Pool School

Pool Math

PoolMathhttp://www.troublefreepool.com/calc.html, is a handy web-based tool that does all of the calculations required to maintain your pool. PoolMath can figure out the amounts of chemicals to add, what appropriate FC levels are for your CYA level, what your calcite saturation index is, and help you estimate your pool volume. It is also possible to make a local copy of the PoolMath web page, and use it without an Internet connection.

To figure out the amount of a chemical to use, enter your pool volume in the orange “Size” field near the top, enter your current test results in the “Now” column and your desired levels in the “Goal” column, then hit tab, or press the “Calculate” button, to see what you need to do to get to your goal levels.

For every level except FC, it is usually best to approach your goal level gradually, instead of adding the full amount all at once. This is especially important with CH and CYA, which are much easier to raise than to lower: be careful not to add too much. There will always be small errors in your pool size, test numbers, and chemical measurements, which will result in levels coming out slightly different from what PoolMath says.

To calculate the number of gallons in the pool, look for the “Estimating pool volume” section, towards the bottom of the PoolMath page. That calculation requires you to enter an average width, length, depth, and basic shape. If the width, length, or depth of your pool varies, you need to estimate the average number. If your shape is not listed, you need to either divide the pool up into sections that match one of the available shapes or select the closest available shape.

It is common for a pool to have more than one depth. If you have a shallow end and a deep end, you can measure each and average the two numbers to get an approximate average depth. For irregularly shaped pools the same thing can be done for the width and length: measure in a couple of places and average the results. For L shaped pools, you can divide the pool into two rectangular sections, calculate the volume of each, and total the two volumes.

PoolMath can suggest appropriate FC levels based on your current CYA level. Enter your current CYA level in the “Now” column of the yellow CYA row and press tab or click on Calculate. PoolMath will then suggest appropriate FC levels in the blue “Suggested FC levels” section towards the bottom.

The “SWG” level is appropriate for day to day use with a salt water chlorine generator. The “Normal” range is suitable for day to day use when using other sources of chlorine. Be sure to keep the pool’s FC level at or above the SWG level, or lower end of the normal range, as appropriate. “Shock” level is for when CC is greater than 0.5 or you have algae. The higher “Mustard Algae Shock” level is sometimes required when fighting persistent mustard algae.

PoolMath can also calculate your calcite saturation index (CSI). Enter all of your available test results and look in the green CSI row, towards the bottom, for your “Now” and “Goal” CSI numbers.

PoolMath takes care of all of the math required for pool maintenance, leaving you more time to swim.

Here is a detailed example, showing how to adjust levels with PoolMath. Let’s say that you have a 16′ x 34′ rectangular vinyl sports pool with 3′, 5′ and 3′ depths, you chlorinate with bleach, and your current test results are:

  • FC 0
  • PH 7.4
  • TA 60
  • CH 30
  • CYA 0

First, you need to calculate the number of gallons in the pool. Look for the “Estimating pool volume” section, towards the bottom of PoolMath page. That calculation requires an average depth, which in this case is going to be the average of 3, 5, and 3, which is about 3.67. Enter the width, 16, length, 34, the average depth, 3.67, and select rectangular from the popup menu. That gives you an estimate of 14,900 gallons. So we enter 14900 (Do not insert the comma) gallons in the orange “Size” box near the top of the page.

Now lookup the recommended levels for this pool in this article at Pool School. For a vinyl pool using bleach the recommended levels are:

  • FC 3-7
  • PH 7.5-7.8
  • TA 50-90+
  • CH 50-350
  • CYA 30-50

Enter your current test results in the “Now” column. Then you need to select a goal level from the ranges suggested. Let’s pick a goal of:

  • FC 6
  • PH 7.5
  • TA 80
  • CH 60
  • CYA 40

to start with and enter those numbers in the “Goal” column.

PoolMath will then say to:

  • Add 185 oz of 6% bleach.
  • Add 9.2 oz of borax.
  • Add 70 oz of baking soda.
  • Add 88 oz of calcium chloride dihydrate.
  • Add 80 oz of stabilizer.

The full amount of bleach should be added at one time. The PH change is small and near the low end of the recommended range, so it doesn’t matter if you go a little over. Therefor the full amount of borax can also be added at once. For TA, you want to be more cautious. 70 oz of baking soda is a bit more than 4 lbs, and baking soda happens to come in 4 lb boxes, so add 4 lbs of baking soda. CYA is very difficult to lower, so start with 3 or 4 lbs instead of the 5 lbs calculated. The CH level should not be raised on the same day you raise PH or TA, so leave the calcium chloride dihydrate until tomorrow. Tomorrow, the full amount of calcium chloride dihydrate can be used because the change is small and it is at the low end of the recommended range. So, it doesn’t matter if CH gets a little higher than calculated.

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