Dipping a test strip is easy. Reading it is where most hot tub owners get stuck - squinting at faded blocks of pink, orange, and green under bad lighting, trying to decide if the pH is 7.2 or 7.8. This chart explains what each color pad is telling you, the ideal range for a hot tub, and exactly what to do when a reading is off.
The Test Strip Color Chart (Hot Tub Targets)
| What it tests | Ideal range | Color direction | What it means / what to add |
|---|---|---|---|
| Free Chlorine | 1-3 ppm | light → pink/purple | Your sanitizer. Too low = unsafe water, add chlorine or shock. Too high = wait for it to drop before soaking. |
| Bromine | 3-5 ppm | light → pink | Bromine's the common spa sanitizer. Low = add bromine tabs / shock. Note: standard chlorine pads read bromine differently. |
| pH | 7.2-7.8 | yellow (low) → orange/red (high) | Comfort + sanitizer effectiveness. Low = add pH increaser (soda ash). High = add pH decreaser (dry acid). |
| Total Alkalinity | 80-120 ppm | light green → dark green | Buffers pH so it stays stable. Low = add alkalinity increaser (baking soda). High = add pH/alkalinity decreaser. |
| Calcium Hardness | 150-250 ppm | light → purple | Protects heater + jets. Low = add calcium hardness increaser. High = partial drain & refill with softer water. |
| Cyanuric Acid (CYA) | 30-50 ppm (outdoor) | light → pink/tan | Chlorine stabilizer - matters mostly for outdoor tubs. Too high locks up chlorine; fix with a partial drain. |
Ranges follow common industry guidance for residential hot tubs. HTReminder is a tracking tool - always follow your chemical and strip manufacturer's instructions.
How to Read a Test Strip (Step by Step)
- Dip 1-2 seconds. Submerge the strip elbow-deep, then pull it straight out. Don't swish it around and don't shake off the water.
- Hold it level. Keep the strip flat and horizontal so reagents don't bleed from one pad into the next.
- Wait 15-30 seconds. Let the colors develop - but read them within that window. Waiting too long is the #1 mistake and gives false readings.
- Compare in good light. Match each pad against the chart on the bottle, in natural or bright white light. Yellow garage bulbs throw the colors off.
- Read every pad. At minimum check sanitizer (chlorine/bromine), pH, and alkalinity, then adjust chemicals as needed.
Tired of Squinting at the Colors?
HTReminder scans your test strip with AI, reads every pad against the right brand chart, and tells you exactly what to add - no color-matching, any brand.
Get HTReminder FreeWhat Each Reading Means (and How to Fix It)
Sanitizer (Chlorine or Bromine)
This is the single most important reading - it's what keeps the water safe. Chlorine should sit at 1-3 ppm, bromine at 3-5 ppm. If the pad barely changes color, your sanitizer is low: add it and re-test in a couple of hours before soaking. If it's off-the-chart high, wait for it to fall to a safe range first.
pH (7.2-7.8)
pH controls both comfort and how well your sanitizer works. A yellow-leaning pad means low pH (corrosive, itchy skin) - add a pH increaser. An orange/red pad means high pH - chlorine gets sluggish and scale forms, so add a pH decreaser. Aim for the 7.4-7.6 sweet spot.
Total Alkalinity (80-120 ppm)
Alkalinity is pH's shock absorber. Get it in range first and your pH stops bouncing around. Low alkalinity (light green) = add baking soda / alkalinity increaser. High (dark green) = bring it down with a pH/alkalinity decreaser.
Calcium Hardness (150-250 ppm)
Too-soft water is corrosive and can eat at your heater; too-hard water leaves scale on jets and the shell. Low = add a calcium hardness increaser. High = partial drain and refill with softer water.
Common Mistakes When Reading Test Strips
- Reading too late. Colors keep shifting after 30 seconds - a strip read at two minutes will lie to you.
- Wet fingers. Water on your hands dilutes the pads. Keep the strips and your fingers dry until the moment you dip.
- Old or heat-baked strips. Test strips expire and hate humidity. Store them cool and dry, cap tight, and check the date.
- Bad lighting. Match colors in bright, neutral light - not under a yellow bulb or in shade.
Which Test Strips Should You Use?
Any 4-way or 7-way strip works for the readings above. If you want brand picks - including options built specifically for bromine and FROG systems - see our guide to the best hot tub test strips.
Does This Chart Work for Pools?
Yes - the same pads mean the same things in a pool. HTReminder and this chart are built around hot tubs and spas, but pools use the identical chemistry; the main differences are that outdoor pools lean harder on cyanuric acid (stabilizer) and often run chlorine a touch higher. The reading technique and color directions are the same.
Never Read a Color Chart Again
Point your phone at the strip. HTReminder does the reading and tells you the exact dose to add.
Download Free on AndroidFrequently Asked Questions
What do the colors on a hot tub test strip mean?
Each pad changes color based on one water level. Matched against the bottle's chart, the color gives you the value - for example, a pH pad shifting toward orange/red means higher pH, yellow means lower. Exact colors vary by brand, so always compare against your own bottle.
What is the ideal pH for a hot tub?
7.2 to 7.8, ideally 7.4-7.6. Below that is corrosive and irritating; above it makes your sanitizer weaker and causes scale.
How long do you wait to read a test strip?
15 to 30 seconds - and read it inside that window. Reading too late is the most common cause of a wrong result.
Can an app read a test strip for me?
Yes. HTReminder scans the strip with your phone camera, reads every pad against the correct brand chart, and tells you exactly what to add - so you never have to eyeball the colors.