How Much Muriatic Acid to Add to Adjust Swimming Pool pH

Updated on May 18, 2019
robhampton profile image

Rob is a pool-service industry professional with over 20 years of experience.

What Is pH?

pH is a measurement of the hydrogen ion concentration in water. The pH of pool water affects the comfort of swimmers as well as pool maintenance.

"pH" stands for "protens hydrogen" or "power of hydrogen." Water conducts electricity and is easily ionized, so it will always contain some level of hydrogen ions. The pH scale is exponential and is set up so that the smaller the pH number measured in the water (or any solution), the more acidic it is. The higher the pH, the more alkaline it is.

Why Does Your Pool's pH Need to Be Balanced?

Controlling the pH is important not only for comfortable swimming, but also for the long life of the materials that make up the pool.

Considering that the pH of teardrops from a human eye is about 7.5, having a pH as close to that number as possible will make swimming more comfortable. As a pool service tech, I sometimes hear people complain that swimmers' eyes hurt because the chlorine level is too high. But in these cases, an unbalanced pH level, not a high chlorine level, is almost always the problem.

What Causes Pool pH to Become Unbalanced?

Many things can affect the pH balance in a swimming pool:

  • disinfectants
  • source water
  • evaporation
  • water treatment chemicals
  • airborne debris
  • swimmer waste
  • aeration

Common Problems Caused by a LOW pH:

  • etching of the pool surface
  • metal corrosion
  • staining
  • chlorine loss
  • wrinkles in vinyl liners
  • eye and skin irritation

Common Problems Caused by a HIGH pH:

  • scaling (which can lead to clogged filters, clogged heater elements, and reduced circulation)
  • cloudy water
  • chlorine inefficiency
  • eye and skin irritation

How to Control Your Pool's pH

To lower the pH, the most common liquid acid used is muriatic acid, also known as hydrochloric acid.

To raise the pH, soda ash (sodium carbonate) is normally used. Sodium bicarbonate (baking soda) can be used to raise pH if the alkalinity is low, but it’s more commonly used to raise alkalinity.

What Is Alkalinity?

Briefly, alkalinity refers to dissolved salts, like carbonates, that keep pH stable. If alkalinity is in the right range, pH and chlorine levels are easier to maintain.

To lower alkalinity, add muriatic acid. Muriatic acid lowers both pH and alkalinity.

To raise alkalinity, add sodium bicarbonate (baking soda). This is different from sodium carbonate (soda ash).

How Much Acid Should I Add to My Pool?

This depends on how high your pool's pH is and the size of the pool. If you are using test strips, it's usually hard to determine exactly how high the pH level is; you just know it's high. A better test kit, or a DPD drop test kit, will use phenol red to give you a better idea of your true pH reading. Some better test kits include an "acid demand" test to help calculate a more precise dosage.

If you have a pH reading of 7.8 or higher, and an average size (15,000 gallons) in-ground pool, you should add 1/4 gallon (a quart) of muriatic acid, and re-test after the water has circulated for an hour. If you have a smaller pool, or an above-ground pool, you would start with one cup of acid, and then re-test and add more if needed. If you have a spa, we are talking only capfuls at a time. Very little acid is needed to adjust pH in a spa.

For a more exact idea of how much acid to add, see the table below, or see this pool calculator that will give you an amount based on estimated pool volume and chemistry.

How Much Acid to Add for Your Starting pH and Pool Volume

pH
1000 gal
5000 gal
10,000 gal
15,000 gal
20,000 gal
25,000 gal
50,000 gal
7.6-7.8
1 1/4 oz.
6 oz.
12 oz.
18 oz.
24 oz.
1 qt.
2 qts.
7.8-8.0
1 1/2 oz.
8 oz.
16 oz.
24 oz.
1 qt.
1 1/4 qts.
2 1/2 qts.
8.0-8.4
2 1/2 oz.
12 oz.
24 oz.
1 1/4 qts.
1 1/2 qts.
2 qts.
1 gal.
Over 8.4
3 oz.
16 oz.
1 qt.
1 1/2 qts.
2 qts.
2 1/2 qts.
1 1/4 gal.
How much muriatic acid to add to your pool. Check the pH with a test kit or test strip and use the pool calculator (link above) to estimate your pool's volume in gallons.

How to Add Acid

When you add acid, pour it into the deepest part of the pool, while the pump is running and circulating the water. Wait at least an hour before using the pool; if you added a lot of acid, wait 24 hours and test the pH before using the pool.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

  • How long do I have to wait to swim after adding muriatic acid?

    If it's small pH adjustment then it is ok to swim after 1 hour of pool circulation. If a large quantity of acid is being added, wait 24 hours then re-test the pH to make sure the water is not too acidic.

  • My above ground pool has a pH of 8.4. It holds around 16500 gallons of water. I have used a gallon of muriatic acid and shocked the pool. It is still green, and the pH is still at 8.4. What should I do?

