Hot Tub Test Strips: How to Read Them & What the Colors Mean

Test strips are the most-used and most-misread tool in hot tub ownership. They’re cheap, fast, and — used correctly — perfectly adequate for routine water care. Used carelessly, they produce confident wrong answers that send owners chasing problems they don’t have. This guide covers what each pad actually measures, the technique that gives accurate readings, the misreads everyone makes, and when strips aren’t enough.

New to water chemistry entirely? Start with our hot tub chemicals beginner guide — then come back here to learn how to actually read the strips.

What each pad measures

A standard 4-way or 5-way spa strip has one pad per parameter, arranged in a fixed order along the strip. Read them in the order the manufacturer intends (usually top to bottom or as labeled on the bottle) so you don’t mix up results.

pH — how acidic or basic the water is

Ideal range for spas: 7.2–7.8. Below 7.2, water is acidic: it stings eyes, corrodes metal parts and heater elements, and eats away at the tub’s plumbing over time. Above 7.8, water is basic: sanitizer works poorly, scale forms on surfaces, and water gets cloudy. pH is the parameter that makes everything else work or fail — sanitizer efficiency, bather comfort, and equipment life all hinge on it.

Free chlorine / bromine — active sanitizer level

Ideal: 3–5 ppm free chlorine, or 4–6 ppm bromine (some strips show a combined sanitizer pad calibrated for both). This is the sanitizer actually available to kill contaminants right now — not the total ever added. If this reads zero, your tub is unprotected regardless of what you added yesterday. Note: strips measure total sanitizer on some pads and free on others; free is the number that matters. Not sure which sanitizer system you’re on? Our chlorine vs bromine guide covers the trade-off.

Total alkalinity — pH’s bodyguard

Ideal: 80–120 ppm. Alkalinity buffers pH against swings — it’s the reason pH stays put instead of bouncing every time someone gets in. Low alkalinity means pH crashes and spikes unpredictably; high alkalinity makes pH stubbornly high and hard to bring down. Always adjust alkalinity before pH — fixing pH first while alkalinity is off is the most common reason owners end up chasing their chemistry in circles.

Calcium hardness — dissolved minerals

Ideal: 150–250 ppm. Too soft (low calcium) and water turns aggressive, leaching minerals from metal components and etching surfaces. Too hard and you get scale — crusty white deposits on the shell, heater, and jets. Inflatable owners on soft municipal water should pay attention here; very soft fill water is common and quietly corrosive.

Cyanuric acid (stabilizer) — on some strips

Some multi-way strips include a stabilizer pad. Cyanuric acid (CYA) protects chlorine from UV breakdown — relevant for outdoor pools, mostly irrelevant for covered hot tubs. If your strip has it and you use dichlor granules (which contain CYA), watch that it doesn’t creep up over time; high CYA “locks” chlorine and reduces its effectiveness. Bromine users can ignore this pad entirely.

How to read a strip accurately

The technique matters more than the brand. Follow this sequence exactly:

1. Run the jets first. Circulate the water for a minute or two so you’re testing mixed water, not a stagnant pocket near the edge.
2. Dip elbow-deep, away from the jets. Push the strip about 18 inches under the surface in the middle of the tub, away from return jets and chemical feeders. Surface water and jet-blasted water don’t represent the tub.
3. One second, then out. Submerge all pads for about one second — no longer. Soaking the strip leaches the reagents and muddies every reading.
4. Don’t shake it dry. Hold the strip horizontally and let excess water sit on the pads (most manufacturers say not to shake; a gentle flick is the most you should do). Shaking flings reagent off the pads.
5. Read within 10–15 seconds. The colors keep developing after you pull the strip out. Reading at 30 seconds or a minute gives a different — wrong — answer. Set the expectation before you dip: pads in, eyes on the chart, read promptly.
6. Natural light, not bathroom light. Compare pad colors to the bottle’s chart in daylight or bright white light. Warm indoor lighting shifts every color and is the single most common cause of “the pH looks fine to me” misreads.
7. Match, don’t interpret. Find the closest color match per pad — don’t average, don’t squint, don’t decide it’s “between” two. If a pad genuinely falls between two chart colors, retest with a fresh strip rather than guessing.

