Is your hot tub ready to soak?
Free calculators for chlorine, bromine, shock, pH and drain day, sized for your tub, inflatable or hard shell. Hot tub limits based on CDC guidance. The same math runs in the Spa Calc app.
Check before you get in
Wait 5 min
Give it at least 15 minutes with the jets running and the cover open, then retest.
- Soak range for free chlorine: 3 ppm to 5 ppm (and at least 7.5% of your CYA), with pH 7.2 to 7.8. Below pH 7.0 or above 7.8 it is not safe. Hot tub limits based on CDC guidance and EPA product labels; your product label comes first.
Calculators
Each one answers a single question with an amount for your product, in US or metric units, and shows its working.
- All-in-one hot tub calculatorEnter a test. Get every dose in order, and whether you can soak.
- Hot tub chlorine calculatorDichlor or bleach, in teaspoons and grams, for your tub.
- Hot tub bromine calculatorBromide reserve for new water, and how to raise bromine.
- Drain interval calculatorDays between drains from your volume and soakers.
- Hot tub volume calculatorGallons or litres for inflatable, round and square tubs.
- Hot tub pH and alkalinity calculatorpH down, pH up or alkalinity up, sized for your tub.
- Ready to soak?Soak, wait or fix first, from your test and your last dose.
- MPS shock calculatorNon-chlorine shock after soaks or weekly.
- Bromine to chlorine converterChlorine reading to bromine, and back (× 2.25).
Test, tap, soak
One hot tub? Every feature, free. Spa Calc checks your readings against the soak range when you lift the cover: soak now, wait, or add this first. It counts down to drain day and keeps your log in your own iCloud.
Spa Calc for iPhoneComing soon to the App StoreGarden spa, 400 gal
Chlorine 4.0 and pH 7.5 are in range.
After the soak
One addition, then wait 15 minutes.
Dichlor granules 56%
FC 1.0 to 4.0
8.1g
gtspoz
Sprinkle in with the jets running.
Leave the cover open 15 minutes.
Check the product label. Strengths vary by brand.
Mark as added
Stabilizer estimate 24 ppm. Switch to bleach at 30.
Drain and refill
Based on 400 gal and 2 soakers a day.
- Flush the lines
- Drain and wipe the shell
- Rinse the filter
- Refill and rebalance
I drained it today
Built for the moment you’re holding the test kit
- Ready-to-soak check
- Sanitizer, pH and your last dose become one answer: ready, wait, add sanitizer first, or not safe. Hot tub limits based on CDC guidance and product labels.
- Chlorine or bromine
- Dichlor-then-bleach with a running stabilizer estimate, bleach only with no stabilizer, or a bromide reserve plus oxidizer. All built in.
- MPS and chlorine shock
- After-soak and weekly shock doses, with the wait before you can get back in.
- Drain countdown
- Days to the next drain from your tub size and how many people soak, plus the drain checklist.
- Inflatable presets
- Pick your inflatable model or enter litres or gallons. Doses come out in grams, teaspoons or ounces.
- Your data stays yours
- No account and no ads. Your log lives on your iPhone and in your own iCloud.
For hosts & hot tub pros
Plus covers up to 10 tubs, for rental hosts. Pro is unlimited, for hot tub service techs and companies. Both include every tool below.
- Clients and properties
- A card per property or client with address, phone, email, access notes and service days. Each tub keeps its own log.
- Today’s route
- The properties due today, in the order you set, ticked off as you finish each stop.
- One screen per stop
- Readings, the doses with “Mark as added”, tasks, photos and notes, all on one screen.
- Branded service report
- A PDF with your logo: readings against target, chemicals added, tasks, photos and the next visit. Send it from your phone by text or email.
- Chemical use and cost
- What each property used and what it cost, from the prices on your own product shelf.
- Business profile and logo
- Company name, phone, email, website and license number, printed on every report.
- Client import
- Bring your client or property list in from a CSV file or from Contacts.
- Bulk CSV export
- Every property’s visits, readings and chemicals in CSV files, ready for your invoicing tool or spreadsheet.
Priced per person, not per tub. No per-tub fees, it works offline, there is no account, and your client data stays in your own iCloud.
Spa Calc does not do invoicing, payments or dispatch. It is the chemistry, the log and the report, on your phone.
One hot tub? Every feature, free.
Free has every hot tub feature for one tub: the soak check, the full calculator, your whole log, charts, reminders, the drain countdown, widgets and export. Pay only to look after more tubs. No weekly plans, no ads.
