Comparison

Spa Calc vs Pool Math for Hot Tubs

Pool Math is a pool calculator that can also hold spa configurations on its Premium plan. Spa Calc is built only for hot tubs: a soak check on the first screen, bromine and chlorine, doses in teaspoons and a drain countdown. If your hot tub is your only body of water, here's how they compare.

Prices, features and ratings checked on 28 October 2026.
App Store rating as shown on 28 October 2026: 3.6 stars from 287 ratings on the US App Store.

The short answer

Pool Math is a solid pool app that can hold a spa. Spa Calc is for people whose hot tub is the main event: it tells you if it's ready to soak, speaks bromine and teaspoons, and counts down to drain day, free for one tub.

At a glance

From each app's own App Store page and website, checked on 28 October 2026. "Not listed" means we found no mention of it there, not that the feature is missing.
Feature	Spa Calc	Pool Math
Price for one hot tub	Free, every feature	Free calculators; spa configurations with $7.99/year Premium
Built for	Hot tubs, swim spas, plunge tubs, inflatables	Pools, with spa configurations
"Ready to soak?" check	Yes	Not listed
Bromine	Yes Bromide bank, then bromine doses	Not listed
Dichlor then bleach tracker	Yes	Not listed
Non-chlorine shock (MPS)	Yes With a wait timer	Not listed
Post-soak top-up dose	Yes "I just soaked"	Not listed
Doses in teaspoons	Yes Down to an eighth	Not listed
Inflatable presets	Yes	Not listed
Drain and refill countdown	Yes From tub size and use	Not listed
Water balance (CSI)	Yes Uses tub temperature	Yes CSI calculator
Test log history	Yes Free	Yes Unlimited with Premium
Reminders	Yes Free	Yes Premium
Backup and sync	Yes Free iCloud backup	Yes Premium
Rental and service tools	Yes Plus and Pro	Not listed
Community forum	No	Yes TroubleFreePool
Platforms	iPhone (iOS 18 or later)	iPhone, iPad, Apple silicon Mac, Android

Pricing

Our app

Spa Calc

Free$0Every feature for one tub: soak check, doses, full history, charts, reminders, PDF and CSV export, iCloud backup.
Plus$1.99 a month, $14.99 a year or $39.99 onceUp to 10 tubs plus the host and pro tools. 1-week free trial on the yearly plan.
Pro$9.99 a month or $79.99 a yearUnlimited tubs plus the host and pro tools. 1-month free trial on the yearly plan.

Prices on the App Store at launch. Pricing is per person, not per tub.

Pool chemistry app with spa configurations

Pool Math

Free$0Calculators for pH, free chlorine, calcium hardness, salt, total alkalinity, borates and CSI.
Premium$7.99/yearAdds unlimited test log history, reminders, cloud and multi-device sync, unlimited pool/spa configurations, and CSV import/export.

Free download with one in-app purchase.

Prices as listed on 28 October 2026. Source

Pool Math Premium at $7.99 a year is cheap and covers pools and spas. Spa Calc is free for one tub with every feature. If you own a pool and a hot tub, Pool Math Premium may be the simpler buy; our pool app, Pool Calc, also covers a spa attached to the pool for free.

Is it safe to soak?

Spa Calc's first screen answers that. Enter sanitizer and pH and it says ready, tells you what to add first, or shows how long to wait after a shock. The ranges come from CDC guidance and product labels: chlorine 3 to 5 ppm, bromine 4 to 6 ppm, pH 7.2 to 7.8.

Hot tub chemistry, built in

Bromine starts with a bromide bank and moves to maintenance doses. Chlorine tubs can start on dichlor and switch to bleach; the tracker estimates the stabilizer that builds up and tells you when. MPS gets its own dose and wait timer, and "I just soaked" gives a top-up after each soak.

Small tubs, small doses

Doses come in teaspoons down to an eighth, tablespoons, grams or your own scoop. Inflatable presets set the volume, and you confirm it at your fill line.

What Pool Math Users Complain About

Paraphrased from public reviews. Each one links to where it was posted and gives the month it was written.

Not enough spa-specific options. A 2022 App Store reviewer asked for non-chlorine oxidizers and more spa-specific options, and was logging MPS as something else or in a note (App Store review, April 2022).

Spa Calc approach: MPS, bromine, dichlor and bleach are all first-class, with their own doses, wait timers and log entries.
Custom targets reset. A 2022 reviewer said custom targets reset to defaults when reopening results (App Store review, June 2022).

Spa Calc approach: custom targets stay until you reset them, per row or all at once.

When Pool Math is the better choice

You have a pool and a spa and want one inexpensive app for both.
You follow the TroubleFreePool method and use its forum.
You want an iPad, Mac or Android version.

Questions people ask

Does Pool Math work for hot tubs?

Pool Math's Premium listing includes unlimited pool/spa configurations, so you can set up a spa. A 2022 App Store reviewer asked for more spa-specific options and for non-chlorine shock. Spa Calc is built only for hot tubs.

Is there a free hot tub chemical app?

Spa Calc gives one hot tub every feature for free, inflatables included: a soak check, doses in teaspoons, a drain countdown, the full log and charts. It is on the App Store at launch.

Can I use the TroubleFreePool method in a hot tub?

Many hot tub owners use bleach with a small amount of stabilizer, or start with dichlor and switch to bleach. Spa Calc has a dichlor-then-bleach tracker that estimates how much stabilizer the dichlor has added and tells you when to switch.

More comparisons

All comparisons

References

Pool Math and TroubleFreePool are trademarks of their respective owners. Spa Calc is not affiliated with them. Information checked on 28 October 2026; prices and features change. Corrections: [email protected]