Paracetamol (Acetaminophen) Unit Converter

Paracetamol and acetaminophen are the same drug, and this converts a plasma concentration between mg/L, µg/mL, µmol/L and mg/dL. The reason to do it here is that the nomogram takes mg/L: a result in µmol/L has to be converted before it can be plotted, and the conversion reproduces the published treatment lines exactly.

Last updated 18 Sep 2026

Paracetamol converter

Mass ⇄ molar

Plasma paracetamol concentration

mg/L and µg/mL are the same number. µmol/L is the mg/L figure × 6.6155; to go the other way, divide by 6.6155 (or multiply by 0.1512). The default is the United Kingdom’s 100 mg/L treatment line at four hours.

Comparison figureThis is not a reference interval and it is not a treatment threshold. It is the peak that the Australian and New Zealand guideline says is not usually exceeded after therapeutic dosing, shown so that a converted number has some scale to it. Whether treatment is needed depends on the hours since ingestion and is decided on the nomogram, not here.

661.6µmol/LExample

Paracetamol 100 mg/L — the United Kingdom’s four-hour treatment line

The factor, derived rather than quoted

µmol/L = mg/L × 6.6155
mg/L = µmol/L ÷ 6.6155 = µmol/L × 0.15117
because 1 mg/L is 1 × 10⁻³ g/L, and 10⁻³ ÷ 151.16 g/mol = 6.6155 × 10⁻⁶ mol/L = 6.6155 µmol/L

151.16: the molecular weight of paracetamol in grams per mole. C₈H₉NO₂ recomputed from IUPAC standard atomic weights gives 151.165, and PubChem prints 151.16; the 0.003% between them moves the factor from 6.6153 to 6.6155, which is below the precision any laboratory reports
6.6155: the whole conversion. Published tables usually round it to 6.62, which changes a result by 0.07% — 200 mg/L becomes 1,324 µmol/L instead of 1,323. Either is fine clinically; this page uses the derived value so that its own arithmetic is internally consistent
mg/L = µg/mL: the same concentration written two ways. One milligram in a litre is one microgram in a millilitre, and there is no factor between them. United States sources usually write µg/mL or mcg/mL; UK and Australasian ones write mg/L
mg/dL: a tenth of the mg/L figure, because a decilitre is a tenth of a litre. Rare for paracetamol but it appears on some US chemistry panels, and a mg/dL read as mg/L is a tenfold underestimate of a potentially lethal level
what the number needs next: a time. A paracetamol concentration has no reference interval and no single toxic threshold; it is interpreted against the hours since a single acute ingestion, on the Rumack-Matthew nomogram, and only between four and twenty-four hours

Worked example

Paracetamol 100 mg/L — the United Kingdom's four-hour treatment line
100 mg/L = 0.100 g/L
0.100 ÷ 151.16 = 6.6155 × 10⁻⁴ mol/L = 661.6 µmol/L
In one step: 100 × 6.6155 = 661.6. With the conventionally published factor of 6.62 it is exactly 662
= 100 µg/mL, the same number, and 10.00 mg/dL
At four hours after a single acute ingestion this is on the UK treatment line, so acetylcysteine is indicated — plotted on the Rumack-Matthew nomogram calculator

