You dip a strip, hold it against the bottle, and every pad lands in the “OK” zone. Then a dosing app — or a spa forum, or this site — tells you to adjust something anyway. It feels like a contradiction, and it’s one of the most common questions new spa owners ask. The short answer: both are right, because they’re answering two different questions. The strip tells you what’s in your water. The target range tells you what that number should be for a spa. Those are not the same job.
The bottle prints broad bands, not spa ideals
The color chart on a test-strip bottle is built to sell one product to everyone — pool owners and spa owners, hot water and cold. To do that, each “OK” band is drawn wide enough to cover all of those cases, so it only flags water that’s clearly out of bounds. It’s a pass/fail sanity check, not a precision target.
Spa-specific ranges are narrower on purpose. Hot, heavily aerated, low-volume water behaves differently from a backyard pool: it off-gasses carbon dioxide and drifts upward in pH, it burns through sanitizer faster, and small chemical additions move the numbers a lot. So the ideal spa window sits inside the strip’s broad band, usually toward one end of it. Here’s how a typical bromine or chlorine strip’s printed zones compare to the spa ideals (based on Pool & Hot Tub Alliance guidance):
| Reading | Strip’s printed “OK / ideal” | Spa ideal target | What’s going on |
|---|---|---|---|
| pH | 7.2–7.8 | 7.4–7.6 | App aims for the middle of the strip’s band — no conflict |
| Total alkalinity | 80–120 ppm | 80–100 ppm | Low end keeps spa pH from drifting up |
| Bromine | 2–5 ppm (ideal ~5) | 4–6 ppm | Closely aligned; both center near 5 |
| Hardness | 250–500 ppm | 150–250 ppm | Different and measuring a different thing — see below |
For pH, alkalinity, and sanitizer, the app simply lands inside the band the strip already calls acceptable — so anything it flags is a small nudge toward ideal, never a reversal. Hardness is the one real outlier, and it’s worth understanding why.
Total hardness vs. calcium hardness — the one that trips everyone up
Look closely at your strip and the hardness pad almost always says Total Hardness. That’s calcium plus magnesium dissolved in the water. But when a spa guide talks about a hardness target, it means Calcium Hardness — just the calcium, because calcium is the part that protects your heater, seals, and shell from aggressive water. Magnesium comes along for the ride but isn’t what the target is about.
Because total hardness counts both minerals, it always reads higher than the pure calcium number. That single fact explains the whole gap in the table: the strip’s “OK” band starts around 250 ppm because it’s scoring total hardness, while a spa’s calcium target tops out around 250 ppm. They look like they disagree, but they’re measuring two different things.
What to do about it in practice: enter whatever your strip reads. On typical fill water the difference is modest, and it’s close enough for dosing — the calcium increaser you’d add on refill day isn’t sensitive to a few dozen ppm. The one time it matters is if your strip reads high-ish total hardness but you’re still seeing foaming or aggressive-water symptoms; that can mean your calcium is genuinely low even though total hardness looks fine. Our calcium hardness guide covers how to read those signs and dose for your fill water.
Let the app hold the ideal ranges for you
SpaCheckup compares each reading to the spa-specific target — not the broad band on the bottle — and tells you exactly what, if anything, to add.
pH and alkalinity: tighter, not contradictory
The other two spots where the app looks stricter than the bottle are pH and total alkalinity, and both come down to the same spa quirk. Hot, aerated water constantly off-gasses carbon dioxide, which pushes pH up over time. Keeping alkalinity toward the low end of the acceptable band (80–100 rather than 80–120 ppm) slows that climb, and aiming pH at 7.4–7.6 instead of anywhere in the strip’s 7.2–7.8 window gives you a little runway before it drifts high again. If you’ve ever chased rising pH week after week, this is why the tighter target is doing you a favor. The alkalinity guide goes deeper on that link.
So which one should you trust?
Trust the strip to measure and trust spa-specific ranges to judge. A good strip, read correctly, gives you an honest number — the technique tips in how to read a test strip matter far more to accuracy than which range chart you use. Once you have the number, compare it to spa targets, not the broad pass/fail band on the label. That’s the whole trick: the bottle is a ruler, and the ideal range is what you do with the measurement.
Stop second-guessing the color chart
SpaCheckup reads your numbers against spa-specific targets and tells you exactly what to add — sized to your tub’s volume, sanitizer, and product strength. No more wondering whether “OK” is actually good enough.
Free to download — unlock everything once. No subscription, no ads.
Frequently asked questions
Why does my test strip say OK but my app says to adjust?
What is the difference between total hardness and calcium hardness?
Should I trust my test strip or the spa target ranges?
Do I enter total hardness or calcium hardness into a dosing app?
Related guides
How to read a hot tub test strip
What each pad measures, how to get an accurate reading, and what the numbers should be for a spa.
Read the guide → Fix itCalcium hardness in hot tubs: why it matters
What hardness protects, why both low and high cause trouble, and how to adjust it for your fill water.
Read the guide → Fix itHot tub alkalinity: the number behind your pH problems
Why total alkalinity makes pH bounce around, the 80–100 ppm target, and how to raise or lower it without wrecking pH.
Read the guide →