To convert teaspoons to milliliters (tsp to mL), multiply by 4.9289216. 1 teaspoon equals 4.9289216 mL, about the same as a teaspoon.
| teaspoons | milliliters |
|---|---|
| 1 teaspoon | 4.9289216 mL |
| 2 teaspoons | 9.8578432 mL |
| 5 teaspoons | 24.644608 mL |
| 10 teaspoons | 49.289216 mL |
| 25 teaspoons | 123.22304 mL |
| 50 teaspoons | 246.44608 mL |
| 100 teaspoons | 492.89216 mL |
| 250 teaspoons | 1,232.2304 mL |
| 500 teaspoons | 2,464.4608 mL |
| 1000 teaspoons | 4,928.9216 mL |
The factor is 4.92892, which is derived rather than defined, so any decimal you see has been rounded somewhere.
For a quick estimate, multiply by 5. That is off by about 1.4%, which is fine for a rough figure and not fine for anything you are paying for or building to. Use the exact factor of 4.92892 when it matters.
The reverse factor is 0.203, so one milliliter is that many teaspoons. Two decimal places is plenty for most purposes, and three is more than a tape measure can justify.
To anchor the scale: 1 teaspoon is about 4.93 mL.