Co-trimoxazole (Septran / Bactrim) Pediatric Dose Calculator
Weight-based co-trimoxazole dosing for children — 8 mg/kg/day in 2 divided doses · max 160 mg/dose. Enter age and weight for the dose in mg and syrup mL or tablets.
Pediatric dosing
Urinary tract infection
Acute otitis media (second line)
PCP treatment or prophylaxis Specialist / refer
Co-trimoxazole (trimethoprim–sulfamethoxazole) dose chart by age and weight
Urinary tract infection. Weights are WHO/IAP reference medians for age — always dose on the measured weight.
| Age | Weight | Dose (mg) | TMP 40 mg / SMX 200 mg per 5 mL | Frequency |
|---|---|---|---|---|
| 3 months | 6.1 kg | 24.4 mg | 3.1 mL | Every 12 hours |
| 6 months | 7.6 kg | 30.4 mg | 3.8 mL | Every 12 hours |
| 9 months | 8.6 kg | 34.4 mg | 4.3 mL | Every 12 hours |
| 1 year | 9.3 kg | 37.2 mg | 4.7 mL | Every 12 hours |
| 18 months | 10.6 kg | 42.4 mg | 5.3 mL | Every 12 hours |
| 2 years | 11.8 kg | 47.2 mg | 5.9 mL | Every 12 hours |
| 3 years | 14.1 kg | 56.4 mg | 7.1 mL | Every 12 hours |
| 4 years | 16.2 kg | 64.8 mg | 8.1 mL | Every 12 hours |
| 5 years | 17.5 kg | 70 mg | 8.8 mL | Every 12 hours |
| 6 years | 19 kg | 76 mg | 9.5 mL | Every 12 hours |
| 8 years | 24.4 kg | 97.6 mg | 12 mL | Every 12 hours |
| 10 years | 31.1 kg | 124.4 mg | 15.5 mL | Every 12 hours |
| 12 years | 39.4 kg | 157.6 mg | 19.5 mL | Every 12 hours |
Strengths in the calculator
- TMP 40 mg / SMX 200 mg per 5 mL (Oral)
- Paediatric tablet TMP 20 mg / SMX 100 mg (Oral)
- Single-strength tablet TMP 80 mg / SMX 400 mg (Oral)
- Double-strength (DS/forte) tablet TMP 160 mg / SMX 800 mg (Oral)
Different strength on your bottle? Use “Different strength on the bottle?” under the result.
Safety
Contraindications and cautions
- Infants under 6 weeks (kernicterus risk) — this calculator starts at 2 months.
- G6PD deficiency, sulfonamide allergy, megaloblastic anaemia, severe renal or hepatic impairment.
Pregnancy and breastfeeding
Legacy FDA category D. Avoid in the first trimester (folate antagonist) and near term (kernicterus risk) when possible.
Avoid in preterm, jaundiced or G6PD-deficient infants.
Clinical notes
Dose on trimethoprim only — never on the combined TMP+SMX strength. Stop at the first sign of rash (Stevens–Johnson syndrome risk). Maintain good fluid intake.
Adult and adolescent (≥40 kg) dose
160 mg — Every 12 hours (max 160 mg per dose).
Adult values refer to trimethoprim mg (one DS tablet = 160 mg TMP + 800 mg SMX), not the combined 960 mg.
Other names and brands
Also known as Trimethoprim-sulfamethoxazole, TMP-SMX, TMP/SMX, Cotrimoxazole.
- India: Septran, Bactrim
- USA: Bactrim, Septra
- UK: Septrin
Brand examples only — products and strengths vary by country. Always check the label.
Frequently asked questions
What is the co-trimoxazole dose for children?
Urinary tract infection: 8 mg/kg/day in 2 divided doses · max 160 mg/dose. Usual duration: 7–10 days (3–5 days for uncomplicated cystitis in older children). Always use the child's measured weight and check the strength on the bottle.
How many mL of TMP 40 mg / SMX 200 mg per 5 mL for an 11.8 kg child?
For urinary tract infection, an 11.8 kg child (about 2 years) needs 47.2 mg per dose — 5.9 mL of TMP 40 mg / SMX 200 mg per 5 mL, every 12 hours. Recalculate with the child's actual weight.
From what age can co-trimoxazole be given?
Age range in PediaRx: ≥2 months – adult. The calculator does not calculate a dose below 2 months. Each indication can have its own age and weight limits, shown with its regimen.
Is co-trimoxazole safe in pregnancy?
Legacy FDA category D. Avoid in the first trimester (folate antagonist) and near term (kernicterus risk) when possible.
Sources and editorial process
- Sulfamethoxazole and trimethoprim oral suspension — DailyMed
- BNF for Children — Co-trimoxazole — NICE / BMJ Group / Royal Pharmaceutical Society
Dosing data last updated 2026-09-27. PediaRx is built and maintained by Dr. Shounak Pal, MBBS (Medical Officer, Government of West Bengal, India — founder and developer of PediaRx). Every regimen is encoded with explicit units, maximum doses and age/weight limits and is checked by automated tests before publication. Report an error: hello@pediarx.com.
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Clinical disclaimer: reference tool for qualified health professionals. Verify every dose against current guidance and the product label before prescribing.
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