The standard recipe for saline solution is 9 grams of salt per liter of water.
Since most people don’t have an accurate scale, we need to convert the nine grams to a volume. For that we need the bulk density of table salt. For fine table salt, the listed bulk density in 1201 kg/m³ or 1,201 g/cm³.
So we need 9/1.201 ≈ 7.5 cm³ fine table salt per liter of water. A “teaspoon”-size measuring cup is 5 ml, so it’s 1½ teaspoon per liter of water.
Most metric measuring spoon sets also have a 1 ml cup. So alternatively, one can use 3 ml of fine table salt in 400 ml of water.