Dichlor Startup Dose for a 400-Gallon Inflatable Hot Tub: First Fill Guide

You just filled your inflatable hot tub for the first time, the jets are bubbling, and now you're holding a bag of dichlor wondering how much to dump in. Too little and you're soaking in unsanitized warm water - a breeding ground for bacteria. Too much and you're stepping into something closer to a bleach bath.

For a 400-gallon inflatable hot tub on first fill, add 2 teaspoons (approximately 14 grams) of granular dichlor to reach a free chlorine level of 3-5 ppm. That is your startup target. This guide walks through why that number is right, how to apply it correctly, how to handle the cyanuric acid buildup that comes with regular dichlor use, and what mistakes trip up first-time owners.

Why First Fill Is Different From a Regular Refill

A brand-new fill of tap water is a blank slate - no chlorine residual, no stabilizer, and often some dissolved metals or organics from the hose. That combination means your first dose of dichlor needs to do two jobs at once: establish a sanitizer residual quickly and begin building a small amount of cyanuric acid (CYA), which stabilizes chlorine against UV breakdown. On a refill you might already have 20-30 ppm of CYA in the water from previous doses, so you need less dichlor to maintain levels. On a first fill, you are starting from zero.

Hot water also accelerates everything. At 100-104 degrees Fahrenheit, chlorine dissipates two to three times faster than it would in a pool at 80 degrees. That means a 400-gallon tub used for one 30-minute soak by two adults can lose 1-2 ppm of free chlorine in a single session. Starting with the right dose matters more than it might seem.

Step 1: Fill, Circulate, and Test Tap Water First

Before you add any chemicals, fill the tub completely, turn the jets on, and let the water circulate for 20-30 minutes. Then test your tap water for pH, alkalinity, and hardness. Most tap water comes in at a pH of 7.2-7.8 and total alkalinity of 80-150 ppm - usually acceptable for an inflatable tub - but you want to know where you are before you add sanitizer. Dichlor itself has a pH around 6.7, so it nudges pH slightly downward with every dose. If your tap water is already acidic (pH below 7.2), bring alkalinity up with baking soda before proceeding.

Step 2: Calculate and Measure Your Dichlor Dose

Here is the math in plain terms. Dichlor (sodium dichloro-s-triazinetrione) is about 56% available chlorine by weight. To raise free chlorine by 1 ppm in 400 gallons, you need roughly 0.3 grams of dichlor. To reach a target of 4 ppm from zero, multiply 0.3 grams by 4 ppm by 1 (for 400 gallons / 400 gallons) and you get approximately 1.2 grams. Round up to account for chlorine demand from the water and organics already present, and a practical startup dose lands at 12-15 grams, or about 2 level teaspoons.

Do not eyeball it from the bag. Use a kitchen scale or a measuring spoon made for chemicals, not the one you use for cooking. Measure into a small cup of warm water from the tub to pre-dissolve the granules, then pour the solution into the water near a jet inlet while the pump is running. Never pour dry granules directly onto the inflatable floor - concentrated dichlor can bleach and weaken the PVC material at the point of contact.

Step 3: Wait, Then Test Before Anyone Gets In

Run the jets for at least 30 minutes after dosing. Then test free chlorine with a reliable test kit or strips. Your target is 3-5 ppm of free chlorine. Do not enter the water above 10 ppm. If your reading comes in under 3 ppm, add another half-teaspoon, circulate for 15 minutes, and retest. If it reads above 5 ppm, just wait - in a 400-gallon tub at 102 degrees, chlorine will drop on its own within a couple of hours.

Also test pH and make sure it sits between 7.4 and 7.6. At a pH above 7.8, chlorine efficiency drops sharply and you need much more sanitizer to do the same job. If pH is low after your dichlor dose, a small amount of sodium carbonate (pH Up) will correct it. Getting these two numbers right on day one makes every future maintenance session easier.

The Cyanuric Acid Problem With Dichlor

Every gram of dichlor you add also adds about 0.5 grams of cyanuric acid (CYA) to the water. In a 400-gallon tub, CYA accumulates fast. A startup dose of 14 grams of dichlor adds roughly 7 grams of CYA, putting your opening level around 4-5 ppm - harmless at that point. But after 6-8 weeks of regular dosing, CYA can climb to 80-100 ppm or higher, which actually suppresses the effectiveness of chlorine and makes the water harder to sanitize. This is sometimes called "chlorine lock."

