The approximate ratio by volume of flour to water in tempura batter is 1:1.
This equal-volume proportion is a useful rule of thumb for a basic tempura batter, although recipes vary according to the ingredients and the texture the cook wants. Some batters use egg, sparkling water, or a different balance of flour and liquid, so 1:1 describes an approximate standard rather than an inflexible formula.
Tempura batter is deliberately mixed lightly. Overmixing develops gluten in the wheat flour, which can make the coating heavy or chewy instead of delicate and crisp. Cold water also helps limit gluten formation and slows the batter’s reaction before it reaches the hot oil.
The batter is usually applied just before frying seafood or vegetables. The contrast between chilled, lightly mixed batter and hot oil helps create tempura’s characteristic irregular, airy crust. This is why the ratio alone does not guarantee the result: temperature, mixing, ingredient moisture, and frying time matter too.