# Pool size reverse calculation

#### Ezun

Gold Supporter
So here’s my thought and a question:
I’ve been trying to determine my exact pool size (my pool builder just doesn’t think it’s important so I never get a solid answer). They’ve said it’s around 33k. BUT, when I add acid to bring down my PH, it seems to WAY overshoot. Granted, I haven’t been measuring to the exact level of acid, but it’s been a guesstimate. I have to add Acid about every 2 weeks to bring my PH down from increases due to a few sheer descents and kids/dog playing in the pool

So, I’m wondering if I could back into a very close approximation of pool size by tightly measuring the Acid I put in the pool and seeing how the PH changes. For example, if I need to lower the PH from 8.0 to 7.6 and I could use the PoolMath app to determine what that would take for a 28,000 gallon pool and add that amount and see what happens. If the PH lands at 7.8 or 7.7, then I know the pool is a bit larger. So the next time I add I could use a 30,000 gallon pool to calculate the add and on and on until I get the expected results from the add of acid.

Thx!

#### jblizzle

TFP Expert
That is pretty much how all of us figure out our pool size. Take a guess. Add some chemicals and over time if consistently over or undershooting then adjust the volume.
You can't really do it with 1 acid addition, because there are many factors at play (like TA, CYA, borates, etc) that impact the pH change acquired with the acid addition.
Salt and calcium are pretty good thing that you can add and test for a change because they are independent of other parameters.
Bleach is a little harder too, because you never really know the strength.

All that said, it really does not matter and there is no need to know within a few gallons. If you can get within 1000 gallons, that is probably fine, although you can likely do a little better than that.

#### JoyfulNoise

TFP Expert
Platinum Supporter
TA is the most accurate way to measure it -

#### calinb

##### Well-known member
TA is the most accurate way to measure it -

It's tough to know the strength of liquid chlorine, because it degrades with sunlight, time, and temperature, but what do you think about weighing out some quick dissolving dichlor or calcium hypochlorite for a test? I bet factory packaging that provides a percentage of chlorine /usable chlorine to a decimal point and a FAS-DPD test would generally offer good accuracy in pool volume determination.

Of course it depends on the test and concentration targets used and measured too. My pool is at 30 ppm CYA. I'd be inclined to use measured dichlor or cal-hypo to raise FC from 2.5 ppm to a 6 ppm target, which is only a little over my normal 5 ppm target. Speaking in terms of the resolution of the method, that's 7 drops "worth" in the FAS-DPD test that limits my accuracy.

If I didn't have a pool stain problem, I would likely bump my chlorine target up to the SLAM level (2.5 ppm to 12 ppm yields 19 drops of change, which is even more compelling in method error analysis), but I don't want to SLAM without good reason, because the chlorine might precipitate stains. I also keep my pH on the low side for this reason.

In using TA, I'd prefer to not increase TA for this purpose more than about 20. That's only two drops "worth" in the TF-100 kit and not very good resolution. To match the 7 drops of the FAS-DPD chlorine based determination, I'd have to add acid afterwards and aerate away my too-high TA that I increased by a whopping 70!

Depending on your pool's chem balance, your pool may be different. Maybe your TA is low and then I'd say go for the TA method.

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#### calinb

##### Well-known member
Also, if you do a formal error analysis of the method (which I've not taken the time to do either), you'll find that a dichlor / calhypo FC based method can provide superior error levels by conducting the test multiple times. You have to add chlorine to your pool anyway and the addition of CYA or calcium won't be a problem until after many tests.

If you calculate the average of the results of several pool volume calculations from tests performed on different days, the "quantization error" of single drop resolution can become very small. The same would be true for a TA based method, but who wants to drive their TA that high? I think the commonly used TA estimation methods also get pretty flaky with pool chemistry far from typical.

#### JoyfulNoise

TFP Expert
Platinum Supporter
For the TA Test you get can get 1ppm accuracy simply by using a 250mL water sample. Sure, you need to use more dechlorinating and dye reagent, but’s it’s far superior to chlorine measurements considering chlorine can degrade rapidly from both sanitation and oxidation. It’s a one-time measurement that will get you accuracy down to the hundreds of gallons as opposed to agonizing over chlorine concentrations and doing multiple chlorine tests.

And if you don’t like raising TA, you can lower it using muriatic acid and get the same accuracy. Muriatic acid is highly stable and does not degrade. It also has a fixed concentration and the decrease in TA as a result of adding acid is as exact as raising TA using baking soda.

#### calinb

##### Well-known member
Yes. If you're willing to measure reagents in some manner that improves upon drop resolution and accuracy (how big is a drop from any given nozzle?), then many tests can be better used to estimate pool volume.

#### Ezun

Gold Supporter
This is all super helpful. I’m going to have to set up on day and do this. Maybe tomorrow - seems strange to say this will be a “fun” project.

#### jcolec

Gold Supporter
I can’t wait to try this methodology on our pool. Fingers crossed it gets plastered tomorrow.

#### SgtWilko

##### Active member
I have a SWG pool so my method of estimating volume is using salt.
• 4 easy steps
• No interactions with other chemicals
• Only dependent on your salt measurement accuracy (and math)
My geometrically calculated volume is around 60,000 liters. With the salt method, I get about 62,000 liters if I use the salt indication on the SWG; I get 66,000 liters if I use the pool store salt test results. Neither measurement is super reliable but its close enough to the geometrical calculation to confirm it. More importantly, Poolmath suggested chemical quantities produce the expected results when I use 60,000 l.

Methodology:

1. Measure starting salt level in ppm (call it Sa)
2. Add n bags of salt (20kg each)
3. Measure salt level in ppm after 24h (call it Sb)
4. Estimated pool volume in liters (call it V) = (n * 20) / (Sb-Sa) x 1,000,000
For gallons, divide liters by 3.7874

The volume estimation above is a slightly simplified calculation. The mathematically correct calculation would be:
V = (n * 20) x (Sb-Sa+1,000,000) / (Sb-Sa)

The simplified calculation is close enough.

I neglect salt purity (99.78% in my case) because its the smallest source of error in the whole process. Factor it in if you wish.

Sgt Wilko

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Ezun

#### Ezun

Gold Supporter
OK, so I just went through this using the TA method and I assume it worked. Looks like I'm about 5,000 gallons less than what my builder told me.

To confirm that it's right, I'm going to start using the new data of 28,000 gallons vs 33,000 gallons to set up the app and calculate chemicals. From there, I'm hoping that the response of PH will be more in line with expectations. I'll report back after the next addition of Muriatic Acid when I need to drop PH - seems to be about every 3-4 weeks.