Connect · auto-log every session · zero backend

Log every workout automatically.
Ten-minute setup. Once.

Step on the scale before class and after it. That's your whole job. When your watch ends the workout the shortcut fires by itself, reads both weights and the duration out of Apple Health, and your formula is waiting in your workout log before you've found your shoes. No account, no subscription, no native app — an Apple Shortcut talking to your browser's local storage.

Weigh in twice and we keep your rate — every estimate after that is yours instead of a population average. For the founder that gap was 35%, which is about 6 g of salt a session. This page exists to make the weighing cost nothing, because a measurement you take twice a week for a year beats a perfect one you take once.

iOS · recommended

Apple Shortcuts + HealthKit

Full auto-log. Reads workout + weight from Health. Build-it-yourself steps below — about 10 min one-time.

Android

Google Fit / Health Connect

Coming later. For now, use the manual calculator; HealthKit is the Phase-0 integration.

Any · fallback

Quick-log bookmarklet

Drag to bookmarks; click after class to prompt for weights and auto-fill the calculator. Works on any browser.

The Apple Shortcut — step by step

We can't host the shortcut file (would require an iCloud account to upload and a per-user token system). It's a one-time 10-minute build. Once it's on your iPhone, it runs in about 3 seconds after each class.

What you'll need

A smart scale that syncs to Apple Health (Withings, Renpho, Eufy — around $30–40 for the cheap ones, and they all write body mass to Health). This is not the optional part, it's the whole point: the scale is the only thing here that measures anything. Everything else on this page is plumbing to save you typing.

Plus: an iPhone with the built-in Shortcuts app, and an Apple Watch if you want it to fire on its own (step 9). Health workout history from any tracker that syncs — Watch, Oura, Whoop — works for the duration.

1

Open the Shortcuts app · tap  +  to create a new shortcut

Name it "Log SweatSciences Session".

2

Add action: Find Workouts

Search for "Find Workouts" in the action list. Set: Sort by Start Date · Order Latest First · Limit: 1.

This grabs your most recently ended workout from Apple Health — whether it's a Watch-detected workout, a manual entry, or synced from a third-party app. You can filter by activity type (e.g., "Yoga") if you want to restrict it.
3

Get the Duration — and turn it into minutes here

Add "Get Details of Workouts" → Detail: Duration. Then add "Calculate" → divide it by 60, then "Round Number" → to the nearest 1. Save that as duration_min.

While you're here, add a second "Get Details of Workouts" → Detail: Type, and save it as activity.

Do the conversion now, not in the URL step — Shortcuts gives you Duration in seconds, and a stray 4,380 landing in t would tell us you did a 73-hour yoga class. Both of these have to reach the URL in step 7; the duration sets your sweat rate and the type decides which activity it calibrates.
4

Find your Body Mass samples (last 24 hours)

Add "Find Health Samples" → set Type: Body Mass, Order: Latest First, Limit: 2.

If you use a smart scale that syncs to Health, the two most-recent readings are typically your before-class and after-class weights. If you only have one reading, we'll prompt you to weigh in now.
5

Pull the two weights

Use Get Item from List twice — first item = weight_after (most recent), second item = weight_before. Then Get Details of Health Sample → Value for each. Health stores weight in your preferred unit (lb or kg). If kg, multiply by 2.20462 to get lb.

6

(Optional) Prompt if you didn't step on the scale after class

Add "If" action — if weight_after is empty or less than 10 minutes old, use it; otherwise show "Ask for Input" with prompt "Weigh in now — what's your weight? (lb)".