The published thresholds in both units — the factor’s own cross-check

Four of these pairings are printed independently by guidelines and reference sources, and the derived factor reproduces every one of them to the last digit they print: 66, 132, 199 and 31. The fifth is the interesting one. 150 mg/L converts to 992 µmol/L, and the guideline prints 1000 — it has rounded, deliberately and sensibly, to a figure a clinician can hold in their head. StatPearls’ 990 is the same number rounded to two significant figures. So a converter whose output disagrees with 1000 by eight is not wrong; it is un-rounded. And the UK’s 100 mg/L line has no published µmol/L twin at all, because the countries that use it report paracetamol in mg/L.
Threshold	mg/L (µg/mL)	× 6.6155 gives	What the source prints
United Kingdom treatment line at 4 hours, single line since 2012	100	661.6	100 mg/L. UK guidance and nomograms print the line in mg/L only; no µmol/L figure for it could be found in a primary source
United States, Canada, Australia and New Zealand treatment line at 4 hours	150	992.3	"150 mg/L (1000 µmol/L)" — the Australian and New Zealand guideline, rounding 992 to 1000. StatPearls independently prints "150 mcg/mL or 990 µmol/L"
The same line at 24 hours	4.69	31.0	"4.69 mcg/mL or 31.3 µmol/L" — StatPearls
Level below which acetylcysteine can be stopped, or a repeat is unnecessary	10	66.2	"10 mg/L (66 µmol/L)" — the Australian and New Zealand guideline, and the same pairing in LITFL
Repeated supratherapeutic ingestion: treat if above	20	132.3	"20 mg/L (132 µmol/L)" — the same guideline, which also gives 20 mg/L as the peak not usually exceeded after therapeutic doses
Therapeutic concentration range	10–30	66.2–198.5	"10 to 30 μg/mL (66 to 199 μmol/L)" — UNITSLAB; Mayo Clinic Laboratories prints the same range as 10–30 mcg/mL
Conventionally quoted toxic concentration	200	1,323.1	"> 200 μg/mL (> 1324 μmol/L)" — UNITSLAB, which is using the rounded factor 6.62
Double the 150 mg/L line, where a higher acetylcysteine dose is used	300	1,984.7	"300 mg/L at 4 h" — the Australian and New Zealand guideline, in mg/L only

You have converted the number. Now which page does it go to?

The first row is why this page is worth building. The nomogram is defined on a plot of milligrams per litre against hours, and every published version of it is drawn that way, so a laboratory that reports in micromoles per litre — which is most of Europe and much of the world outside the mg/L countries — hands the clinician a number that cannot be plotted as it stands. Divide by 6.6155 first. A µmol/L figure entered on a mg/L axis is high by more than sixfold and will put a harmless level above the treatment line.
What you have	Where it goes	The unit that page takes
A concentration and a known time since a single acute ingestion, 4 to 24 hours	Rumack-Matthew nomogram calculator	mg/L — convert here first if your result is in µmol/L
A reported amount swallowed and a body weight, no level yet	paracetamol toxic dose calculator	mg and kg, giving mg/kg against the 200 or 150 mg/kg threshold
A decision to treat, and a body weight	N-acetylcysteine dose calculator	kg, giving the three-bag 21-hour regimen
A rising ALT and ALP and a question about the pattern of liver injury	R-factor liver injury calculator	U/L and each enzyme’s upper limit of normal
A co-ingestion and an unexplained acid-base picture	salicylate converter	mg/dL, mg/L or mmol/L

When the converted number cannot be interpreted at all

The unifying point is that paracetamol is one of the few assays where the number without a clock is worthless, and one of the few where a reassuring number can be actively dangerous. A concentration of 30 mg/L — 198 µmol/L — is below every treatment line if it was drawn at four hours and well above the line if it was drawn at sixteen. The converter gives you the value in the unit the decision tool wants; the decision still needs the history.
Situation	Why the concentration does not answer the question
Less than 4 hours after ingestion	Absorption is incomplete, so the concentration is still rising and a low value predicts nothing. The nomogram starts at 4 hours for this reason, and a level drawn earlier cannot be plotted even by extrapolation.
Time of ingestion unknown or unreliable	The nomogram needs an interval, not just a number. Underestimating the interval moves the point left and can make a treatable concentration look safe; StatPearls’ rule for an unknown time is to treat a detectable concentration above 10 mg/L rather than to guess a position on the curve.
Staggered or repeated supratherapeutic ingestion	The nomogram was derived for a single acute overdose and does not apply. Risk assessment shifts to the dose taken and the transaminases, with treatment indicated above 20 mg/L (132 µmol/L) or an ALT above 50 U/L.
Modified-release preparation	Absorption continues for hours, so a single concentration can fall below the line while the true peak has not yet occurred. Repeat sampling is needed.
More than 24 hours after ingestion	Beyond the nomogram entirely. Management is driven by the transaminases, the INR, the pH and the creatinine rather than by the paracetamol concentration, which may by then be low or undetectable in a patient with established liver injury.

