You filled the tub, you bought the dichlor granules, and now you're standing there wondering: how much do I actually add? The bottle says "use as directed" and the directions say things like "dose to achieve 3–5 ppm" — which tells you what the target is, but not how to hit it.
This guide gives you concrete numbers. Not ranges to call your pool store about — actual teaspoons, tablespoons, and gallons, organized by tub size and scenario. After reading this, you'll know exactly how much chlorine to add on day one, after each soak, and when you shock weekly.
Why Getting the Dose Right Matters
Chlorine works in a narrow window. Too little (below 1 ppm free chlorine) and bacteria — including the kind that causes hot tub folliculitis and, in rare cases, Legionella — can multiply faster than the sanitizer eliminates them. Too much (above 5 ppm during normal use) causes skin and eye irritation, bleaches bathing suits, and accelerates wear on the tub's shell, seals, and cover.
The challenge with hot tubs specifically is that this window closes faster than in a pool. A 400-gallon hot tub at 102°F with three bathers loses chlorine several times faster than a 15,000-gallon pool at 80°F with the same bathers. The math changes: your doses need to be frequent, precise, and tested.
Your Target: Free Chlorine 1–3 ppm
For routine soaking, keep free chlorine between 1 and 3 ppm. This range kills bacteria on contact, stays below irritation thresholds for most people, and gives you a buffer before the next test.
- Below 1 ppm: sanitizer is ineffective — dose before soaking
- 1–3 ppm: safe zone for regular use
- 3–5 ppm: acceptable during or just after heavy use; wait until it drops to 3 before soaking if you're sensitive
- Above 5 ppm: wait — don't soak until it drops; ventilate the tub
For shocking, you target 10 ppm and wait until it returns to 3 or below before anyone enters. More on shock timing in the weekly routine section below.
Combined chlorine (chloramines — the irritating breakdown products) should stay below 0.5 ppm. If combined chlorine reads high but free chlorine looks normal, you need to shock, not add more sanitizer. See the non-chlorine shock vs. chlorine shock guide for which shock to use in that situation.
What Type of Chlorine to Use
For hot tubs, use dichlor granules (sodium dichloro-s-triazinetriene). All the doses in this guide assume dichlor at approximately 56–62% available chlorine.
Do not use trichlor (the slow-dissolving pool tablets) — they crash pH and load cyanuric acid into your water at a rate your 400-gallon tub cannot handle. For the full explanation, see the dichlor vs. trichlor guide.
If you use cal-hypo shock or liquid bleach for the weekly shock portion, the doses differ from dichlor — see the shock section below.
The Dosing Reference Table
The most useful thing you can have on your phone: how much dichlor raises the chlorine level in your tub by how much. These numbers assume 56% available chlorine dichlor and are rounded to practical measurements:
| Hot Tub Size | 1 tsp dichlor raises Cl₂ by | 1 tbsp dichlor raises Cl₂ by |
|---|---|---|
| 200 gallons | ~1.5 ppm | ~4.5 ppm |
| 300 gallons | ~1.0 ppm | ~3.0 ppm |
| 400 gallons | ~0.75 ppm | ~2.25 ppm |
| 500 gallons | ~0.6 ppm | ~1.8 ppm |
| 600 gallons | ~0.5 ppm | ~1.5 ppm |
How to use this table: Test your current free chlorine level. Subtract from your target. Divide by the per-teaspoon raise for your tub size to get the dose. Example: 400-gallon tub at 0.5 ppm, target is 3 ppm — you need 2.5 ppm of raise. At 0.75 ppm per teaspoon, that's about 3.5 teaspoons, rounded to 4.
If you don't know your tub's exact volume, check the manual or use the physical dimensions: length × width × average depth in feet × 7.48 = gallons (or × 7.5 for a quick estimate). Round, inflatable, and irregular-shape tubs are usually specified on the box or in the product listing.
Scenario 1: First Fill (Startup Dose)
When you fill a hot tub for the first time with fresh tap water, chlorine starts at zero. You want to bring it to about 3–5 ppm as your baseline before the first soak.
Startup dose by tub size:
| Tub Size | Dichlor Startup Dose |
|---|---|
| 200 gallons | 1.5 tbsp (3 tsp) |
| 300 gallons | 2 tbsp |
| 400 gallons | 2.5 tbsp |
| 500 gallons | 3 tbsp |
| 600 gallons | 4 tbsp |
Process:
- Fill the tub and balance pH first (target 7.4–7.6) — chlorine works significantly better in balanced water
- Run the jets to circulate
- Pre-dissolve the dichlor in a small container of warm water before adding; never pour granules directly onto the shell
- Add the dissolved dose with jets running
- Wait 30 minutes, then test
- Adjust up or down based on the reading before anyone gets in
Your tap water may have trace chlorine already (~0.5–1 ppm from municipal treatment), which slightly reduces the needed dose, but at startup it's easier to dose as if starting from zero and adjust after testing.
