Free Beta-hCG Unit Converter

Convert the free β subunit of hCG between ng/mL, µg/L and pg/mL. Before anything else: this is not total hCG. Free β-hCG is a different measurand with its own reference figures, and reading a free β result against a total hCG interval — or against the pregnancy-test numbers everyone knows — is the mistake this page exists to stop.

Last updated 18 Sep 2026

Free β-hCG converter

Mass units only

Free β-hCG subunit

ng/mL and µg/L are the same number; pg/mL is that number × 1,000. mIU/mL is deliberately not on this ladder — for the two commonest platforms it happens to equal the ng/mL figure, and the table below explains why that is a calibration rather than a conversion.

Gestational weekFifth to ninety-fifth centiles from one manufacturer’s expected-values study (842 first-trimester samples, Siemens ADVIA Centaur / Atellica IM, reported in IU/L, which that assay states is the same number as ng/mL). They are shown so you can see the scale and the steep fall with gestation — they are not what a screening report uses, which is a multiple of your own laboratory’s median.

44.0ng/mLExample

Free β-hCG 44 ng/mL at 11 weeks

The ladder, and the unit that is not on it

1 ng/mL = 1 µg/L (identical)
1 ng/mL = 1,000 pg/mL
and, for an assay traceable to WHO IRP 75/551 only: 1 ng/mL = 1 mIU/mL = 1 IU/L

ng/mL = µg/L: one nanogram in a millilitre is one microgram in a litre. The prefixes and the volumes both differ by a thousand and cancel, so there is no factor between these two labels at all
pg/mL: a thousand picograms make a nanogram, so pg/mL is the ng/mL figure multiplied by 1,000. Some research assays and some older reports use it; a free β-hCG of 44 ng/mL is 44,000 pg/mL
no pmol/L: the free β subunit is a variably glycosylated protein measured against a protein calibrator, so it has no single grams-per-mole to divide by. A molar unit for it would be an assumption dressed up as an identity, and none is offered here
mIU/mL: off the dropdown on purpose. It is a calibration against a World Health Organization standard, not a metric prefix. For the two platforms in the table below the calibration works out to exactly one, for a reason that is definitional rather than coincidental — but it is still a property of that standard, not of the molecule
not total hCG: the whole hormone is an α–β heterodimer and is what a pregnancy test and a quantitative hCG measure. This assay measures only the unbound β subunit, a small and variable fraction of it. Different number, different units in practice, different reference figures

Worked example

Free β-hCG 44 ng/mL at 11 weeks
44 ng/mL is 44.0 µg/L — the same number, because the two units are identical
44 × 1,000 = 44,000 pg/mL
On an assay traceable to WHO IRP 75/551 it is also 44 mIU/mL, which is 44 IU/L — see the calibration table below before relying on that
It sits almost exactly on the 11-week median of 44.11 for one platform, and 3% above the 42.8 median another platform publishes for the same week — which is why the screening laboratory must use its own
A total hCG at 11 weeks would be in the tens of thousands of mIU/mL. If your number looks like that, you are holding the total hCG converter, not this one

The mIU/mL relation: a calibration that happens to equal one

The factor is one, and it is one for a reason: the International Unit of the free β subunit was defined as the activity in a microgram of it, so a milli-international-unit per millilitre and a nanogram per millilitre are the same quantity by construction. That is still a fact about the 1975 reference preparation rather than about the molecule, which is why mIU/mL is tabled here instead of put on the dropdown. If your report gives mIU/mL and does not name its standard, the number is almost certainly interchangeable with ng/mL — but the sentence that makes it so is in the package insert, not in the SI.
Assay	Units it reports	Standard it is traceable to	Relation it states
Roche Elecsys free βhCG (cobas)	IU/L, mIU/mL or ng/mL	NIBSC 75/551 — International Reference Preparation of chorionic gonadotrophin β subunit	IU/L × 1 = mIU/mL; mIU/mL × 1 = ng/mL
Siemens Atellica IM / ADVIA Centaur FBHCG	IU/L, convertible to ng/mL or mIU/mL	WHO 1st International Reference Preparation, NIBSC 75/551	1 IU/L = 1 ng/mL or 1 mIU/mL
The standard itself, NIBSC 75/551	International Units	—	70 IU per ampoule "by definition", containing approximately 70 µg of β subunit, because one IU is the activity in one microgram of the pure subunit
Any assay standardised against something else	whatever it reports	not 75/551	unknown — do not assume one

Free β-hCG by gestational week, two platforms, in ng/mL (= IU/L)