The fix is a water change. Most inflatable hot tub owners on dichlor should plan to drain and refill every 60-90 days. Some switch to a non-chlorine shock oxidizer once or twice a week to handle bather load, reducing how often they rely on dichlor and slowing CYA buildup. That combination - dichlor for a sanitizer residual, non-chlorine shock for oxidation - keeps the water clear longer between drains. Biofilm is a related concern in inflatable tubs: warm stagnant water is exactly where it takes hold, and you can read more about what causes biofilm in hot tubs and how to get rid of it if you start noticing slippery walls or persistent cloudiness after your startup.

Ongoing Maintenance Doses After First Fill

After the first fill, your goal is to keep free chlorine between 3 and 5 ppm at all times. In a 400-gallon inflatable tub with moderate use (two adults, three to four sessions per week), plan to add 1 teaspoon of dichlor every 1-2 days. Test before every use, not on a fixed schedule - actual chlorine demand varies with bather load, water temperature, and sun exposure.

Common mistakes to avoid:

• Dosing by habit, not by test results: Adding dichlor on a timer without testing lets CYA climb while chlorine levels swing unpredictably. Always test first.
• Pouring dry granules on the floor: Concentrated granules sitting on inflatable PVC will bleach and degrade the material. Always pre-dissolve in a cup of water.
• Ignoring pH: Dichlor's slightly acidic pH will slowly drag your water's pH down. Check pH every few days and nudge it back to 7.4-7.6 as needed.
• Using too many chemicals at once: Add one chemical at a time, circulate for 15 minutes, then test before adding the next. Dumping in everything at once makes it impossible to know what changed what.

AquaDoc's granular dichlor is sized for small-volume soaks like this - the scoop markings are calibrated for tub dosing, not pool dosing, which makes measuring for a 400-gallon tub a lot less guesswork. If you have ever noticed a persistent chlorine smell that does not go away even after dosing correctly, the problem is usually chloramines rather than excess free chlorine, and it is worth reading why hot tub water smells bad even after shocking to pinpoint the cause.

Quick Reference: Dichlor Doses for a 400-Gallon Inflatable Hot Tub

Why These Differences Matter

The first-fill dose is higher than a maintenance dose because the water has zero chlorine residual and unknown organic demand. The shock dose pushes chlorine to 10 ppm intentionally - high enough to break down chloramines and kill bacteria quickly - but requires you to wait until the level drops back to 5 ppm or below before using the tub. Using these as a reference rather than rigid rules is the right approach: always confirm with a test reading before you act.

Frequently Asked Questions

How much dichlor do I add to a 400-gallon hot tub on first fill?

Add 2 teaspoons (about 14 grams) of dichlor to a 400-gallon inflatable hot tub at startup to reach roughly 3-5 ppm of free chlorine. Test with a reliable kit after 30 minutes and adjust up or down from there. Do not enter the water until the reading falls between 3 and 5 ppm.

Can I use regular pool chlorine instead of dichlor in an inflatable hot tub?

Avoid it. Trichlor tablets are too acidic for the small water volume of an inflatable hot tub and will crash your pH fast. Cal-hypo adds calcium hardness that foams and stresses inflatable walls. Dichlor is the right choice because it dissolves quickly, has a near-neutral pH, and does not load the water with extras.

How often do I add dichlor to a 400-gallon inflatable hot tub after startup?

Test your water every day or every other day. In a 400-gallon tub used by 2-3 people, free chlorine typically drops 1-2 ppm per day. Add 1 teaspoon of dichlor to restore the level to 3-5 ppm whenever it reads below 3 ppm.

What is the correct free chlorine level for an inflatable hot tub?

Keep free chlorine between 3 and 5 ppm in a hot tub. Levels below 3 ppm leave the water under-sanitized, especially at 100-104 degrees F where chlorine burns off quickly. Above 10 ppm the water can irritate skin and eyes, so do not over-dose.

Why does my hot tub water smell like chlorine right after I add dichlor?

A strong chlorine smell after dosing usually means combined chlorine (chloramines) is present, not excess free chlorine. This happens when sanitizer reacts with swimmer waste, body oils, or biofilm. If the smell persists after 30 minutes, shock the tub with a non-chlorine oxidizer or a double dose of dichlor and retest in an hour.

For a 400-gallon inflatable hot tub on first fill, 2 teaspoons of granular dichlor gets you to a safe 3-5 ppm target - test after 30 minutes, confirm the number, then get in.

If you have questions about your specific setup or want help picking the right dichlor for a small-volume tub, we are here. AquaDoc carries sanitizers and test kits sized for inflatable hot tubs, so you can get the right products without guessing - and get back to actually enjoying the water.