Make a buttermilk substitute in five minutes with milk and an acid when you have none on hand.
Put 1 tablespoon of lemon juice or white vinegar into a liquid measuring cup. This is the ratio for every 1 cup of buttermilk you need.
Pour in whole or 2% milk until the total reaches 1 cup. Whole milk gives the closest texture; in a pinch, lower-fat milk works too.
Stir once and let it sit at room temperature for 5-10 minutes. The acid curdles the milk slightly — it will look lightly clabbered and thicken a touch.
Give it a final stir and use it anywhere a recipe calls for buttermilk — pancakes, biscuits, fried chicken brine, or cornbread. Use it the same day.