Mash thickness — the ratio of water to grain during the mash — has a real but secondary effect on fermentability and extraction, and most homebrewing recipes cluster in a fairly narrow practical range rather than varying wildly batch to batch.
The typical working range
1.25-1.5 qt/lb (roughly 2.6-3.1 L/kg) covers the great majority of single-infusion homebrew mashes — thick enough to keep the mash tun's thermal mass and heat loss manageable, thin enough to stir easily and lauter without difficulty. Most homebrew recipe software defaults somewhere in this band unless a recipe specifically calls for something different.
What moves outside that range, and why you'd do it
A thicker mash (below roughly 1.25 qt/lb) concentrates enzymes and can modestly favour certain enzymatic reactions, and is sometimes used deliberately for a decoction mash or when working with a particularly starch-heavy adjunct-rich grain bill. A thinner mash (above roughly 2.0 qt/lb) gives enzymes more room to work and is sometimes used for a highly fermentable, driest-possible result, or simply because a given system's equipment doesn't comfortably support a thicker ratio. Neither extreme is wrong — they're deliberate choices for a specific outcome, not a mistake to correct.
What thickness does to your strike-water calculation
For 5.44 kg (12 lb) of grain at 68°F (20°C) targeting a 152°F (66.7°C) mash: a thick 1.25 qt/lb ratio needs 74.1°C strike water; a typical 1.5 qt/lb needs 72.9°C; a thin 2.0 qt/lb needs 71.3°C. Check any of these directly — thicker mashes consistently need hotter strike water for the same target, because there's less water relative to grain to carry the same total heat.
The trade-off that matters more than the small fermentability effect
In practice, mash thickness's effect on fermentability is real but modest compared to mash temperature, which is the dominant lever — see mash temperature and body for the bigger effect. Where thickness matters more directly is in strike-water logistics: a very thick mash needs a hotter, smaller-volume strike addition (easier on a system with limited kettle capacity, but requiring careful temperature control since a small volume cools and heats faster); a very thin mash needs a larger, cooler strike addition (more forgiving of small temperature misses, but needing a bigger vessel).
Picking a number for your own system
Unless a specific recipe or technique calls for something else, 1.25-1.5 qt/lb (2.6-3.1 L/kg) is a safe, well-tested default — plug it into the strike water calculator alongside your actual grain weight and starting temperature, and adjust from there based on how your specific system's stirring, lautering and thermal behaviour respond.