Pain Relief Dose Calculator

Pain Relief Dose Calculator — evidence-based tool to check your pain relief dose. Includes formula, tips, and interpretation guide.

A medication dosage calculator covers three areas of pharmaceutical mathematics: weight-based antibiotic dosing (amoxicillin, co-amoxiclav, cefalexin, clarithromycin), analgesic dosing for paracetamol, ibuprofen, and aspirin including the aspirin contraindication check for under-16s, and pharmacokinetic timing calculations (dosing interval, time to steady state, time to elimination). All dosing ranges are sourced from the BNF for Children (2024) and FDA prescribing information. This calculator is for information only — always follow the prescribed dose. BMI Calculator and Daily Calories Calculator offer related health calculators.

Weight-based dosing is essential in paediatrics because a child's mg/kg dose may be significantly different from the adult dose once renal clearance, protein binding, and hepatic enzyme maturity are considered. For the antibiotics in this tool, adult dose caps apply when weight exceeds 40 kg — the calculator flags this automatically. Pharmacokinetic timing calculations are useful for understanding drug accumulation during a course and how quickly a drug clears after the last dose.

Note the medical disclaimer: this tool is for education only — always use the dose prescribed by the clinician.
Antibiotic Dose: enter the patient's weight and age, then select the antibiotic. The tool shows the daily dose, per-dose amount, and number of doses per day. If weight exceeds 40 kg, adult caps are shown.
Pain Relief: enter weight and age, then select the analgesic. For aspirin and a patient under 16, a Reye's syndrome warning is shown immediately — choose paracetamol or ibuprofen instead.
Drug Timing: enter the drug's half-life (from the BNF monograph or patient information leaflet), doses per day, and treatment duration. The tool calculates dosing interval, time to steady state, time to elimination, and total course doses.

Medication dosage formulas

Daily antibiotic dose = weight (kg) × mg/kg/day rate. Per-dose amount = daily dose ÷ doses per day. Adult cap applies if per-dose amount exceeds the maximum adult dose: amoxicillin 500 mg, co-amoxiclav 500 mg, cefalexin 500 mg, clarithromycin 500 mg.

Paracetamol: 15 mg/kg per dose, maximum 1000 mg/dose, maximum 4000 mg/day, every 4–6 hours. Ibuprofen: 5–10 mg/kg per dose, maximum 400 mg/dose (paediatric), 600 mg (adult), every 6–8 hours.

Dosing interval = 24 ÷ doses per day (hours). Time to steady state ≈ 4 × half-life (plasma concentration within 6% of steady state). Time to elimination ≈ 5 × half-life after last dose (<3% of dose remains). Worked example: amoxicillin half-life ~1 hour, three-times-daily → dosing interval 8 h, steady state ~4 h, elimination ~5 h after last dose.

Understanding medication dosing results

When adult caps override weight-based calculations

For a child weighing 50 kg on amoxicillin 25 mg/kg/day in 3 doses: weight-based dose = (50 × 25) ÷ 3 = 416.7 mg per dose. Since this is below the adult cap of 500 mg, no cap applies. For a 60 kg child: (60 × 25) ÷ 3 = 500 mg — exactly at the adult cap. For 80 kg: (80 × 25) ÷ 3 = 667 mg — capped at 500 mg. The tool flags the cap applied automatically.

Drug timing: a drug with a 12-hour half-life taken twice daily reaches steady state in approximately 48 hours (4 × 12 h). If the same drug is taken for 7 days, the last dose eliminates to below clinically relevant concentrations (~3%) within 60 hours (5 × 12 h) of the final dose. This matters for planning drug interactions, surgical procedures, or pregnancy testing windows.

Health tips and best practices

Always read the drug label or patient information leaflet for the specific product prescribed — licensed dose ranges and available formulations vary. Suspension strengths (e.g. amoxicillin 125 mg/5 mL vs 250 mg/5 mL) require different volumes for the same dose.
Paracetamol overdose is the most common cause of acute liver failure in the UK, and overdose can occur by exceeding the maximum daily dose across all paracetamol-containing products — including combination cold and flu remedies that contain paracetamol.
Ibuprofen should be taken with food or milk to reduce GI irritation, and is contraindicated in the third trimester of pregnancy, dehydrated children, and patients with active peptic ulcer disease.
Antibiotic courses should always be completed even if symptoms resolve early. Stopping antibiotics prematurely leaves a partially treated bacterial population that may regrow — often with increased resistance.
Drugs with very short half-lives (caffeine ~5 h, ibuprofen ~2 h) leave the body quickly; drugs with long half-lives (fluoxetine ~1–4 days) accumulate significantly and take weeks to clear. This distinction matters for managing drug interactions and stopping a course safely.
For drug timing: the steady-state calculation assumes regular dosing at equal intervals. Taking doses late or missing doses causes dips below the therapeutic concentration, especially for antibiotics where consistent drug levels are important for efficacy.

Amoxicillin is the most commonly prescribed antibiotic in UK primary care, accounting for approximately 21% of all antibiotic prescriptions. [UKHSA English Surveillance Programme, 2023]
Paracetamol is involved in approximately 40,000 hospital admissions per year in the UK — making it the most common cause of acute liver failure and deliberate self-poisoning in Europe. [Medicines and Healthcare products Regulatory Agency, 2023]
Aspirin is contraindicated under age 16 in the UK due to the risk of Reye's syndrome — a rare but potentially fatal condition causing liver and brain swelling. [MHRA, 2021 guidance]
Antimicrobial resistance is estimated to cause 700,000 deaths globally per year, rising to 10 million by 2050 if trends continue. Unnecessary antibiotic prescribing is the primary driver. [O'Neill Review on Antimicrobial Resistance]

Common mistakes to avoid

Using the weight-based formula for neonates without specialist input — neonatal pharmacokinetics differ dramatically from older children due to immature renal and hepatic function. Neonatal dosing must always be confirmed with a prescriber or pharmacist.
Applying the adult paracetamol maximum (4 g/day) to children — a 20 kg child's maximum is 15 mg/kg × 4 doses = 1200 mg/day, not 4000 mg. Using the adult figure leads to significant overdosing.
Ignoring drug-drug interactions — ibuprofen significantly reduces the antiplatelet effect of aspirin and increases bleeding risk with anticoagulants. Clarithromycin inhibits CYP3A4 and can raise plasma levels of statins and other drugs to toxic levels.
Confusing half-life with duration of action — a drug's therapeutic effect can outlast its half-life (e.g. proton pump inhibitors irreversibly bind their target; the biological effect persists after the drug has cleared).

Antibiotic and analgesic dose ranges in this calculator are sourced from the BNF for Children 2024, FDA prescribing information, and NICE guidelines. Pharmacokinetic half-life values used in the Drug Timing tab must be entered by the user from the relevant drug's data sheet or BNF monograph. This calculator is for informational and educational purposes only and does not constitute medical, pharmaceutical, or prescribing advice. Prescribing decisions must be made by a qualified clinician with access to the patient's full medical history, allergy record, concurrent medications, and renal/hepatic function. In the United Kingdom, antimicrobials are prescription-only medicines (POM). Do not obtain antibiotics without a valid prescription.