How much boiling water raises a 6 kg mash in 16 L from 62 to 68°C?

The infusion equation treats the mash as thermal mass: dry grain counts as 0.41 L of water equivalent per kilogram.

Thermal mass: 0.41 × 6 + 16 = 18.5 L equivalent.
With 98°C water: (68 − 62) × 18.5 ÷ (98 − 68) = 3.7 L.

Stir while adding and confirm with a probe: real tuns eat a degree into the vessel walls, so landing half a degree low is normal, and a small top-up corrects it. The new total of about 19.7 L should still fit the tun with headroom before you commit to a mashout step, which takes far more water than people expect.