Work out how much stain, oil, varnish, or topcoat you need for your project before you head to the store.
Coverage rates are approximate — actual consumption varies with surface porosity, application method (brush vs. wipe vs. spray), and temperature. End-grain absorbs 2–3× more finish than face grain; always apply a seal coat first.
As a rule of thumb: buy one size up from your calculated need. A quart covers most small to medium projects; a gallon is worth it if you're finishing multiple pieces or need 3+ coats on a large surface.