sievert are separate derived units with the same dimensions of joules per kilogram, kept distinct precisely because they express different quantities, and that the becquerel is a reciprocal second
NCRP Report No. 160 — Ionizing Radiation Exposure of the Population of the United StatesNational Council on Radiation Protection and Measurements · verified 2026-09-08 · The representative effective doses used for scale on this page — chest radiography, computed tomography and natural background — and the point that these are whole-body effective doses rather than organ doses

Conversion note

This converts units and applies published weighting factors. It does not assess exposure, and no unit conversion can: working out a dose from a source needs the isotope, the geometry, the distance, the shielding, the exposure time and whether the material was inhaled or ingested, which is a radiation protection assessment carried out by a qualified person. The dose references are representative published figures for scale — a real procedure varies substantially with the equipment, the protocol and the patient, and the same nominal scan can differ several-fold between hospitals. Effective doses are whole-body figures computed with tissue weighting factors and are not what any single organ received. Neutron weighting is a continuous function of energy and the two values shown are only points on it. Nothing here is medical advice or a basis for a decision about a scan; that conversation belongs with the clinician who ordered it and, for occupational exposure, with the radiation protection adviser.

How we calculate · Found an error? email us

Authorship & verification

Written and maintained by Jay Sudha, a business operator who builds spreadsheet-based calculators.

Formula and examples verified on 8 September 2026
Self-reviewed — written and checked by the same person, with no independent reviewer
Educational estimate only
Formula tested against 40 automated cases · last run 2026-09-07
How calculators are verified

What's changed (11 updates)

Published 8 September 2026

Published the Gray to Sievert Converter: gray, milligray and rad for absorbed dose; sievert, millisievert, microsievert, rem and millirem for equivalent dose; becquerel, kilo- and megabecquerel, curie, milli- and microcurie for activity.
Groups the units by which QUANTITY they measure and refuses every cross-quantity conversion, because the gray and the sievert are both joules per kilogram -- identical dimensions, different quantities -- so nothing about the arithmetic would stop a converter bridging them silently.
Bridges absorbed to equivalent dose only through an explicit radiation weighting factor, and shows the spread that makes it necessary: one gray is one sievert of gamma and twenty sieverts of alpha.
Explains the physical reason -- energy deposited over a short track does more harm than the same energy spread along a long one -- and why that makes an alpha emitter comparatively harmless sealed and serious once inhaled.
Names the coincidence that trains the wrong intuition: medical imaging is X-rays and gamma where the factor is one, so milligrays and millisieverts come out numerically equal there and nowhere else.
Refuses activity-to-dose outright rather than approximating it, since getting from becquerels to sieverts needs the isotope, geometry, distance, shielding, exposure time and route of intake -- a dose assessment rather than a unit conversion.
Pairs the old units correctly: rad with gray and rem with sievert, both exactly a hundredth, and warns that converting a rad figure to sieverts silently asserts a weighting factor of one.
Records that the curie is exactly 3.7 times ten to the tenth becquerels, fixed at that value so the unit stopped depending on a measurement of radium.
Distinguishes effective dose from organ dose, noting that an effective dose is a whole-body figure computed with tissue weighting factors and therefore understates what the tissue in the beam received.
States that neutron weighting is a continuous function of energy and the two values shown are points on a curve rather than categories, and that the dose references are representative published figures that vary several-fold between hospitals.
Verified by 40 automated cases, asserting that EVERY same-quantity pair converts and EVERY cross-quantity pair refuses -- all of them rather than a sample -- and that activity refuses against every dose unit in both directions.

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