We’ve also provided a formula you can use to calculate the number of boards you need and how to estimate fastener requirements. 17 mars 2023 · Use our deck stain calculator to determine how much stain you need based on your deck’s square footage. How to Estimate the Amount of Stain You Need for Your Deck Enter the length and width of the deck, as well as the number of coats to calculate the amount of stain needed: Length (ft): Width (ft): Stain Needed:.