Bactrim/Septra labels give you both the sulfamethoxazole concentration and the trimethoprim concentration. For tablets, there is 400 mg of sulfamenthoxazole and 80 mg of trimethoprim in each tablet. The oral liquid is half of that, 200 mg of suflamethoxazole and 40 mg of trimethoprim per 5 mL (or 40 and 8 per mL). The injectable is 400/80 per mL. The dosing is always based on the trimethoprim component. The usual dose for peds patients is 8 mg/kg/day of the TMP, given in two divided doses. So a 5 kg kid would be ordered 40 mg TMP per day, or 20 mg per dose and the amount of oral liquid you'd give would be... 2.5 mL per dose. Make sense?