How Many Gallons Is an 18×36 Inground Pool and What It Means for Chemicals

You picked up a jug of algaecide, flipped it over, and the label says “add 16 oz per 10,000 gallons.” Now you’re standing at the edge of your pool doing the mental math, and you’re not entirely sure whether your pool is closer to 20,000 or 30,000 gallons. It matters more than most people think.

An 18×36 inground pool holds approximately 24,300 gallons at a standard average depth of 5 feet. That number is the foundation for every single chemical dose you add all season. This article walks through how to calculate your exact volume if your depths differ, and then shows you the real-world doses for chlorine, shock, pH adjusters, alkalinity, cyanuric acid, and salt – all based on that 24,300-gallon baseline.

How to Calculate the Exact Volume of Your 18×36 Pool

The standard formula for a rectangular inground pool is: length x width x average depth x 7.5. The 7.5 converts cubic feet into gallons. For an 18×36 pool at a flat 5-foot depth, that’s 18 x 36 x 5 x 7.5 = 24,300 gallons.

Most inground pools are not flat-bottomed, though. If your pool slopes from a 3.5-foot shallow end to an 8-foot deep end, your average depth is (3.5 + 8) / 2 = 5.75 feet. Plug that in: 18 x 36 x 5.75 x 7.5 = 27,945 gallons. That extra 3,600 gallons means every underdose or overdose is compounded across a larger volume. Always use your actual average depth before buying chemicals.

Two common mistakes here: using only the deep-end depth (which inflates the number), and ignoring steps and benches (which reduce usable water by 200 to 500 gallons in most pools). For most chemical dosing, neither error is large enough to matter, but it is worth knowing your number is approximate to within about 5 percent.

Chlorine: Your Weekly Maintenance Dose

Free chlorine in a 24,300-gallon pool should stay between 1 and 3 ppm, with 2 ppm being a practical daily target. To raise chlorine by 1 ppm in this volume, you need roughly 2.5 oz of 90% trichlor granules or about 20 oz of liquid chlorine at 10% strength.

For tablet feeders, most 18×36 pools burn through 2 to 4 standard 3-inch trichlor tablets per week in summer. Sun exposure, bather load, and whether you have a cover all shift that number. A pool with no cover that hosts weekend parties will sit at the high end; a covered pool with light use might stay sanitized at 2 tabs. Test twice a week and adjust your feeder, not your expectations.

One important note: trichlor tabs also add cyanuric acid (stabilizer) every time they dissolve. In a 24,300-gallon pool, each 8-oz trichlor tab raises CYA by roughly 0.5 ppm. Over a full season that adds up, which is why CYA tends to creep high in pools that rely exclusively on tabs – something worth watching if you’re troubleshooting persistent chlorine loss. If shock keeps falling short even after you add it, check out why pool shock fails before assuming you have algae.

Shock: Maintenance Doses vs. Problem Doses

Shocking a 24,300-gallon pool with calcium hypochlorite (68% available chlorine) follows a simple baseline: 1 lb per 10,000 gallons for maintenance, so about 2.5 lbs per shock treatment. That raises free chlorine by roughly 6 to 8 ppm, enough to break apart combined chloramines and give the water a reset.

For a green or visibly algae-affected pool, double the dose to 5 lbs and plan to brush the walls before and after. Retest at 24 hours. If chlorine has already dropped back below 1 ppm and the water is still hazy, you likely need a second shock treatment before the problem is actually resolved. AquaDoc’s pool shock is one option pool owners reach for in this situation, because it dissolves fast and doesn’t leave a residue that muddies the water further.

pH and Total Alkalinity: What the Numbers Look Like at Scale

Target pH is 7.4 to 7.6. Target total alkalinity is 80 to 120 ppm. At 24,300 gallons, the doses to hit those targets are large enough that getting them wrong is expensive.

• Lowering pH with muriatic acid: To drop pH by 0.2 units, add roughly 12 to 14 oz of muriatic acid (31.45%). Always pre-dilute in a bucket of pool water, add near a return jet with the pump running, and retest after 4 hours before adding more.
• Raising pH with soda ash: To raise pH by 0.2 units, add approximately 12 oz of sodium carbonate (soda ash). Dissolve it in a bucket first to prevent cloudy spots near the point of addition.
• Raising total alkalinity with sodium bicarbonate: To raise TA by 10 ppm in a 24,300-gallon pool, add about 20 oz (1.25 lbs). If your TA is 60 ppm and you want 100 ppm, you’re looking at roughly 5 lbs total, added over two sessions 24 hours apart to avoid a pH spike.

