Lemon juice and lemon zest do different jobs

Lemon juice adds liquid and tartness. Zest adds the aroma of the outer peel. They are useful in different ways, so replacing one with the other can change a recipe more than you expect.

For a ginger drink, add juice in small measured amounts. For a dressing or yogurt sauce, a little finely grated zest can give lemon character without adding as much liquid. Avoid grating deeply into the white pith when you want a cleaner peel aroma.

If you plan to use both, wash the lemon and zest it before squeezing; a whole fruit is easier to hold. Start with less zest than the recipe's juice quantity suggests. A teaspoon of zest is not the flavor equivalent of a teaspoon of juice.