Pool chemistry for beginners: the 5 numbers that matter
You just got a pool. Or you've had one for years but you've been winging it with whatever the pool store tells you. Either way, pool chemistry can feel overwhelming — there are dozens of things you could test for.
The good news: you really only need to understand five numbers. Get these right and your pool will be clear, safe, and gentle on your equipment. Ignore them and you'll spend more money on chemicals, repairs, or both.
The big picture
Think of pool chemistry as a balancing act. Your water needs to do three things at once:
- Stay sanitized — Kill bacteria and algae before anyone gets sick
- Stay comfortable — Not irritate eyes, skin, or swimsuits
- Stay balanced — Not damage your pool surfaces, plumbing, or equipment
The five numbers below cover all three. Here they are, in order of how often you'll need to check them.
Free Chlorine (FC) 2 – 4 ppm
Free chlorine is your sanitizer — the active chlorine that kills bacteria, destroys algae, and oxidizes contaminants. It's the single most important number in your pool.
Without enough FC, your pool can turn green overnight. Too much and it's harsh on skin and swimwear, though this is less common than most people think.
How to raise it: Add liquid chlorine (sodium hypochlorite) or granular shock. For everyday dosing, liquid chlorine is the simplest option.
How to lower it: Sunlight breaks down chlorine naturally. Just wait — or if you're in a hurry and FC is very high, a small dose of sodium thiosulfate works.
Test frequency: 2–3 times per week in summer, weekly in cooler months.
pH 7.2 – 7.8
pH measures how acidic or basic your water is, on a scale from 0 (very acidic) to 14 (very basic). Pool water should sit in a narrow range that's comfortable for swimmers and effective for chlorine.
This is important: chlorine works much better at lower pH. At pH 7.2, about 65% of your chlorine is in its active killing form. At pH 8.0, only about 20% is active. Same amount of chlorine, dramatically different effectiveness.
How to raise it: Add soda ash (sodium carbonate). This also raises alkalinity slightly.
How to lower it: Add muriatic acid (hydrochloric acid). Go slowly — a little goes a long way.
Test frequency: 2–3 times per week. pH drifts constantly and is affected by rain, swimmers, and even your chlorine source.
Total Alkalinity (TA) 80 – 120 ppm
Alkalinity is pH's bodyguard. It acts as a buffer that keeps your pH from swinging wildly every time you add a chemical, get rained on, or have a pool party.
When alkalinity is too low, your pH becomes unstable — it'll bounce around and you'll constantly be chasing it with acid or soda ash. When it's too high, pH gets stubbornly locked high and your water can turn cloudy.
How to raise it: Add sodium bicarbonate (baking soda). Cheap and easy.
How to lower it: Add muriatic acid slowly with the pump running. You may need to aerate afterward (run water features, point a return jet up) to let pH recover while TA stays down.
Test frequency: Weekly. Alkalinity moves slowly compared to pH and chlorine.
Calcium Hardness (CH) 200 – 400 ppm
Calcium hardness measures how much dissolved calcium is in your water. This one matters for your pool's long-term health.
Water that's too low in calcium is "hungry" — it will pull calcium from your plaster, grout, and concrete surfaces, slowly dissolving them. Water that's too high in calcium deposits white scale on surfaces, inside pipes, and on heat exchangers and salt cells.
How to raise it: Add calcium chloride. Dissolve it in a bucket of water first, then pour around the pool with the pump running.
How to lower it: There's no chemical to remove calcium. You'll need to partially drain and refill with fresh water.
Test frequency: Monthly, or after adding fresh water. Calcium doesn't change much day to day.
Cyanuric Acid (CYA) 30 – 50 ppm
CYA is sunscreen for your chlorine. Without it, the sun destroys free chlorine in a matter of hours. With the right amount of CYA, your chlorine lasts all day.
But there's a catch: too much CYA weakens chlorine's killing power. It essentially locks up the chlorine so it can't do its job efficiently. This is why many pool experts recommend keeping CYA moderate and adjusting your FC target based on your CYA level.
How to raise it: Add cyanuric acid (sold as "stabilizer" or "conditioner"). It dissolves slowly — put it in a sock in the skimmer or a dissolving feeder.
How to lower it: Like calcium, there's no chemical fix. Partial drain and refill is the only way.
Test frequency: Monthly. CYA is very stable — it doesn't evaporate or get used up. It only leaves your pool when water leaves your pool.
What about everything else?
You might see other numbers on your test strips — salt, phosphates, iron, copper, TDS. Those can matter in specific situations, but they're not part of the core five. If your five key numbers are in range, your pool is in good shape.
Salt pools
If you have a salt water generator (SWG), you'll also want to track your salt level (usually 2700–3400 ppm depending on your unit). But the SWG is just a different way of making chlorine — the five core numbers still apply exactly the same way.
How often to test
| Parameter | Summer | Off-Season |
|---|---|---|
| Free Chlorine | 2–3x per week | Weekly |
| pH | 2–3x per week | Weekly |
| Total Alkalinity | Weekly | Bi-weekly |
| Calcium Hardness | Monthly | Monthly |
| CYA | Monthly | At open/close |
The pattern is simple: test sanitizer and pH often (they change fast), test the slower-moving numbers less often.
The one rule that saves the most headaches
Test before you add. Don't guess. Don't assume. The five-minute test today prevents the five-hour problem this weekend. Most pool problems happen because someone added chemicals based on a hunch instead of a reading.
Once you're in the habit of testing, the rest becomes routine. You'll start to notice patterns — pH always creeps up after rain, chlorine drops faster on hot sunny days, alkalinity stays stable for weeks. Those patterns make pool care predictable instead of stressful.
Start tracking the 5 numbers
PoolChem Tracker lets you log all five key readings in seconds, gives you a health score, and tells you exactly what to add and how much — based on your pool, not generic advice.