Scenario 2: After-Soak Maintenance Dose
Every soak consumes chlorine. Three people in a 400-gallon tub for 30 minutes can drop free chlorine from 3 ppm to near 0.5 ppm. You need to dose after each use to bring it back up before the next soak.
Approximate after-soak dose (restoring to 3 ppm after typical use):
| Tub Size | After-Soak Dose (1–2 people, 30 min) | After-Soak Dose (3–5 people, 30–60 min) |
|---|---|---|
| 200 gallons | 1–1.5 tsp | 2–3 tsp |
| 300 gallons | 1.5–2 tsp | 3–4 tsp |
| 400 gallons | 2–3 tsp | 4–5 tsp |
| 500 gallons | 2.5–3.5 tsp | 5–6 tsp |
| 600 gallons | 3–4 tsp | 6–7 tsp |
These are starting points. Test before every soak, not by guessing based on the last dose. Bather load, swim suit residue, sunscreen, and body oils vary enormously. A heavy-use evening and a light solo soak need very different follow-up doses.
The one rule that matters most: test BEFORE you get in, not the morning after. Bacteria grow overnight in under-sanitized water. A 5-minute test before each soak with a test strip or liquid kit is the habit that keeps the water safe.
For accurate results, use a reliable kit. The Taylor K-2006 is the gold standard for FAS-DPD testing — it gives separate free and combined chlorine readings in one step, which matters for diagnosing chloramine buildup. The AquaChek 7-in-1 strips are faster for daily checks, though somewhat less accurate at low ppm levels.
Scenario 3: Weekly Shock
Shocking is not the same as regular sanitizing. It's a deliberate overdose — usually 10 ppm — designed to oxidize chloramines, kill any bacteria that built up resistance, and reset the water to a clean baseline. It should happen once a week regardless of how often the tub was used.
Chlorine shock dose (dichlor, raising to ~10 ppm):
| Tub Size | Weekly Shock Dose (dichlor) |
|---|---|
| 200 gallons | 3–4 tbsp |
| 300 gallons | 4–5 tbsp |
| 400 gallons | 5–6 tbsp |
| 500 gallons | 6–7 tbsp |
| 600 gallons | 7–8 tbsp |
Important: if you're already using dichlor for regular maintenance, weekly dichlor shock accelerates cyanuric acid (CYA) accumulation. Each tablespoon of dichlor adds roughly 57% CYA by weight. Once CYA passes 50 ppm, it begins neutralizing chlorine's effectiveness (chlorine lock) — you'll see normal free chlorine readings but reduced sanitizing power.
The solution: use non-chlorine shock (MPS) for your weekly oxidation, and reserve dichlor shock for true contamination events (illness, heavy bather load, cloudy water). MPS adds no CYA, allows re-entry in 15–30 minutes, and handles the chloramine oxidation job that weekly shocking is actually for. See the MPS vs. chlorine shock guide for when to use each type.
After shocking:
- Leave the cover off for at least 20–30 minutes to allow off-gassing
- Wait until free chlorine drops below 3 ppm before soaking (typically 4–8 hours for dichlor shock; 15–30 minutes for MPS)
- Test before re-entry — don't time-guess
How Temperature and CYA Affect Your Dose
Two variables significantly change how much chlorine you'll burn through:
Water Temperature
Higher temperature accelerates chlorine decomposition. A tub running at 104°F consumes chlorine roughly twice as fast as one kept at 98°F. If you keep your tub on the hotter end of the range, expect to dose slightly more frequently or add 10–20% more per dose during warm-weather use.
CYA Level
Cyanuric acid (the stabilizer inside dichlor) bonds with chlorine and slows its release. As CYA accumulates (which it will, over months of dichlor use), the "effective" free chlorine drops even when the measured reading looks fine.
At 30 ppm CYA, chlorine is reasonably active. At 50 ppm CYA, a 3 ppm chlorine reading behaves more like 1 ppm in actual bacteria-killing power. At 80 ppm CYA, you may have detectable chlorine that is effectively doing very little.
The practical implication: if your water starts feeling slimy or you smell chloramines despite normal chlorine readings, test CYA. If it's over 50 ppm, partial drain and refill is the only solution — no additive removes CYA from water. Once refilled, switch your shock product to MPS or liquid bleach (sodium hypochlorite) to stop adding more CYA.
The Weekly Maintenance Routine
Here's a practical schedule that integrates all three scenarios:
Before every soak (2–5 minutes):
- Test free chlorine — if below 1 ppm, dose and wait 20 minutes before entering
- Glance at pH — if below 7.2 or above 7.8, adjust before dosing chlorine
- Add the after-soak dose after you're done, while jets are running
Once a week:
- Run MPS shock (or chlorine shock if needed — see the decision guide linked above)
- Leave cover off 20–30 minutes after adding
- Test monthly: CYA, total alkalinity, calcium hardness — not just chlorine
Every 3–4 months:
- Drain, clean, and refill
- Restart the dosing routine from the startup dose
This is the routine that keeps a hot tub clean without chemistry headaches. Every problem hot tub owner ends up at — slimy water, persistent smell, eye irritation, cloudy water — usually traces back to one of three gaps: not testing before soaking, not shocking weekly, or letting CYA accumulate unchecked.
