How the deck stain calculator works
Coverage by product type
Transparent sealers (~250 sq ft/gal) show grain and need annual renewal; semi-transparent stains (~150) add pigment that blocks UV and last 2–3 years; solid stains (~100) behave like thin paint, hiding grain and lasting longest. The product choice is a maintenance-frequency decision wearing a color card.
The surfaces that decide rot
Board ends, undersides, and rail tops absorb water fastest and gray first — yet most DIY staining covers only the walking surface. Back-priming (staining all sides before installation) and end-sealing cuts is the difference between a deck that lasts 20 years and one that cups and splinters in 8. The calculator counts the hidden surfaces on purpose.
New wood wants to drink
Kiln-dried PT lumber arrives thirsty: the first coat penetrates deeply and often looks "done" — then fades in a season because there was no second coat to seal. Two coats on new wood (wet-on-wet where the product allows) is the standard; the calculator's wood-condition multiplier encodes exactly this.