Water-to-grain ratio — also called mash thickness — is the volume of strike water used per unit weight of crushed grain, typically expressed in L/kg or qt/lb. It's one of the handful of numbers, alongside grain weight and target temperature, that a strike-water calculation genuinely needs to get right.
The number that decides your strike water temperature, not just your mash's texture
Because grain and water absorb and release heat differently, the same target mash temperature calls for a different strike-water temperature depending on this ratio — a thicker mash needs hotter strike water than a thinner one to land at an identical target rest temperature. Run your own grain weight, starting grain temperature, target mash temperature, and this ratio through the strike water calculator rather than reusing a strike temperature from a previous recipe at a different ratio.
Thin and thick mashes each have real, situational advantages
A thinner ratio generally lauters faster and marginally favors better fermentability; a thicker ratio holds heat more efficiently and is the traditional starting-rest choice for a decoction mash, where a portion gets pulled and boiled separately. Neither is universally correct — the right ratio depends on equipment, technique, and which of those trade-offs matters more for a given recipe.