The common misreads

Reading too late. The colors keep changing. A strip read at 60 seconds can show pH a full point higher than the same strip read at 15 seconds. The chart is calibrated for the specified read time — after that, it’s fiction.

Wet fingers on the pads. Touching the pads with wet fingers before dipping contaminates the reagents. Handle strips by the dry end only, with dry hands.

Old or badly stored strips. Heat, humidity, and light degrade the pads. Keep the bottle capped tight, stored cool and dry — not on the tub’s edge in the sun. Once opened, most strips should be used within about 90 days. Expired strips read low on sanitizer, which sends owners overdosing.

Testing right after dosing. Adding chemicals and immediately testing tells you nothing useful — the water isn’t mixed yet. Wait at least 15–30 minutes with the jets running (an hour is better) before retesting after an adjustment.

Testing right after shocking. Shock oxidizers — especially chlorine-based shock — will blow sanitizer readings sky-high for hours. Wait until the next day to get a meaningful sanitizer reading after shocking.

Confusing total and free sanitizer. Some strips show total chlorine (free + combined). A high total with zero free means your sanitizer is all spent chloramine — the tub looks “fine” on the strip while being unprotected. Know which pad your strip uses; free sanitizer is the number that matters.

When to use a liquid test kit instead

Strips are fine for routine weekly checks. They’re not fine for everything:

• When something is wrong and strips say everything is fine. Cloudy water, foam, smell, or irritation with “normal” strip readings means you need better resolution — a liquid drop kit (reagent drops + comparator tube) reads in finer increments and is less fooled by interfering substances.
• When dialing in a new fill. Fresh fills need precise balancing; the first week deserves liquid-kit accuracy. Once the water is stable, strips are fine for maintenance checks.
• When CYA or calcium need real numbers. Strip pads for stabilizer and hardness are coarse. If you’re troubleshooting scale or chlorine lock, a liquid kit gives you numbers you can act on.
• When strips disagree with each other. Two strips, two different answers — trust neither, and break out the drops.

A good setup for most owners: strips for the weekly routine, a basic liquid kit (pH + sanitizer + alkalinity) in the cupboard for the moments above. That’s under $30 total and covers everything an inflatable owner will ever need to diagnose. If problems persist past what testing explains, our troubleshooting guide walks through cloudy water, foam, and smell in diagnostic order.

How often to test

New tub or new fill: daily for the first week — you’re learning the water’s personality and catching the initial balance drift. Established tub, regular use: 2–3 times a week minimum; before every soak is ideal and takes 30 seconds. After heavy use, rain, or topping up: always test — bather load and fresh water both move chemistry. After any chemical adjustment: retest after 30–60 minutes of circulation to confirm the change landed where you aimed. Our dosing calculator tells you exactly how much to add once you know what the strip says.

Build testing into the routine from our maintenance schedule — the weekly 15-minute check is mostly just this: dip, read, adjust, done.

Frequently asked questions

Are cheap test strips accurate enough?

For routine maintenance, yes — provided they’re fresh, stored properly, and read correctly (on time, in daylight). The accuracy gap between cheap and expensive strips is smaller than the accuracy gap between good and bad technique. Spend your attention on the procedure, not the brand.

Why do my strips always show low sanitizer?

Three usual suspects: expired or heat-damaged strips (they read low), testing too long after your last dose (sanitizer genuinely depleted — dose more often), or high CYA locking your chlorine (test CYA; if it’s high, a partial drain and refill is the fix). Rule them out in that order.

Can I cut strips in half to save money?

Don’t. Cutting exposes the pad edges, changes how reagents wet, and voids any calibration the chart assumes. Strips are already the cheapest part of hot tub ownership — this is not the place to economize.

Do digital test readers work better?

They remove the color-matching guesswork, which helps people who struggle with the chart. But they read the same chemistry with the same limitations — a digital reader on an expired strip is still wrong, just more confidently. Fix technique and freshness first; buy the gadget only if color-matching is genuinely your bottleneck.

How we wrote this guide

No lab testing was performed for this article. Reading technique, ideal ranges, and storage guidance reflect manufacturer instructions (AquaChek, Intex) and widely published spa-industry practice; the misread catalog is synthesized from common owner errors documented across spa forums and dealer guidance. Last verified: October 2026.