MTX (Methotrexate) Dose Calculator

This calculator turns a prescribed weekly methotrexate dose given in mg per square metre of body surface area into a dose in milligrams, and shows how many 2.5 mg tablets that is. It is intended for BSA-based paediatric dosing in non-cancer conditions such as juvenile idiopathic arthritis.

How the dose is calculated

1. Body surface area (BSA) is estimated with the Mosteller formula: BSA (m²) = √(height in cm × weight in kg ÷ 3600).
2. Weekly dose = prescribed dose (mg/m²) × BSA.
3. The dose is rounded to the nearest 2.5 mg, since tablets come in 2.5 mg strength, and is capped at 25 mg per week, the usual maximum.
4. If you enter the strength of an oral solution, the volume is dose (mg) ÷ strength (mg/mL).

If the calculated dose is under 1.25 mg (less than half of one tablet), the calculator shows the unrounded figure and warns you that tablets can’t deliver it accurately.

Usual dose ranges

UK shared-care guidance for paediatric rheumatology gives 10–15 mg/m² once weekly initially in juvenile idiopathic arthritis and related conditions, increased if necessary to a maximum of 25 mg/m² once weekly, adjusted by the specialist according to response. International consensus recommendations for juvenile idiopathic arthritis also suggest 10–15 mg/m² per week, with the subcutaneous route preferred at the higher end because of better absorption. Other conditions (for example Crohn’s disease or psoriasis) have their own dosing, so follow the specialist’s prescription.

Safety points for prescribers and pharmacists

• Once weekly only. Write the day of the week in full on the prescription and label. Avoid weekend or Monday administration only if your local protocol says so.
• Tablet strength: in the UK only 2.5 mg tablets should be prescribed, in multiples of 2.5 mg, because 10 mg tablets have been involved in dosing errors.
• Folic acid is usually co-prescribed to reduce gastrointestinal and blood-count side effects; schedules differ between centres.
• Monitoring: full blood count, kidney and liver function are checked regularly, more often when starting or changing the dose.
• Interactions: avoid trimethoprim and co-trimoxazole. Some other medicines, including NSAIDs at high dose, need care.
• Handling: carers should wear gloves, and pregnant people should avoid handling the tablets.

Frequently asked questions

How often is methotrexate taken?

For arthritis and related conditions it is taken once a week, on the same day each week. Taking it daily by mistake can be very dangerous.

What is the usual paediatric dose?

Commonly 10–15 mg/m² once weekly to start, increased if needed to a usual maximum of 25 mg/m² per week, as directed by a specialist.

Why does the calculator round to 2.5 mg?

Tablets come in 2.5 mg strength, so a practical dose is a multiple of 2.5 mg.

Why is the dose capped at 25 mg a week?

That is the usual adult maximum for these conditions. Larger children can calculate above it on a BSA basis, so the calculator caps the result and tells you when it has done so.

Can this be used for cancer chemotherapy?

No. Cancer regimens use very different and much higher doses and schedules. This calculator is for low-dose weekly use only.

Medical disclaimer

Sources & references

• Health and Social Care Northern Ireland. Shared care guidance: Methotrexate oral — paediatric and adolescent. ipnsm.hscni.net
• Consensus recommendations on the use of methotrexate in juvenile idiopathic arthritis. Pediatric Rheumatology, 2018. pmc.ncbi.nlm.nih.gov
• NICE. British National Formulary for Children (BNFc) — methotrexate monograph. bnfc.nice.org.uk
• Mosteller RD. “Simplified calculation of body-surface area.” New England Journal of Medicine, 1987;317:1098.