How to Shock a Pool

A raised pool.

The process to shock a pool is usually a last resort. Over the course of time, chemical imbalances, environmental conditions, overuse, operational deficiencies (due to an inefficient filtration system, for example) can all contribute to getting your pool out of balance. Testing and adding base or acidic agents are more like fine-tuning an engine. But occasionally, you’re going to need an overhaul. Next to replacing your engine (i.e. emptying and replacing your entire pool water), the most drastic measure you can take is to shock your pool. This should occur once you’ve taken all other precautionary measures, including checking out your filtration system.

Shocking Your Pool

When you check the chlorine, level you may find that it’s adequate. But actually, the chlorine has attached itself to the contaminants in the water (“combined chlorine”) and can do no more sanitizing. Your “free” chlorine (the molecules that are still able to neutralize the contaminants) is not available. This is where shocking plays a role.

Adding more chlorine raises the level of the “free” chlorine which will attack the contaminants.

A shock dosage is about 2 lbs per 10,000 gallons of water for calcium hypochlorite (powdered shock), or 3 gallons per 10,000 gallons of water for sodium hypochlorite.

If many swimmers have been in the pool, you should do this at least every other week. The proper balance for pool water is between 7.2 and 7.8 pH, but this range is actually a bit narrower during shock treatment. You should aim for a pH level between 7.4 and 7.6. Over 7.6 pH and the chlorine will not be as effective. Lower than 7.4 and the chlorine becomes over-active. The goal is to get your total alkalinity within the proper range.

There are many over the shelf “shock treatment” products available through pool stores, home improvement centers, and hardware stores.

With regard to how much to use, it’s critical to know the volume of your pool. You may not know or have forgotten, but volume determines how much shock treatment your pool should get.

Determining the Size of Your Pool

Here are some simple formulas to help you calculate the volume of your pool.

Rectangular pool: Volume = Length x Width x Average depth

Oval pool: Volume = Width (radius from center of pool to shortest side) x Length (radius from center of pool to longest side) x 3.14 x Average depth.

Circular pool: Volume = Radius (from center of pool to the side) x 2 x Average depth.

Long Oval (Rectangular pool with rounded corners): Calculate this in sections. Imagine a squared off rectangle within the oval, and measure the length and width (Length x Width = Area).Next measure from where the rectangle “ended” at either side of the pool’s length. Measure from that imaginary line to the edge of the pool. Essentially you’re creating an imaginary semi-circle at either end of the pool, where the rectangle ended, and then you measure the radius of a semi-circle. Add these two figures together and multiply by 3.14 (Radius x Radius x 3.14). Combine the rectangular and semi-circular results together and multiply by the average depth of the pool to get the volume. The equation would look like this: (Length x Width) x (Radius x Radius x 3.14) x (Average Depth).

Kidney or Bean Shape: Measure the area on either side of the pool (where the kidney/bean shaped area is). Measure each area from one side of the pool to the other along the width axis. Multiply both of these widths by the entire length of the pool. Both Width’s x Length x 0.45 = Area of these pockets. Multiply this result by the average depth and that will provide the volume. The equation would look like this: A (width of kidney shaped area) + B (width of 2nd kidney shaped area) x Length of entire pool X 0.45 = Area. Area x average depth of pool = Volume.

Follow the instructions on your shock treatment kit and hopefully, you'll wind up with a balanced pool. And remember that test kits do not reveal the difference between "combined" vs. "free" chlorine.