Rumack-Matthew Nomogram Calculator

Plot a paracetamol concentration against the 150 mg/L or the United Kingdom's 100 mg/L treatment line between 4 and 24 hours after a single acute ingestion, and see when the nomogram does not apply at all.

Last updated 2 Sep 2026

Rumack-Matthew nomogram

3 inputs → × treatment line

Paracetamol concentration (mg/L)

Hours post-ingestionThe nomogram is valid only from 4 to 24 hours after a single acute ingestion of an immediate-release preparation with a known time.

Treatment line

1.13× treatment lineExample

Paracetamol 120 mg/L taken 6 hours after ingestion, plotted against the 150 mg/L line

Formula

Treatment line (mg/L) = L × 2^{(4 − t) ÷ 4}
Result = measured concentration ÷ treatment line

L: the treatment-line concentration in mg/L at 4 hours — 150 in Australia, New Zealand and the United States, 100 in the United Kingdom
t: hours since ingestion, valid only from 4 to 24
÷ 4: the line falls with a 4-hour half-life, halving every 4 hours to reach 4.7 mg/L at 24 hours on the 150 line and 3.1 mg/L on the 100 line
result: 1.00 or more means the concentration sits on or above the line and acetylcysteine is indicated

Worked example

Paracetamol 120 mg/L taken 6 hours after ingestion, plotted against the 150 mg/L line
Treatment line at 6 h = 150 × 2^{(4 − 6) ÷ 4} = 150 × 2^−0.5 = 106.1 mg/L
120 ÷ 106.1 = 1.13 × the treatment line
1.13 is at or above 1.00 → start acetylcysteine
The same concentration against the United Kingdom's 100 mg/L line gives 1.70 × the line

Both treatment lines by time since ingestion

Both lines halve every 4 hours. Concentrations in mg/L are multiplied by 6.61 to give µmol/L, so the 150 line starts at roughly 1000 µmol/L and the 100 line at roughly 660 µmol/L.
Hours post-ingestion	150 mg/L line (mg/L)	100 mg/L line, UK (mg/L)
4	150	100
8	75	50
12	37.5	25
16	18.8	12.5
20	9.4	6.3
24	4.7	3.1

When the nomogram does not apply

In all of these situations treatment decisions are made on other grounds — dose history, transaminases, INR and a poisons centre discussion — not on the nomogram.
Situation	Why the plot fails
Concentration taken before 4 hours	Absorption is incomplete, so the value understates the peak and cannot be plotted
Staggered or repeated ingestion	There is no single time point to plot against
Unknown time of ingestion	The line cannot be positioned
Modified-release preparation	Absorption continues for many hours, so one concentration does not define the curve
Presentation beyond 24 hours	The nomogram stops at 24 hours

Which line applies, and where the nomogram stops working

The Rumack-Matthew nomogram plots a single paracetamol concentration against the time since ingestion to decide whether acetylcysteine is needed. Australia, New Zealand and the United States use a line starting at 150 mg/L at 4 hours, reported in the United States as 150 mcg/mL. The United Kingdom does not: in September 2012 the Commission on Human Medicines removed risk-factor stratification and moved every patient onto a single line starting at 100 mg/L, having judged the evidence for stratifying by risk factor poor and inconsistent. Before that change the United Kingdom ran a 200 mg/L normal-risk line alongside a 100 mg/L high-risk line. Whichever line applies, it falls with a 4-hour elimination half-life, reaching 4.7 mg/L at 24 hours on the 150 line and 3.1 mg/L on the 100 line.

Units cause avoidable errors. Paracetamol has a molecular weight of 151.16, so a concentration in mg/L is multiplied by 6.61 to give micromol per litre: the 150 line starts at roughly 1000 µmol/L and the 100 line at roughly 660 µmol/L. Laboratories do not all report in the same unit, and a number that looks reassuring in one is far above the line in the other. Check the unit before plotting anything.

The nomogram is narrower in scope than its familiarity suggests. It does not apply to staggered or repeated ingestions, to an unknown time of ingestion, to modified-release preparations, or to a presentation beyond 24 hours, and in all of those situations treatment is decided on other grounds. A concentration drawn before 4 hours cannot be plotted either, because absorption is incomplete and the value understates the peak. A low result at 2 hours is not reassurance; it is an uninterpretable result.

Paracetamol poisoning is time-critical. Acetylcysteine started within 8 hours of ingestion is close to fully protective, and its benefit falls away steadily after that, so treatment should not wait on a confirmatory result when the history suggests significant ingestion. Management should be guided by a poisons centre or clinical toxicology service, and this calculator supports that conversation rather than replacing it.

Frequently asked questions

Which paracetamol treatment line should I use, 100 or 150 mg/L?

Use the line your jurisdiction has adopted. Australia, New Zealand and the United States plot against a line starting at 150 mg/L at 4 hours; the United Kingdom has used a single 100 mg/L line since September 2012. Both fall by half every 4 hours.

Why does the United Kingdom use a lower line than Australia and the United States?

It is a policy judgement about the balance of harms, not a disagreement about the pharmacology. The Commission on Human Medicines judged the evidence for risk-factor stratification poor and inconsistent, and a lower line treats more people and accepts more unnecessary acetylcysteine in exchange for fewer missed cases of hepatotoxicity.

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

Multiply by 6.61, derived from the molecular weight of 151.16. That puts the 150 mg/L line at roughly 1000 µmol/L and the United Kingdom’s 100 mg/L line at roughly 660 µmol/L.

Can I use the nomogram at 2 hours, or after a staggered overdose?

No to both. Absorption is incomplete before 4 hours, so an early concentration understates the peak and cannot be plotted; repeat it at or after 4 hours. Staggered ingestions, unknown ingestion times, modified-release preparations and presentations beyond 24 hours all fall outside the nomogram, and treatment is decided on dose history, transaminases and INR instead.

Should I contact a poisons centre?

Yes. Paracetamol poisoning is time-critical and management should be guided by a poisons centre or clinical toxicology service. This calculator supports that discussion and never replaces it.

Related calculators

References

Rumack BH, Matthew H. Acetaminophen poisoning and toxicity. Pediatrics. 1975;55(6):871–876.
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.
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.
Bateman DN. Paracetamol poisoning: beyond the nomogram. Br J Clin Pharmacol. 2015;80(1).