Same drug, two names, and a number that needs a clock

Paracetamol and acetaminophen are the same molecule, C₈H₉NO₂, and the two words are simply the international non-proprietary name and the United States adopted name for it. Acetaminophen is standard in North America and paracetamol nearly everywhere else, which is also roughly the line between reporting a plasma concentration in micrograms per millilitre and reporting it in milligrams per litre — and those two are the same number. The real split in the units is not between the two drug names but between mass and molar reporting: milligrams per litre against micromoles per litre, and the factor between them is 6.6155, which is a thousandth of a gram per litre divided by a molecular weight of 151.16 grams per mole and scaled to micromoles.

That factor is the most cross-checkable number on this site, because the treatment thresholds for paracetamol poisoning are published in both units by different authorities who did not coordinate. The Australian and New Zealand guideline prints its stopping rule as 10 mg/L (66 µmol/L), its repeated-supratherapeutic threshold as 20 mg/L (132 µmol/L) and the concentration not usually exceeded after therapeutic doses as 20 mg/L (132 μmol/L). Reference tables give the therapeutic range as 10 to 30 µg/mL, or 66 to 199 µmol/L. StatPearls gives the twenty-four-hour point of the American treatment line as 4.69 mcg/mL or 31.3 µmol/L. Multiply each mass figure by 6.6155 and you get 66.2, 132.3, 198.5 and 31.0. Every published pair is reproduced to the digit the source prints. A converter that regenerates the thresholds a reader already knows is a converter whose other answers can be trusted.

There is one apparent exception and it is instructive. The four-hour treatment line used in the United States, Canada, Australia and New Zealand is 150 mg/L, and the Australasian guideline prints it as "150 mg/L (1000 µmol/L)". The exact conversion is 992.3. The guideline has rounded, which is exactly right for a figure that has to be recalled at three in the morning, and StatPearls’ 990 is the same rounding done to two significant figures. So if your laboratory reports 995 µmol/L, you are above the American line by the arithmetic and below the round number that gets quoted — which is a difference of about one millimetre on the printed nomogram and no difference at all to the decision. The United Kingdom’s single line, set at 100 mg/L when the Commission on Human Medicines removed the high-risk line in 2012, has no published micromolar twin, because the countries that use it report in mg/L. Its exact conversion is 661.6 µmol/L, or 662 if you use the rounded factor.

All of which is in service of one practical fact: the Rumack-Matthew nomogram is drawn with milligrams per litre on its vertical axis. Every published version of it is. If your report gives micromoles per litre — and most of Europe’s do — the number cannot be plotted until it has been divided by 6.6155, and a µmol/L value read onto a mg/L axis is more than six times too high, which will place a harmless concentration far above the treatment line. That is the single most useful thing this page does, and it is why it links straight to the Rumack-Matthew nomogram calculator rather than trying to duplicate it.

The last thing to say is the most important and it is not about units. A paracetamol concentration has no reference interval, and a single value means nothing without the time since ingestion. The nomogram is valid only from four to twenty-four hours after a single acute ingestion of an immediate-release preparation with a reliable history; before four hours absorption is incomplete and a low level predicts nothing at all, which is why a sample drawn at two hours cannot be used even if the result is reassuring. Thirty milligrams per litre is far below every treatment line at four hours and clearly above it at sixteen. Where the history is staggered, the preparation modified-release, the timing unknown or the presentation beyond twenty-four hours, the concentration stops being the decision variable and the transaminases, the INR, the pH and the dose taken take over — which is what the paracetamol toxic dose calculator, the R-factor liver injury calculator and a poisons centre are for. Convert the number here, then give it a time.

Frequently asked questions

Are paracetamol and acetaminophen the same drug?

