It's a good idea to have seven limes on hand for this recipe, but you may not need all of them.

Select 3 limes with fewest blemishes and scrub well. Rinse and dry thoroughly. Using vegetable peeler, remove zest of washed limes in strips. Transfer zest to bowl and add sugar and salt. Toss to combine. Using potato masher, mash zest, sugar, and salt vigorously until mixture is very fragrant, about 2 minutes. Add hot water and stir until sugar is dissolved, about 1 minute. Set aside until cool, about 30 minutes. While mixture cools, juice enough limes to yield ¾ cup juice.

Stir juice into zest mixture until combined. Strain through fine-mesh strainer set over 2-cup liquid measuring cup; discard zest. Refrigerate until chilled, about 1 hour. (If not using concentrate right away, transfer to airtight container and refrigerate for up to 2 weeks.)

TO MAKE LIMEADE: For single serving, pour ½ cup cold water and ½ cup concentrate over ice and stir gently to combine; if necessary, dilute with additional water to taste. For 4 servings, fill pitcher with ice, add 2 cups cold water and all of concentrate, and stir to combine; if necessary, dilute with additional water to taste. For sparkling limeade, substitute plain seltzer for cold water.