Centiles and medians in columns two to four are Siemens’ expected values from 842 first-trimester samples; the last column is Roche’s median for the same completed week. The two disagree by up to 13% — 75.5 against 67.08 at nine weeks — on the same measurand in the same unit. That is not an error in either: it is why a screening laboratory has to derive its own gestation-specific medians on its own analyser, and why no table like this one can be used to compute a multiple of the median.
Gestational week	5th centile	Median	95th centile	A second platform’s median
Week 8+0 to 8+6	39.79	79.51	174.28	70.7
Week 9+0 to 9+6	26.60	67.08	169.39	75.5
Week 10+0 to 10+6	23.69	58.08	169.40	57.3
Week 11+0 to 11+6	16.18	44.11	131.86	42.8
Week 12+0 to 12+6	13.41	36.02	95.06	34.5
Week 13+0 to 13+6	11.46	28.40	81.93	29.5

Free β-hCG against total hCG: the comparison the reference interval depends on

The last row of numbers is the one that matters. A free β-hCG of 40 is unremarkable at 11 weeks; a total hCG of 40 at 11 weeks would be a catastrophe. They are three orders of magnitude apart, so a free β result read against a total hCG interval looks profoundly and alarmingly low, and a total hCG read against these centiles looks impossibly high. The two tests are also chosen for different windows: the ACMG technical standard notes that before 11 weeks free β-hCG discriminates while total hCG does not, and that by 13 weeks total hCG may perform slightly better.
	Total hCG	Free β-hCG
What the assay detects	The intact α–β heterodimer, usually plus free β and some fragments	Only β subunit that is not bound to an α subunit
Typical unit on the report	mIU/mL or IU/L	ng/mL or µg/L, sometimes mIU/mL
Order of magnitude at 11 weeks	Tens of thousands	Tens
What it is used for	Confirming and monitoring a pregnancy, gestational trophoblastic disease, germ cell tumours	First-trimester combined screening, as one of two serum markers
How it is reported in screening	Second-trimester quadruple test, as a multiple of the median	First-trimester combined test, as a multiple of the median
Its own page	total hCG converter	this page

One measurand, three labels, and the test it is constantly confused with

The conversion itself is over in a line. A nanogram per millilitre and a microgram per litre are the same concentration written two ways — a thousandfold in the prefix cancels a thousandfold in the volume — and a picogram per millilitre is a thousandth of a nanogram per millilitre, so pg/mL is the ng/mL figure times a thousand. No measured constant appears anywhere. If relabelling the number is all you came for, it is done above.

What actually goes wrong with a free β-hCG result is that it gets read as though it were an hCG result. They are not the same test. Human chorionic gonadotropin is a heterodimer of an α subunit shared with the other glycoprotein hormones and a β subunit that is specific to it, and a quantitative hCG assay measures the assembled hormone. This assay measures only the free β subunit — β chains circulating unpaired, a small and variable fraction of the total. The consequence is arithmetical and brutal: at eleven weeks a total hCG runs into the tens of thousands of mIU/mL while a free β-hCG sits in the tens of nanograms per millilitre. Put a free β result next to a total hCG reference interval and it looks like a pregnancy that has failed. It is not. It is the wrong denominator.

Then there is the mIU/mL question, which is the commonest search that lands on a page like this one, and it has a genuinely interesting answer. An international unit is a calibration against a reference preparation, not a metric prefix, so in general there is no arithmetic between a mass and an IU. For the free β subunit the reference preparation is NIBSC 75/551, and the institute’s own instructions say each ampoule holds seventy international units by definition and approximately seventy micrograms of subunit, because one unit was defined as the activity in one microgram of the pure material. Both Roche and Siemens standardise their assays against it and both state in their inserts that one IU per litre is one nanogram per millilitre is one milli-international-unit per millilitre. So the factor is one, and it is one on purpose. It still does not go on the unit selector above, because a selector presents its entries as identities and this one is an identity only as long as your assay is traceable to that standard. Read it off the table, check the insert, and you have the number with the condition attached to it.

The deeper point about this measurand is that a concentration is not the result. First-trimester combined screening does not look at nanograms per millilitre; it converts the value to a multiple of the median for the exact gestational age — the ACMG technical standard requires that each result in mass or international units be turned into a MoM for a given gestational age before anything else happens — and then combines that with PAPP-A, nuchal translucency and maternal characteristics. The conversion matters because the marker moves fast: free β-hCG falls by roughly twenty to forty per cent between eleven and thirteen completed weeks in unaffected pregnancies, so the same number means different things a fortnight apart. And the median has to be the laboratory’s own, on its own analyser: the two manufacturers’ published medians in the table above differ by up to thirteen per cent for the same week. That is why this page gives you centiles to orient yourself and refuses to give you a MoM threshold. A threshold without the laboratory’s median behind it would be a number with no meaning, and the risk figure on a screening report is the output of a validated algorithm, not of a cut-off.