Yes — one molecule, C₈H₉NO₂, with two approved names. Paracetamol is the international non-proprietary name and acetaminophen the United States adopted name, and the same convention splits the reporting units: US sources usually print µg/mL or mcg/mL while UK and Australasian ones print mg/L. Those two units are also the same number, so a result of 100 µg/mL is 100 mg/L.

How do I convert paracetamol from µmol/L to mg/L?

Divide by 6.6155, or equivalently multiply by 0.15117. So 1,000 µmol/L is 151.2 mg/L, 662 µmol/L is 100.1 mg/L and 132 µmol/L is 20.0 mg/L. The factor is the molecular weight of paracetamol, 151.16 g/mol, applied to a concentration: one milligram per litre divided by 151.16 grams per mole is 6.6155 micromoles per litre.

What is the conversion factor for paracetamol — 6.62 or 6.6155?

Both, and it does not matter clinically. 6.6155 is the value derived from the molecular weight of 151.16 g/mol; published conversion tables round it to 6.62. The difference is 0.07%, which moves 200 mg/L from 1,323 to 1,324 µmol/L. This page uses the derived figure so that its arithmetic agrees with the molecular weight it quotes, and the table above shows where the rounded factor produces a published number.

Is the UK’s 100 mg/L treatment line the same as 662 µmol/L?

It is 661.6 µmol/L by exact conversion, and exactly 662 if you use the conventionally published factor of 6.62. No primary source printing the UK line in micromoles per litre could be found — the countries that use the 100 mg/L line report paracetamol in mg/L — so treat 662 as the correct conversion of a mg/L threshold rather than as a quoted threshold in its own right.

Can I plot a µmol/L result on the Rumack-Matthew nomogram?

Not without converting it. Every published version of the nomogram has milligrams per litre on its concentration axis, so a micromolar value has to be divided by 6.6155 first. Entering it unconverted overstates the concentration more than sixfold and will put a level that needs no treatment well above the treatment line.

Why is a paracetamol level taken before 4 hours useless?

Because absorption is not complete, so the concentration is still climbing and a low value does not predict where it will peak. The nomogram is defined from 4 to 24 hours after a single acute ingestion for that reason. A sample drawn at two hours cannot be plotted and cannot exclude toxicity; repeat it at or after four hours from the stated time of ingestion.

Related calculators

References

Chiew AL, Reith D, Pomerleau A, et al. Updated guidelines for the management of paracetamol poisoning in Australia and New Zealand. Med J Aust. 2020;212(4):175–183. "The 150 mg/L (1000 µmol/L) at 4 hours nomogram is currently used in the USA, Canada, Australia and New Zealand"; "Paracetamol concentration < 10 mg/L (66 µmol/L)"; "> 20 mg/L (132 µmol/L)"; "Peak serum concentrations after therapeutic doses do not usually exceed 20 mg/L (132 μmol/L)".
Agrawal S, Khazaeni B. Acetaminophen toxicity. In: StatPearls. NCBI Bookshelf NBK441917. "The treatment threshold is 150 mcg/mL or 990 µmol/L at 4 hours and 4.69 mcg/mL or 31.3 µmol/L at 24 hours"; ">10 mcg/mL (66 µmol/L)".
Medicines and Healthcare products Regulatory Agency and Commission on Human Medicines. Benefit–risk profile of acetylcysteine in the management of paracetamol overdose — drug safety alert, 3 September 2012. The change to a single 100 mg/L treatment line in the United Kingdom.
Commission on Isotopic Abundances and Atomic Weights (IUPAC). Standard atomic weights. C₈H₉NO₂ recomputed as 151.165 g/mol; PubChem Compound Summary for CID 1983 prints 151.16.
UNITSLAB. Acetaminophen unit conversion. Therapeutic "10 to 30 μg/mL (66 to 199 μmol/L)"; toxic "> 200 μg/mL (> 1324 μmol/L)".
Mayo Clinic Laboratories. Acetaminophen, Serum (test 37030). Therapeutic 10–30 mcg/mL; toxic ">150 mcg/mL 4 hours after dose"; homogeneous enzyme immunoassay.

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.