    If the pH is still 8.4 after adding a gallon of acid, there is a more severe chemistry issue. Also be sure that the test kit being used is giving accurate readings. I would suggest brushing the pool and adding a floccing agent such as "Drop-Out" or "Drop-Down" which will sink all algae to the bottom of the pool. It will need to be then vacuumed to waste (this is sometimes difficult to accomplish with above ground pools). Some water will be lost during this process. After this is done, the water chemistry can then be re-balanced. If the pump/filtration system is not set up to vacuum to waste or "backwash," other options would be to keep the pH around 7.2, elevate the chlorine and filter out the algae cleaning the filter frequently. If all else fails, drain the pool and re-fill. Try this chemical dosage calculator to adjust chemicals.

    https://www.poolcalculator.com

  • I’ve added a half a gallon of muriatic acid to a 10000-gallon pool and my Ph and alkalinity are still very high, what do I do?

    Since muriatic acid will lower BOTH pH and alkalinity, the answer is simple. Continue adding muriatic acid (1/4 gallon at a time) let circulate for a few hours and retest until the pH is at the correct level (7.4-7.6). If the alkalinty is still a little high at that point, it will come down eventually on it's own.

  • What do I do after adding too much shock to my pool?

    If you've added too much shock, then there's a risk of the chlorine level being dangerously high. To lower the chlorine level, you can approach this in two different ways. To lower it immediately, sodium thiosulfate can be used (sometimes called "After-shock" or simply known as chlorine reducer). The other option would be to wait several days and let the level come down on its own.

© 2012 Rob Hampton

Comments

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  • robhampton profile imageAUTHOR

    Rob Hampton 

    5 months ago from Port Richey, Florida

    Hi David, you need to dilute the water..Drain some, add fresh.

  • profile image

    David 

    5 months ago

    I put too much metal out in my pool by Doheny's my pool is only 10,000 gallons what should I do???

  • profile image

    Sean 

    6 months ago

    I have a waterfall feature about 5' in length, 3" deep and 4' high (that's just the waterfall portion)..Where the water falls into is about 3.5' wide, 1' deep with a 1' depth (it's basically a rectangle tub where the water falls into it and is pumped back over the waterfall).--nothing swims in it. It flows over natural stone and i'm wondering (because i used to have a pool) if i can just use muratic acid and chlorine to keep the water clean or i can just use muratic acid to keep it clean from algae, etc.? I used those 2 chemicals for 4 years while I had a pool, monitored the pH balance and NEVER had a problem. Can i easily do the same for this and if so, about how much inital chlorine and/or muratic acid would I need to use for about 4-5 volume of water?

  • profile image

    Arturo Abascal 

    7 months ago

    I just resurfaced the Pool a month and a half ago. It’s a 25000 gallon pool underground. I used city water to fill the Pool. My water is clear, I run the Pool 8 hours a Day. It’s a salt water pool. My ph has been around 8.00 and for the last few weeks I have been dumping Muriatic acid on a weekly basis to bring the ph down. I have been using the amounts recommended by pinch a penny. The Pool supplier tells me that it will take some time till the ph stabilizes it’s a new pool. Other factors like alkalinity and chlorine are correct, but I wonder how long will I continue using Muriatic acid and if the assumption from pinch a penny is correct. My salt level reads 3,000.

    I thank you for your reply

  • robhampton profile imageAUTHOR

    Rob Hampton 

    8 months ago from Port Richey, Florida

    Laura, Citric acid is not used to drop pH. I also can not recommend pH to be any lower than 7.4, Unless you are doing some sort of stain removal.

  • profile image

    Laura 

    8 months ago

    Starting ph is 7.2 in 2500 gallons of water, how much citric acid is used to drop ph to 5

  • profile image

    MARK 

    10 months ago

    I just added half a litter off acid hydrochoric into a friends pool as they have been away for 3weeks how long until i can swim in it, it was still clear before i add the acid

  • robhampton profile imageAUTHOR

    Rob Hampton 

    11 months ago from Port Richey, Florida

    Hi Patsy, thanks for reading! So the most common alkalinity increaser is sodium bicarbonate (baking soda) and yes, it can be used to raise pH as well. If you have high alkalinity and low pH, soda ash (sodium carbonate) is best. If both alkalinity AND pH is low, than sodium bicarbonate is ideal.

  • profile image

    Patsy Pakeerah 

    11 months ago

    If my ph in my pool is too low could i add some total alkalinity to balance it?

  • profile image

    jarle stensrod 

    12 months ago

    we use paint instead of tiles in our pool but now the water is so cloudy 'whats the cause of it and how to treat it.thank you...

  • profile image

    Beth 

    15 months ago

    My above ground pool around the bottom ring is dark color - looks like rust. It feels grainy. Chlorine is 2/4, pH is 8.4, alkalinity is 40, hardness is 150 & cyanuric acid is 50. How can I get rid of the ring?

  • profile image

    Joshua Lentz 

    16 months ago

    Thank you this was very helpful

  • robhampton profile imageAUTHOR

    Rob Hampton 

    17 months ago from Port Richey, Florida

    ocpool...Thank you for your comment. This article addresses specifically pH. High chlorine can also be an issue. If you have started a pool company, it might be a good idea to brush up on pool chemistry knowledge.

  • profile image

    ocpool 

    17 months ago

    Hi we just started our own pool company and I had no idea pH is usually the problem and not high chlorine levels. This will really come in handy, thanks a lot!

  • robhampton profile imageAUTHOR

    Rob Hampton 

    5 years ago from Port Richey, Florida

    Chris, just make sure the pump is running when you add the muriatic acid, you can swim in 45 minutes. It doesn't take long for it to dilute.

  • profile image

    chris 

    5 years ago

    how long after you add the acid that you can swim

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