Where the 4.93 Comes From: 1/768 of a Wine Cylinder
The US teaspoon's chain is the same as every US customary volume unit: the gallon. 1 US gallon = 231 cubic inches = 128 fluid ounces. 1 fluid ounce = 2 tablespoons = 6 teaspoons. So 1 teaspoon = 1/768 of a gallon = 231/768 = 0.30078125 cubic inches. Multiply by 16.387064 (the exact cubic cm per cubic inch after the 1959 inch definition) = 4.92892159 cm³ = 4.92892159 mL. The chain is long —gallon to quart to pint to cup to fluid ounce to tablespoon to teaspoon —but every link is exact. The teaspoon is the finest link in the US customary volume chain. Below it, there are only fractions: 1/8 tsp ("a pinch"), 1/16 tsp ("a dash"), and 1/32 tsp ("a smidgen") —terms that measuring-spoon manufacturers print on spoons but that have no legal definition.
The metric teaspoon (5 mL) is simpler: a round number, adopted by the UK, Canada, South Africa, Australia, New Zealand, and most of the metric world. It is not legally defined in metric law —the metric system recognizes only the liter and its SI prefixes. The "metric teaspoon" is a customary accommodation: an informal unit that governments allow on measuring spoons and in cookbooks because banning it would be unenforceable and unpopular. The Australian metric teaspoon (5 mL) and the Australian metric tablespoon (20 mL) form a clean 1:4 ratio. The US teaspoon (4.9289 mL) and US tablespoon (14.7868 mL) form a clean 1:3 ratio. Each system is internally consistent. The conflict occurs when the two systems meet —which they do every time an American cook uses a European recipe or a Canadian pharmacist fills a US prescription written in "teaspoon" doses.
Baking Powder: The Teaspoon That Deflates a Cake
Baking powder is the ingredient where teaspoon precision matters most. A standard American cake recipe calls for 1 to 2 teaspoons of baking powder. 2 US teaspoons = 9.86 mL of baking powder. 2 metric teaspoons = 10 mL. The difference is 0.14 mL —about one-quarter of a 1/4-teaspoon measure. In a cake with 2 cups of flour, that 1.4% difference in leavening agent is below the threshold of detection. But in a recipe scaled to commercial quantities —200 cakes, 400 teaspoons of baking powder —the difference accumulates to 28 mL (nearly 6 US teaspoons) of extra baking powder. At industrial scale, the 1.4% discrepancy changes the product's rise, crumb texture, and the CO₂release rate during baking. Commercial bakeries bypass the teaspoon entirely: all leavening agents are weighed in grams. The teaspoon is a home-cook's unit. The gram is the industrial unit. The scale eliminated the 1.4% problem by eliminating the spoon.
mL = tsp (US) × 4.92892159 | mL = tsp (metric) × 5
Common Teaspoons to mL
| US tsp | mL (exact) | Kitchen context |
|---|---|---|
| 1/4 tsp | 1.232 mL | "A pinch" in older recipes. The smallest mark on most US measuring spoon sets. Salt for a single portion. |
| 1/2 tsp | 2.464 mL | Baking soda for a small batch. The half-teaspoon line. Vanilla extract for cookies. |
| 3/4 tsp | 3.697 mL | Common in spice blends: "3/4 tsp cinnamon, 1/4 tsp nutmeg." The teaspoon that requires dirtying two spoons. |
| 1 tsp | 4.929 mL | The definition. Salt for pasta water. Baking powder for a standard cake. Vanilla for a batch of brownies. |
| 2 tsp | 9.858 mL | Baking powder for a layer cake. Two teaspoons is 2/3 of a tablespoon —the conversion that trips cooks mid-recipe. |
| 3 tsp | 14.787 mL | One US tablespoon. The bridge between the two spoon units. A recipe that says "1 tbsp OR 3 tsp" is mathematically correct. |
Frequently Asked Questions
Why does my measuring spoon set say 1 tsp = 5 mL?
Because most measuring spoon sets sold in the US are manufactured to dual-standard tolerances: the spoon's volume must land between 4.93 mL (US legal definition) and 5.0 mL (metric rounding). The spoon maker stamps "1 tsp / 5 mL" on the handle because it's easier to sell one spoon set globally than to manufacture a US-only set and a metric-only set. The spoon itself is usually closer to 5 mL than 4.93 mL —because it's simpler to QC a round-number mold dimension. The spoon says 1 tsp. It delivers roughly 5 mL. The US definition says 4.93 mL. All three statements coexist on the same utensil. Nobody is being lied to. The spoon is just a more honest device than its label.
How do I convert 2.5 mL to teaspoons for a child's medicine?
2.5 mL = 0.507 US teaspoons. Do not use this conversion. Use the 2.5 mL marking on the oral syringe. The syringe is the primary measurement. The teaspoon equivalent is for the parent who lost the syringe and is trying to approximate with a measuring spoon under duress. In that situation: 2.5 mL is slightly more than 1/2 US teaspoon (2.464 mL). The difference is 0.036 mL —about 0.7% of a teaspoon. For a 12-pound infant receiving a weight-based dose, this error is clinically trivial. What matters more: the measuring spoon must be clean, leveled, and confirmed to be a US measuring teaspoon, not a flatware spoon. Better yet: keep the syringe. Tape it to the medicine bottle. Buy a spare. The syringe costs $0.15 to manufacture and eliminates the teaspoon conversion problem entirely. Pediatric dosing should never involve the word "teaspoon" —only "milliliter." For the reverse direction and more on pediatric dosing, see mL to teaspoons.
Engineering Context
The US teaspoon is 1/768 US gallon = 0.30078125 in³ = 4.92892159 cm³. Every digit in that value is exact —derived from the 231 in³ gallon and the 2.54 cm/inch definitions, both fixed by international treaty. The metric teaspoon (5 mL) is not a legal metrology unit —it is an informal accommodation recognized by custom but absent from the SI brochure. In regulated industries (pharmaceuticals, infant formula, laboratory reagents), the teaspoon is never used as a primary measurement —mL is the only valid unit. The teaspoon survives in the home kitchen and the cultural memory of recipe writing. Its 1.4% error relative to the metric teaspoon is a metrological curiosity that matters most at the extremes: infant dosing (where 0.5 mL increments matter) and industrial scaling (where 1.4% of a ton of baking powder is 14 kg of extra leavening). For related converters: mL to tsp, tbsp to mL, mL to tbsp, cups to mL, and the Volume & Cooking Guide.