Product Table
These are the products referenced in this guide, with affiliate links:
| Product | Use |
|---|---|
| In The Swim Dichlor Granules | Primary sanitizer — daily dosing and shock |
| Leisure Time Spa 56 Chlorinating Granules | Dichlor alternative — same application |
| Leisure Time Renew (MPS Shock) | Weekly non-chlorine shock — no CYA added |
| Taylor K-2006 Test Kit | FAS-DPD liquid test kit — measures free and combined separately |
| AquaChek 7-in-1 Test Strips | Fast daily test strips — good for routine checks |
Common Dosing Mistakes
Adding chlorine to a hot tub with the cover on. Trapped chlorine gas builds under the cover, attacks the underside foam (destroying insulation), and creates a concentrated cloud when you open it. Always add chemicals with jets running, lid open.
Pouring granules directly onto the shell. Undissolved dichlor sitting in one spot can bleach the acrylic permanently. Pre-dissolve in a small cup of warm water before adding, or at minimum add while jets are running at full strength and broadcast evenly across the surface.
Dosing without testing first. You might already be at 4 ppm. Adding more could push to 6 or 7 ppm. Test first, every time.
Dosing right after a soak and immediately covering. The bathers consumed some chlorine but also added organic load (oils, sunscreen, dead skin cells) that chlorine is now oxidizing. Covering immediately traps that reaction, reduces off-gassing, and can create a film on the water surface. Leave the cover cracked or off for 20–30 minutes after dosing.
Using the same dose regardless of bather count. One person in for 20 minutes versus five people in for an hour are completely different chemical loads. Adjust by testing — the table above is a starting point, not a fixed recipe.
Letting chlorine run out overnight. This is the highest-risk mistake. Bacteria don't wait for morning. If you soak at 10 p.m. and don't dose until the next evening, you have 20+ hours of no sanitizer. Dose immediately after every soak.
FAQ
How quickly does chlorine raise after dosing? Dichlor granules dissolve within 5–10 minutes in warm water with jets running. You'll see the full reading within 15–20 minutes. Test at 20 minutes — if you test too soon, you'll see an artificially high reading from undissolved granules near the test sample point.
Can I add chlorine while people are in the tub? No. Even 3 ppm rising to 5 or 6 ppm temporarily during dissolution is irritating. Dose after soaking, not during. The only exception is adding a tiny correction dose (under 1 tsp) to a large tub with jets running — the dilution handles it — but the safer habit is always to dose after.
My test strips keep reading zero even though I just added chlorine. Why? Usually one of three causes: (a) the strip is expired or stored improperly (heat and humidity destroy strip accuracy fast — check the expiration); (b) CYA is high enough that it's masking the reading; or (c) the organic load was high enough that chlorine demand consumed all of it immediately. Test with a liquid FAS-DPD kit if strips read zero despite known dosing.
How often should I change the water? Every 3–4 months as a baseline, less often if you use the tub infrequently. CYA accumulation is the primary driver — once CYA exceeds 50 ppm with dichlor as your primary sanitizer, partial or full drain is the only path back. See the CYA guide for the full math.
What if I'm using bromine instead of chlorine? The same testing-before-soaking and weekly-shocking principles apply, but the doses are different (bromine is less concentrated per gram), and MPS shock is mandatory for bromine systems — not optional. See the bromine vs. chlorine guide for the setup details.
Is it safe to hot tub every day if I'm dosing properly? Yes. Daily soaking with proper testing and after-soak dosing is safe. The frequency increases the importance of weekly shocking and monthly CYA testing — a daily-use tub accumulates chemical load much faster than one used twice a week. Drain and refill every 3 months if you soak daily.
The goal of this guide is to replace guessing with a routine. Test before you soak. Dose after you're done. Shock weekly. Test CYA monthly. Drain quarterly. That five-step cycle, done consistently, keeps hot tub water clean without requiring a chemistry degree — just a test kit and a tablespoon.
Found this helpful?
Share it with others shopping for hot tubs
Related Articles
Hot Tub Chlorine Tablets vs. Granules: Which Form to Use
Dichlor tablets and granules are the same compound in different forms — but they're not interchangeable. This guide covers dosing precision, shell-damage risk, and which form is right for your hot tub.
Dichlor vs. Trichlor: Why Pool Tablets Ruin Hot Tubs
Pool chlorine tablets (trichlor) will crash your hot tub's pH and flood it with CYA. Here's the chemistry behind it and what to use instead.
Non-Chlorine Shock vs. Chlorine Shock for Hot Tubs: Which to Use
MPS or chlorine shock — the wrong choice costs you hours of downtime. This guide gives you the exact decision tree for every hot tub situation.