1 US customary teaspoon in metric teaspoons

Conversion

1 US customary teaspoon is equivalent to 0.98578431875 metric teaspoons.^[1]

Conversion in the opposite direction

The inverse of the conversion factor is that 1 metric teaspoon is equal to 1.01442068105529 times 1 US customary teaspoon.

It can also be expressed as: 1 US customary teaspoon is equal to $\frac{1}{1.01442068105529}$ metric teaspoons.

Approximation

An approximate numerical result would be: one US customary teaspoon is about zero point nine nine metric teaspoons, or alternatively, a metric teaspoon is about one point zero one times one US customary teaspoon.

Units involved

This is how the units in this conversion are defined:

Us Customary Teaspoons

"In the United States one teaspoon as a unit of culinary measure is 1⁄3 tablespoon, that is, 4.92892159375 ml; it is exactly 1 1⁄3 US fluid drams, 1⁄6 US fl oz, 1⁄48 US cup, and 1⁄768 US liquid gallon and 77⁄256 or 0.30078125 cubic inches. For nutritional labeling on food packages in the US, the teaspoon is defined as precisely 5 ml."

Wikipedia page of US customary teaspoons

Metric Teaspoons

"A teaspoon is an item of cutlery, a measuring instrument, of approximately 5ml, or a unit of measurement of volume (usually abbreviated tsp.). The metric teaspoon as a unit of culinary measure is 5 mL,[12] equivalent to 5 cm3; that is, 1⁄3 UK/Canadian metric tablespoon, or 1⁄4 Australian metric tablespoon."

Wikipedia page of metric teaspoons

Footnotes

[1] The precision is 15 significant digits (fourteen digits to the right of the decimal point).

Results may contain small errors due to the use of floating point arithmetic.