To convert milliliters to teaspoons (mL to tsp), multiply by 0.20288. 1 milliliter equals 0.20288 tsp, about 20% of a teaspoon.
| milliliters | teaspoons |
|---|---|
| 1 milliliter | 0.20288 tsp |
| 2 milliliters | 0.40577 tsp |
| 5 milliliters | 1.0144207 tsp |
| 10 milliliters | 2.0288414 tsp |
| 25 milliliters | 5.0721034 tsp |
| 50 milliliters | 10.144207 tsp |
| 100 milliliters | 20.288414 tsp |
| 250 milliliters | 50.721034 tsp |
| 500 milliliters | 101.44207 tsp |
| 1000 milliliters | 202.88414 tsp |
The factor is 0.202884, which is derived rather than defined, so any decimal you see has been rounded somewhere.
For a quick estimate, multiply by 0.2. 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 0.202884 when it matters.
Going the other way, one teaspoon is 4.929 mL, which is usually the easier direction to work in. Because the forward factor is small, watch the decimal place rather than the digits: a factor-of-ten slip is the common error, not a rounding one.
To anchor the scale: 5 milliliters is about 1.01 tsp.