How this instrument works
A US teaspoon holds exactly 1⁄6 of a fluid ounce by customary definition, which comes to 4.92892159375 ml once traced back through the same 231-cubic-inch gallon and 1959 inch definition that fixes every other American cooking measure. One millilitre therefore equals precisely 0.202884136211 teaspoons — an exact figure, though not a round one, since 4.92892159375 shares no clean relationship with the metric system's powers of ten.
Regulators long ago simplified this for consumer safety. US nutrition labelling defines a teaspoon at a flat 5 ml, about 1.4% larger than the precise customary value, and liquid over-the-counter medicine dosing devices moved even further: FDA guidance issued in 2011 pushed manufacturers of children's liquid medicine toward metric-only markings on dosing cups and syringes, phasing out household teaspoon and tablespoon language entirely because mismatched kitchen spoons had been linked to real dosing errors.
That regulatory history means a printed '15 ml' dose and a printed '3 teaspoons' dose are not describing quite the same volume once you compare the customary teaspoon against the rounded 5 ml figure — 15 ml is 3.04326204317 teaspoons exactly, a hair over the tidy 'three' a dosing cup's second scale might display. The gap is trivial for a single dose, but it explains why a metric-only dosing cup and a household teaspoon can disagree slightly if you ever compare them side by side.
- Type your volume into the Millilitres (ml) field; it opens at 15, a common single dose, as a reference point.
- Read your result on the US teaspoons (tsp) line, recalculated to six decimal places on every keystroke.
- For a marked dosing cup or syringe, always follow its own printed millilitre line rather than converting by eye.
- Reversing direction? Multiply your teaspoon figure by 4.92892159375 to return to millilitres exactly.
- Scaling up to tablespoons? Divide your teaspoon result by 3, since three US teaspoons make one tablespoon.
Worked example — a 15 ml dose on a medicine label
An over-the-counter liquid medicine label states a single adult dose as 15 ml, and the dosing cup that came in the box shows both a millilitre scale and an older teaspoon scale from before metric-only labelling became standard. Type 15 into Millilitres (ml) and US teaspoons (tsp) reads 3.04326204317 — just a fraction over the familiar 'three teaspoons' figure many people learned growing up.
That small gap comes from rounding, not error: the customary teaspoon used here is 4.92892159375 ml, while the older kitchen-spoon rule of thumb assumes teaspoons of exactly 5 ml. Divide 15 by 5 and you get a clean 3; divide 15 by the true customary value and you get 3.04326204317 — close enough that nobody using an actual spoon would ever notice, but different enough to explain why the two conventions never quite line up.
Questions
Is one US teaspoon exactly 5 ml?
Not by customary definition, though it is close. A precise US teaspoon holds 4.92892159375 ml, about 1.4% less than 5. Food and drug labelling regulation rounds this to exactly 5 ml for consistency on nutrition panels and medicine dosing instructions, a legal simplification rather than a change to what an actual measuring teaspoon holds. Both figures are correct within their own context — one for a kitchen drawer, one for a printed label.
Why did medicine dosing cups switch away from teaspoons?
Because household teaspoons vary significantly in actual volume depending on the spoon, and confusing a kitchen teaspoon with a tablespoon — a threefold error — has been a documented cause of accidental medication overdose in children. FDA guidance issued in 2011 encouraged manufacturers of liquid over-the-counter medicines to mark dosing cups, cups and syringes in millilitres only, moving dosing instructions toward a single, unambiguous metric scale rather than household spoon language.
How many teaspoons are in a tablespoon?
Exactly three, since a US tablespoon is defined as three teaspoons by customary measure — 14.78676478125 ml divided by 4.92892159375 ml comes out to exactly 3.0 with no remainder. That clean ratio holds regardless of which rounded labelling figure — 5 ml or 15 ml — you compare instead, since both are scaled the same threefold way for nutrition-label purposes.
Is an Australian teaspoon the same as a US teaspoon?
Broadly yes for teaspoons specifically — Australia also uses a 5 ml teaspoon under its metric measuring system — but the difference shows up one step higher: an Australian tablespoon holds four teaspoons, 20 ml, rather than the US and UK convention of three teaspoons, 15 ml. A recipe using Australian tablespoons but a shared teaspoon size can therefore mismatch badly at the tablespoon level even though the teaspoon itself lines up closely.
Why does this converter show so many decimal places?
Because every digit is exact rather than estimated. One US teaspoon is defined as precisely one sixth of a US fluid ounce, and since that ounce traces cleanly back to a 231-cubic-inch gallon and a 25.4 mm inch, the chain terminates at 4.92892159375 ml with nothing approximated along the way. Round the displayed answer to whatever a real spoon or dosing device can actually measure — the underlying number itself carries no hidden error to inherit.
Should I ever measure liquid medicine with a kitchen teaspoon?
It is best avoided where a proper dosing device is available. Ordinary kitchen cutlery varies in actual capacity from one spoon to another, sometimes by a wide margin, which is precisely why regulators pushed toward marked, millilitre-only dosing cups and oral syringes for anything given to children. This page converts between the two scales for reference and label-reading purposes, not as a recommendation to substitute a household spoon for a proper measuring device.