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.
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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.
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.