Two practical notes. There is no molar unit here and there should not be: the subunit is glycosylated with variable chains, so it has no fixed grams per mole, and a pmol/L would be an assumption pretending to be a unit. And if you are tracking a pregnancy rather than screening one, the free β subunit is the wrong marker entirely — use the total hCG converter and, if you are watching a trend, the hCG doubling-time calculator. The companion marker for screening is the PAPP-A converter, which moves in the opposite direction for the commonest indication and is the other half of the same blood draw.

Frequently asked questions

Is ng/mL the same as mIU/mL for free beta-hCG?

For an assay standardised against WHO reference preparation 75/551 — which includes the Roche and Siemens assays — yes, exactly: both manufacturers state in their inserts that 1 IU/L equals 1 ng/mL equals 1 mIU/mL. The reason is definitional, because that standard’s international unit was defined as the activity contained in one microgram of the pure β subunit. It is a calibration rather than a unit conversion, so check that your assay names that standard before relying on it.

Is free beta-hCG the same as hCG?

No. hCG is the intact hormone, a heterodimer of an α and a β subunit, and that is what a pregnancy test and a quantitative hCG measure. This test measures only β subunit circulating unpaired, which is a small fraction of the total. At eleven weeks a total hCG is in the tens of thousands of mIU/mL and a free β-hCG is in the tens of nanograms per millilitre, so reading one against the other’s reference interval is out by about three orders of magnitude.

What is a normal free beta-hCG?

It depends entirely on the gestational week and on the platform, which is why the table above gives both. On one manufacturer’s expected-values study the 5th to 95th centile range is 16.18 to 131.86 ng/mL at eleven completed weeks, 13.41 to 95.06 at twelve and 11.46 to 81.93 at thirteen. Those are a guide to the scale, not a decision rule: screening uses a multiple of your own laboratory’s median instead.

Why does my report give a MoM instead of a concentration?

Because the concentration on its own is not interpretable. Free β-hCG falls by roughly 20 to 40 per cent between eleven and thirteen completed weeks in unaffected pregnancies, so the laboratory divides your result by its own median for your exact gestational age and reports the ratio. The multiple of the median is then combined with PAPP-A, nuchal translucency and maternal factors by a risk algorithm. A MoM from one laboratory’s median cannot be recomputed from another’s table.

Does a high free beta-hCG mean Down syndrome?

No single marker value means that. A raised free β-hCG with a low PAPP-A is one of the patterns first-trimester combined screening weights, but the output is a risk estimate from an algorithm that also uses nuchal translucency, gestational age and maternal characteristics, and a screening risk is not a diagnosis. Diagnostic testing is chorionic villus sampling or amniocentesis; cell-free DNA is a more accurate screen. Interpret the risk figure on the report, not the marker.

Why is free beta-hCG not reported in pmol/L?

Because it has no single molar mass to divide a mass concentration by. The subunit is glycosylated with variable oligosaccharide chains, so its grams per mole is a range rather than a constant, and the assay is calibrated against a protein preparation instead. A picomole-per-litre figure for it would be an assumed mass presented as a unit conversion, which is why no molar unit appears on this page.

Related calculators

References

Roche Diagnostics. Elecsys free βhCG method sheet. Reports "IU/L, mIU/mL or ng/mL"; "IU/L x 1 = mIU/mL", "mIU/mL x 1 = ng/mL"; standardised against NIBSC code 75/551; medians by completed gestational week.
Siemens Healthineers. Atellica IM Free Beta Human Chorionic Gonadotropin (FBHCG) instructions for use. "Conversion formula: 1 IU/L = 1 ng/mL or 1 mIU/mL"; traceable to the WHO 1st International Reference Preparation, NIBSC 75/551; expected values from 842 first-trimester samples.
National Institute for Biological Standards and Control. IRP Chorionic Gonadotrophin Beta Subunit, Human, for Immunoassay, 75/551 — instructions for use. "Each ampoule contains 70 INTERNATIONAL UNITS (by definition)"; "HCG-Beta subunit approximately 70µg"; one International Unit is the activity contained in one microgram of the pure subunit preparation.
American College of Medical Genetics. Technical standards and guidelines: prenatal screening for Down syndrome that includes first-trimester biochemistry and/or ultrasound measurements. Genet Med. 2009.

Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.