Free
$0
Every feature, for one tub
- One hot tub, inflatable included
- Full dose calculator and soak check
- Your whole log and every chart
- Unlimited reminders and the drain countdown
- Your own product shelf
- PDF and CSV export
- Widgets, themes and app icons
- Free iCloud backup
Spa Calc Plus
$14.99 a year
with a free first week, or $1.99 a month, or $39.99 once
Rental hosts and homes with more than one tub
- Up to 10 tubs
- Everything in Free, for every tub
- All the pro tools
- Family Sharing
Spa Calc Pro
$79.99 a year
with a free first month, or $9.99 a month
Hot tub service techs and companies
- Unlimited tubs
- Everything in Plus
- All the pro tools
- One subscription per technician
Guides
Plain answers to the questions people actually search, each with the calculator built in.
- Bromine vs Chlorine for Hot TubsBromine vs chlorine for hot tubs, honestly compared: how each works, cost, smell, skin feel, testing, inflatables, and why switching back needs a drain.
- Cloudy Hot Tub Water: Causes and FixesWhy hot tub water turns cloudy (low sanitizer, high pH or alkalinity, dirty filter, old water, biofilm) and the order to fix it, including after a shock.
- Hot Tub Foam: Causes and FixesWhat makes hot tub water foam (soaps, lotions, detergent, old water, low sanitizer, soft water) and how to fix it for good, not just with defoamer.
- Hot Tub pH Too High: Why It Rises and How to Lower ItWhy hot tub pH climbs (jets, high alkalinity, fresh fill water), how to lower pH and alkalinity with pH decreaser, and how borates keep it stable.
- Hot Tub Shock: When, Which Type, and How Long to WaitChlorine shock vs non-chlorine (MPS) shock, how often to shock a hot tub, how much to use, and what to test before you get back in.
- How Many Gallons in a Hot Tub?Typical hot tub volumes, how to calculate gallons or litres for square, round and inflatable tubs, and two ways to measure your tub's real volume.
- How Much Bromine to Add to a Hot TubHow to start a bromine hot tub with a bromide bank, how much tablets or granules to use, the right bromine level, and how to fix low or high readings.
- How Much Chlorine to Add to a Hot TubChlorine amounts per 1 ppm for dichlor, liquid chlorine and bleach, plus the dichlor-then-bleach method, after-soak dosing and shock for any hot tub.
- How Often Should You Drain a Hot Tub?The water replacement formula the spa industry uses, worked examples for acrylic and inflatable tubs, and the signs your water needs changing early.
- Inflatable Hot Tub Chemicals: The Complete GuideWhich chemicals an inflatable hot tub needs (Lay-Z-Spa, Intex PureSpa, SaluSpa, MSpa), how much for your volume, first-fill steps and a simple routine.
About the app and this site
Are the calculators on this site the same as the app?
Yes. Both use the same chemistry engine and are tested against the same reference results.
Does it work for inflatable hot tubs?
Yes. Pick your model or enter your volume. Doses are given in grams, teaspoons or ounces, which suits small inflatable tubs.
Chlorine or bromine?
Both. Bromine uses a sodium bromide reserve and a small oxidizer dose. Chlorine can use the dichlor-then-bleach method, which keeps stabilizer to about 30 to 40 ppm, or bleach only with no stabilizer at all, which is what CDC recommends for hot tubs.
When is a hot tub ready to soak?
Soak when free chlorine is 3 to 5 ppm and at least 7.5% of your CYA (bromine 4 to 6) and pH is 7.2 to 7.8, at least 15 minutes after adding anything (20 after pH decreaser); after chlorine, bromine or a shock, test again first. Above 5 ppm chlorine or 6 ppm bromine, wait and retest in an hour or two; never soak above 10 ppm chlorine or 8 ppm bromine, or with pH below 7.0 or above 7.8. Below 3 ppm chlorine (4 bromine), add sanitizer first; at pH 7.0 to 7.2, raise pH first; with CYA above 40 ppm, drain and refill. Hot tub limits based on CDC guidance and EPA product labels.
Do I need an account?
No. There is no sign-up. Your data stays on your iPhone and in your own iCloud.
What does the free version include?
Every hot tub feature, for one tub: the soak check, the full dose calculator, your whole log and every chart, unlimited reminders and the drain countdown, your own product shelf, PDF and CSV export, widgets and iCloud backup. The pro tools (clients, route, service reports, chemical costs) come with Plus and Pro.
How much are Plus and Pro?
Plus (up to 10 tubs) is $1.99 a month, $14.99 a year with a free first week, or $39.99 once. Pro (unlimited) is $9.99 a month, or $79.99 a year with a free first month. Both include the pro tools. There is no weekly plan.
Is it priced per tub?
No. You pay per person, not per tub: one subscription on your Apple Account covers every tub in your plan. Plus can be shared with your family; Pro is one technician per subscription. Archived tubs don’t count.
What happens if I stop paying?
Nothing is deleted. Your main tub stays fully usable, and any extra tubs become view and export only until you subscribe again or archive some.
Is there an Android version?
Not yet. The calculators on this site work in any browser.
Spa Calc for iPhone
One hot tub? Every feature, free. No account. Free iCloud backup. No ads.
Spa Calc for iPhoneComing soon to the App Store