A practical reminder: always adjust total alkalinity before pH. TA acts as a buffer that stabilizes pH, so fixing TA first makes pH much easier to hold in range. For a deeper explanation of why the order matters, the breakdown in this cloudy water guide covers how chemistry imbalances layer on each other.

Cyanuric Acid and Salt: The Big One-Time Additions

Cyanuric acid (stabilizer) should sit between 30 and 50 ppm for a chlorine pool. To raise CYA by 10 ppm in 24,300 gallons, you need about 20 oz of granular stabilizer. If you’re starting a freshly filled pool from zero, bringing CYA to 40 ppm takes roughly 4.8 lbs. Add it in the skimmer basket with the pump running and plan for 24 to 48 hours for it to fully dissolve and register on a test.

For saltwater pools, an 18×36 pool at 24,300 gallons needs approximately 600 lbs of pool-grade salt to reach a target of 3,200 ppm starting from fresh water. That’s typically 12 to 13 standard 50-lb bags. Test your fill water first – city water often contains 300 to 500 ppm of dissolved minerals already, which could reduce your initial salt requirement by 1 to 2 bags.

A Quick Reference: 18×36 Pool Doses Side by Side

Chemical	Goal	Dose for 24,300 Gallons
Trichlor tablets	Maintain 2 ppm chlorine	2 to 4 x 3-inch tabs per week
Cal-hypo shock (68%)	Maintenance shock	2.5 lbs per treatment
Cal-hypo shock (68%)	Algae treatment	5 lbs per treatment
Muriatic acid (31.45%)	Drop pH by 0.2	12 to 14 oz
Soda ash	Raise pH by 0.2	12 oz
Sodium bicarbonate	Raise TA by 10 ppm	20 oz (1.25 lbs)
Granular CYA stabilizer	Raise CYA by 10 ppm	20 oz
Pool-grade salt	Reach 3,200 ppm (fresh water start)	~600 lbs (12-13 bags)

Why These Numbers Matter at the End of the Season Too

Getting volume right isn’t just a summer concern. When you close your pool, every winterizing chemical – from the algaecide to the phosphate remover to the shock dose – is calculated against pool volume. Underdosing at closing sets you up for a green swamp in spring. If you’re approaching the end of the season, reviewing a fall pool maintenance checklist against your 24,300-gallon baseline will tell you exactly what to buy before you put the cover on.

Frequently Asked Questions

How many gallons is an 18×36 inground pool?

An 18×36 inground pool holds approximately 24,300 gallons at an average depth of 5 feet. If your shallow end is 3.5 feet and your deep end is 8 feet, your average depth is 5.75 feet and your volume rises to roughly 27,945 gallons. Always calculate using your own depths before dosing any chemical.

How much chlorine do you add to an 18×36 inground pool?

To raise free chlorine by 1 ppm in a 24,300-gallon pool, add roughly 2.5 oz of trichlor granules or about 20 oz of liquid chlorine at 10% strength. For weekly maintenance, most 18×36 pools need 2 to 4 standard 3-inch trichlor tablets per week in summer, depending on sun exposure and bather load.

How much shock do I need for an 18×36 inground pool?

For a standard maintenance shock, use 2.5 lbs of 68% calcium hypochlorite per treatment. For a green or algae-affected pool, double that to 5 lbs, brush the walls, and retest at 24 hours before allowing swimmers back in.

How much muriatic acid does an 18×36 pool need to lower pH?

To drop pH by 0.2 units in a 24,300-gallon pool, add 12 to 14 oz of muriatic acid (31.45%). Pre-dilute in a bucket of pool water, distribute near a return jet with the pump running, and retest after 4 hours before adding more. Never add more than the calculated dose in a single treatment.

How much salt do I need to convert an 18×36 inground pool to saltwater?

Starting from fresh water, an 18×36 pool at 24,300 gallons needs approximately 600 lbs of pool-grade salt (about 12 to 13 standard 50-lb bags) to reach a target salinity of 3,200 ppm. Test your fill water first, since municipal water often already contains 300 to 500 ppm, which could reduce your initial requirement by a bag or two.

Your 18×36 inground pool holds roughly 24,300 gallons – and that number is the one figure worth memorizing, because every chemical label, every seasonal product, and every dose recommendation you’ll ever read is built on it.