A Very Important First Inch

How much water is in the first inch of your pool? Lost or gained, the capacity of that top inch (OK -- any other inch in the first few feet for that matter), is a very significant value. A sharp pool operator has it memorized or, along with area, volume and dose values, has the number painted on the pumproom wall. A leak, a fill, daily evaporation, surge flow, even kid displacement can all be quantified by knowing how many gallons reside in an observable inch of water.

Calculating it couldn’t be much easier. The pool’s area -- its length times its width for a rectangular pool -- is simply multiplied by 7.5 (giving you the gallons in the first foot) then divided by 12 to trim down to that desired first inch.

Try this in a typical, short-course high-school pool: 75 feet by 42 feet gives us 3,150 square feet of area. Multiplying that surface area by 7.5 and dividing by 12 results in a surprisingly handy value, just 30 short of 2,000 gallons. Rounding to an even two grand is reasonable, of course, as it is well within two percent of dead on. As a matter of fact, we routinely use 2,000 gallons for pools from 40 to 45 feet wide; precision just isn’t necessary. Each extra foot of width adds just 40 gallons to the total.

Design engineers like to average us all out to a displacement value of two cubic feet, or 15 gallons for each immersed swimmer no matter how under or overfed. Get 100 of us in your 25-yard-pool’s water and we’ve ostensibly ousted 1,500 gallons. That’s about three-quarters of an inch, recalling the fact that 2,000 gallons is an inch’s worth. It either raises the water level by that much (in a skimmer pool) or it dumps that much in the surge pit in a gutter pool.

That amount of static surge will half fill the surge pit if the gutter is not big enough to hold much of the surge itself. As surge pits are often sized at a minimum of one gallon for each square foot of pool surface area, you can see that a well-loaded pool could easily fill the pit, causing a backup of the gutters as the surge pit fills. Coaches don’t like this; wave quelling is no longer happening.

How about hot whirlpools or spas? Another easy number to remember -- this time for a ten-foot-diameter circular vessel -- is 50 gallons per inch. (Check it out: (p r2 x 7.5) / 12 = gallons per inch in any circular spa.) Divide that by our 15-gallon body size and we see that just over three and one-quarter average bodies (a quarter person?) would elevate the surface of the ten-footer an inch, and ten soakers would displace three inches worth. In a smaller spa, a crowd like this will quickly overflow the spa and get them all in trouble!

Besides human displacement, leaks and other losses are likewise easy to estimate, as long as evaporation is taken into account by measuring equal-time deficits in a floating bucket.

Figure your first inch. You’ll soon be able to estimate pool capacities, without a pencil, in a flash. And you’ll impress your patrons and your boss as well!