Paracetamol Dosage Calculator

Most people reach for paracetamol by reading the box, which gives flat dose ranges like take 1 or 2 tablets. The problem is that two tablets at 500 mg each delivers the same dose to a 45 kg person as to a 95 kg person, even though their bodies process and distribute the drug very differently. Paracetamol works best when the dose is matched to body weight, particularly for children where the margin between an effective dose and a damaging one is much narrower.

The calculation is straightforward: multiply body weight in kilograms by 15 mg to get the target single dose. That figure is then compared against age-appropriate caps. For adults, the cap is 1,000 mg per dose regardless of weight, which is why a 90 kg adult gets the same 1,000 mg as a 70 kg adult - the formula hits the ceiling. For children and infants, the cap is lower and weight remains the controlling factor because a 10 kg toddler genuinely needs a different dose than a 25 kg child.

The daily maximum matters as much as the single dose. Paracetamol is metabolised by the liver, and the 4,000 mg daily ceiling for adults reflects how much the liver can safely process in 24 hours under normal function. Spread across four doses, each taken 4 to 6 hours apart, the liver has time to clear each dose before the next. Shortening those intervals - or unknowingly taking a second paracetamol product simultaneously - is what causes accidental overdose.

Use this calculator when you are dosing a child and want to go beyond the flat age-band instructions on the box. Weight-based dosing is more accurate than age-based dosing because children vary significantly in size within the same age range. A tall 4-year-old and a small 4-year-old may differ by 8 kg or more, which translates to a meaningful dose difference.

It is also useful for adults who weigh significantly less than the 60 to 80 kg range that most over-the-counter instructions are implicitly designed for. If you weigh under 50 kg, following the standard adult instructions may result in a higher mg-per-kg exposure than intended.

This calculator is not appropriate for people who have already taken paracetamol in the past 24 hours and need to know whether it is safe to take more - that requires knowing the exact total already consumed. It is also not a substitute for clinical prescribing in hospital settings, where patient-specific factors like renal clearance, concurrent medications, and IV formulations require pharmacist review. If a child shows signs of a serious reaction or fever above 40 degrees Celsius lasting more than 24 hours, the right action is medical advice, not a dose calculation.

The most common mistake is treating the box instructions as universal. Flat-dose guidance assumes a weight range that may not apply to you. A 45 kg adult following standard adult instructions receives the same dose as a 90 kg adult, but proportionally twice the mg-per-kg exposure. For low-weight adults and older teenagers still in adult dosing territory, this can tip exposure closer to the threshold where liver enzymes begin to be affected.

The second mistake is not accounting for paracetamol in combination products. Cold and flu sachets, some prescription painkillers, and branded combination tablets frequently contain 325 mg to 500 mg of paracetamol per dose. Someone taking two standard paracetamol tablets and a cold remedy sachet simultaneously can easily exceed 2,000 mg in a single sitting without realising it. Checking the active ingredients on every concurrent product is not optional - it is the single step that prevents most accidental overdoses.

A subtler mistake specific to this calculator: using the weight in pounds without changing the unit selector. Entering 154 in the kg field instead of switching to lbs gives a calculated dose of over 2,000 mg per dose - more than double the correct amount. The unit selector is the first field for a reason.

The core formula is: Single Dose (mg) = Weight (kg) x 15 mg/kg, capped at the age-appropriate maximum.

For a 32 kg child in the 6 to 11 age group: 32 x 15 = 480 mg. The cap for this group is 750 mg, so 480 mg is the dose. The daily maximum uses the separate formula: Daily Max (mg) = Weight (kg) x 60 mg/kg, capped at the group ceiling. So 32 x 60 = 1,920 mg, well below the 3,000 mg group cap, making 1,920 mg the daily limit.

For an adult at 75 kg: 75 x 15 = 1,125 mg, but the adult single-dose cap is 1,000 mg. The dose is therefore 1,000 mg. Daily maximum is 4,000 mg for healthy adults, or 2,000 mg if liver function is reduced. The dose rounding step - rounding to the nearest 10 mg - reflects that liquid formulations come in set concentrations (typically 120 mg/5 ml or 250 mg/5 ml) and tablets come in 500 mg units. An exact decimal dose is not measurable in practice.

The 15 mg/kg dose target assumes normal hepatic first-pass metabolism, which is precisely where the formula breaks down at the edges. In children under 3 months, glucuronidation - the primary clearance pathway for paracetamol in older patients - is immature, shifting reliance to sulphation. This changes both the effective dose and the overdose threshold, which is why the under-3-month exclusion is absolute rather than weight-adjusted. In adults with chronic alcohol use, induction of CYP2E1 increases production of NAPQI, the toxic metabolite, meaning the liver becomes more vulnerable to damage at doses well within the standard daily cap. The 2,000 mg ceiling for liver-risk patients is a conservative clinical floor, not a precise individual threshold.