Baby bottle sizing in oz and mL, and the concentration ratio
Every baby bottle I own has both oz and mL printed on it, but the two scales are almost never at the same nice round numbers on the same line. This is not sloppy — it's because 2 oz = 59 mL, not 60 mL, and manufacturers refuse to fake it. The half-milliliter differences add up.
US baby bottles show oz (fluid ounces) prominently because US formula is scooped by ounces of water. Metric bottles show mL. 1 US fl oz = 29.57 mL, so a 4 oz feeding is 118 mL, not 120. On the bottle, the 4 oz line and the 120 mL line are slightly different heights — the mL line is a hair higher.
The concentration rule
Standard US infant formula is 1 level scoop per 2 fl oz of water, measured with the scoop provided in the can. The scoop delivers a fixed mass of powder (usually 8.7 g); the concentration is what matters, not the total volume. If you're making a 6 oz bottle you use 6 oz water plus 3 scoops. NEVER 6 oz reconstituted formula — that would be under-concentrated.
Why concentration is dangerous to get wrong
Too dilute (extra water) means underweight babies and, at the extreme, hyponatremia — water intoxication that causes seizures in infants. Too concentrated means kidney load and dehydration. The 1:2 scoop:oz ratio is not approximate. Metric-labeled formula uses grams:mL ratios; different brands differ; always read the specific can.
The mixing order
Water first, then powder. If you add powder first the water displaces to more than the target volume and you end up under-concentrated. This is why every formula manufacturer prints 'water first' in bold.
Recap
oz and mL are both on the bottle but they're separate scales. Concentration ratios are exact and non-negotiable — 1 scoop per 2 oz for standard US formula.
Try the converters mentioned in this article
FAQ
Are UK oz different?+
Yes — 1 UK fl oz = 28.4 mL vs US 29.57 mL. A 6 UK oz bottle is 170 mL; a 6 US oz bottle is 177 mL. Match to your formula brand's country.
Ready-to-feed formula?+
Pre-mixed, no water added. Pour and warm. More expensive per calorie but foolproof.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-23 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.