Paracetamol Toxic Dose Calculator
Paracetamol Toxic Dose Calculator
Convert a reported paracetamol ingestion into mg/kg against the 200 mg/kg or 150 mg/kg risk-assessment threshold, and see why the absolute dose matters as much as the weight-based figure.
Paracetamol ingested dose
3 inputs → mg/kg12 000 mg ingested by a 70 kg adult, assessed against the 200 mg/kg threshold
Formula
- 200 mg/kg or 10 g
- the Australian and New Zealand risk-assessment threshold — whichever of the two is less
- 150 mg/kg or 7.5 g
- the conventional threshold used in much of the rest of practice, including the United States
- body weight
- use lean or ideal body weight in obesity; actual weight understates the mg/kg figure
Worked example
12 000 mg ingested by a 70 kg adult, assessed against the 200 mg/kg threshold
12 000 ÷ 70 = 171.4 mg/kg
171.4 is below the 200 mg/kg weight-based threshold
But the absolute dose is 12 g, which exceeds the 10 g threshold — risk assessment applies and a concentration is measured
Risk-assessment thresholds for a single acute ingestion
| Threshold | Weight-based | Absolute dose | Where used |
|---|---|---|---|
| Australia and New Zealand | 200 mg/kg | 10 g | MJA paracetamol poisoning guidelines |
| Conventional | 150 mg/kg | 7.5 g | Widely used, including United States practice |
How the pattern of ingestion changes the assessment
| Pattern | Assessment |
|---|---|
| Single acute ingestion, known time | mg/kg or absolute dose, then a concentration at 4 hours or later plotted on the nomogram |
| Repeated supratherapeutic ingestion | Assessed on cumulative dose over 24 and 48 hours, transaminases and paracetamol concentration — the nomogram does not apply |
| Staggered ingestion over hours | Treated on dose history and transaminases; there is no single time point to plot |
Turning a dose history into a risk assessment
The first question in a paracetamol overdose is whether the reported ingestion crosses the threshold for investigation. Australian and New Zealand guidelines use 200 mg/kg or 10 g, whichever is less; much of the rest of practice, including the United States, uses 150 mg/kg or 7.5 g. The phrase whichever is less is the part that gets lost. A 90 kg adult reaches 10 g at only 111 mg/kg, so the absolute dose triggers assessment well before the weight-based figure does, which is why this calculator flags any ingestion of 10 g or more separately.
Weight itself needs care. In obesity, actual body weight inflates the denominator and produces a falsely reassuring mg/kg figure, so lean or ideal body weight is used instead. At the other end, in small children the weight-based figure is what governs, and a modest number of tablets can represent a large mg/kg dose. The absolute-dose threshold and the weight-based threshold are each there to catch what the other misses.
The dose history is frequently unreliable. Patients under-report and over-report, tablet counts are approximate, co-ingestants are forgotten, and intent affects what is said. Because of this, a paracetamol concentration is measured whenever an ingestion is plausibly above threshold, and the calculated mg/kg figure is treated as a trigger for testing rather than a decision in itself. A reassuring calculation with an unreassuring history should not stop a concentration being sent.
Repeated supratherapeutic ingestion — several days of excessive dosing rather than one overdose — is assessed differently, on cumulative dose, transaminases and concentration together, and the nomogram does not apply to it. In every pattern, paracetamol poisoning is time-critical and management should be guided by a poisons centre or clinical toxicology service; this calculator supports that discussion and does not replace it.
Frequently asked questions
What dose of paracetamol is toxic in an adult?
Australian and New Zealand guidelines use 200 mg/kg or 10 g, whichever is less; the conventional threshold used elsewhere is 150 mg/kg or 7.5 g. Crossing either triggers risk assessment and a paracetamol concentration, not automatic treatment.
Why does the absolute dose matter if I have already calculated mg/kg?
Because the threshold is whichever figure is less. A 90 kg adult reaches 10 g at only 111 mg/kg, so a heavy patient meets the absolute-dose threshold while the weight-based figure still looks reassuring.
Which weight should I use in an obese patient?
Lean or ideal body weight. Using actual weight inflates the denominator and produces a falsely low mg/kg result.
Is repeated supratherapeutic ingestion assessed the same way?
No. It is assessed on cumulative dose over 24 and 48 hours together with transaminases and a paracetamol concentration, and the Rumack-Matthew nomogram does not apply to it.
Do I still need a poisons centre if the dose is below threshold?
Poisoning management is time-critical and should be guided by a poisons centre or clinical toxicology service, particularly when the history is uncertain or the preparation was modified-release. This calculator supports that advice and never replaces it.
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.
- Rumack BH, Matthew H. Acetaminophen poisoning and toxicity. Pediatrics. 1975;55(6):871–876.
- Agrawal S, Khazaeni B. Acetaminophen toxicity. In: StatPearls. Treasure Island (FL): StatPearls Publishing.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/