This webpage will calculate water usage for batch sparging. Enter your numbers for grain bill, desired pre-boil wort total, mash ratio for mash thickness, grain absorption rate, and mash/lauter tun deadspace if any. The calculator will compute your required mash water, grain absorption, runoff totals, mash infusion (*see below), and total required water. For detailed information on batch sparging, see these two links: Denny Conn and Ken Schwartz.
*Because batch sparging is ideally done with two equal runoffs, you might need to add some heated make-up water (mash infusion) to bump your first run up to 1/2 the desired pre-boil wort total. I would recommend doing this only if your calculated mash infusion is large. Otherwise a first run of 3.25 gallons and second run of 3.75 gallons should be a perfectly acceptable way to achieve 7 gallons of pre-boil wort.