7

Build the URL

Add "Text" action with this template. The tokens in orange get replaced by your Shortcut variables.

https://sweatsciences.com/?m=p&wb={weight_before}&wa={weight_after}&wd=0&t={duration_min}&a={activity}&i=hard&s=1100&auto=1#calc
Don't drop t. You captured the duration back in step 3 — it has to travel. Your sweat rate is litres ÷ hours, and that rate is what we keep and reuse for every future estimate. Send a 73-minute class without t and it records against a default of 60 minutes: 22% too fast, plausible enough to pass our sanity checks, and wrong in your log for good. a matters for the same reason — we calibrate each activity separately, so a weighed yoga class shouldn't teach us anything about your bike.
m=p = precise (weigh-in) mode — the calculator computes your loss straight from the weight delta · wb = weight before (lb) · wa = weight after (lb) · wd = fluid drunk during class (oz; 0 if none) · s = how salty you sweat (800 light → 1,800 very salty; 1,100 is average — adjust once and it sticks) · auto=1 = compute and save to your log automatically.
8

Add Open URL at the end

Pass the built URL to "Open URLs". Done. The browser opens to the calculator with everything pre-filled. It computes your formula, saves the workout to your local log, and you see your recipe.

9

Stop tapping it — make it fire when the class ends

This is the step that turns a gadget into a habit. In Shortcuts, open the Automation tab → + → Apple Watch Workout. Choose your workout type (or Any Workout), select Ends, and set it to run your shortcut. Turn Ask Before Running off.

Now the sequence is: finish class, step on the scale, done. Nothing to remember, nothing to tap.

Requires your iPhone paired with an Apple Watch — the trigger comes from the watch ending the workout. Weigh yourself within a few minutes of the end so the scale reading lands inside the window the shortcut looks at.

Why bother — the honest version

Not so we can nail today's number. Today's number can't be nailed: in a 105°F room at 68% humidity your sweat rate is set by that day's air, and your sweat sodium quietly drifts down over a season of practice while your rate drifts up. The number moves. Nothing on your wrist can see it either — no watch has a sweat sensor, and the ones that model it from calories run 25–33% off, because in a room that humid your sweat mostly drips instead of evaporating and stops telling the truth about heat.

The scale is the only instrument in the building that knows. Done properly it's good to about 5%. This shortcut exists to make using it cost nothing — because a measurement you take twice a week for a year beats a perfect one you take once and never repeat. We keep your rate, we track your drift, and every estimate you get afterwards is yours instead of a population average.

10

Check it once, then trust it

Run the shortcut manually one time and watch what lands. Open your sessions and check three things: the duration matches the class you actually did, the activity is right, and the L/hr rate is somewhere a human could plausibly sweat (roughly 0.5–2.5; above 3 is rare enough to be suspicious).

Worth the two minutes, because this number now sticks: we reuse your measured rate for every estimate afterwards. We take the median of your last five weighed sessions and ignore anything outside 0.2–2.4 L/hr, so one bad reading can't drag you — but a shortcut wired up wrong every single time would teach us the wrong thing patiently.

Also add it to the share sheet while you're here (share icon → "Show in Share Sheet") so you can fire it by hand on days you skip the watch.

That's it

First-time setup: ~10 minutes. Every session after: zero friction. The data lives in your browser (no account needed). Your workout log at sweatsciences.com/workouts fills up automatically.

Or — the 30-second bookmarklet (any browser)

No iPhone or don't want to build a shortcut? Drag the button below to your bookmarks bar. After each class, click it; it asks for your weights + duration and opens the calculator pre-filled.

⬛ SweatSciences Quick-Log

How to install the bookmarklet

On desktop: drag the blue button into your bookmarks bar.

On mobile: tap-and-hold the button → "Add Bookmark." Then just tap the bookmark anytime to log a session.

Privacy + what we actually see

Nothing leaves your device. The Shortcut sends your weights + duration to the SweatSciences URL as query parameters, which our JavaScript reads and processes entirely in your browser. Your workout log is stored in your browser's localStorage — we do not have a server that holds any of it. Export / backup via the Export JSON button on the Workouts page.

When we ship account sync later (Phase 1), you'll opt in explicitly and your data gets encrypted. Until then, this is the most private possible integration — your HealthKit data never touches a cloud we control.

What's coming after Phase 0

Phase 1

Strava OAuth

For runners + cyclists who don't use Apple Watch. Covers Android too via web OAuth.

Phase 1

Studio partner API

Mindbody / ClassPass integration — your class duration + room temp come in automatically from the studio's schedule.

Phase 2

Native iOS app

Apple Watch complication for "log last workout." Background-syncing. Ships when the roadmap supports the build.