Sweat labs charge $130–200 to hand you your electrolyte number. Then the branded tabs run up to $6.50 a session — $700+ a year. But that number was never theirs: it comes from your own weight, your workout, and a little physiology. Here's the whole stack — free number, ~$2 mix, and a weekly review that reads your own watch — in six moves. No subscription. No lab.
The old way
$200 + $6/session
a lab reading your own sweat, then a markup on salt
→
This build
~$2/session
your exact number, real ingredients at cost, your hardware
Fastest: open the calculator, pick your sport, length and how salty you sweat. Your Na/K/Mg appears instantly.
Most exact: weigh yourself before and after one session (towel off, same clothes). The drop is your fluid loss — the calculator turns it into milligrams.
Hands-free: your Apple Watch already logged the workout. On iPhone, a Shortcut can read its length and open your number pre-filled — no typing. Set up the auto-log →
Why this beats the lab: a one-time sweat test gives you one number on one day. Your losses change with heat, fitness and the season — a free calculator you can re-run every time is the honest tool.
Payoff: your exact electrolyte number, free.
Move 2 · 2/6
See the gap
Find out what your current drink actually replaces — and what it misses.
Before you buy anything new, measure what you already sip against your number. Most tabs cover a fraction of the sodium and skip potassium and magnesium entirely.
Open the “what do you use now?” tool and choose your drink — LMNT, Nuun, Liquid I.V., Gatorade and more, every number from the label.
Or scan the barcode of whatever's in your bag — it looks the product up and measures it against your saved sessions.
Read the gap: exactly how many milligrams short, and the pinch of salt that closes it for pennies.
The honest comparison is on the recipe page — LMNT covers ~91% of an average sodium session; most bottled drinks under 20%. Good products, one-size doses.
Payoff: the exact gap, and the pennies that close it.
Move 3 · 3/6
Build the mix
The real recipe — dosed to you, not to a label.
A real-food base for flavor and absorption, topped with bulk minerals to hit your exact number. Sealed from the brands you already trust — nothing repackaged.
Per session: a flavor base + ~1.8 g pink salt, ~0.6 g potassium chloride, ~2.5 g magnesium malate in 20–34 oz water. The calculator gives you your grams.
Why the little sugar in the base matters: it switches on the sodium-glucose co-transporter, so you absorb the sodium and water instead of just drinking them. The physiology →
Your whole recipe to one Amazon cart — sealed, at cost.
This is the loadout. Instead of a proprietary powder, your exact ingredients from the brands that make them, added to one cart at the quantities the calculator worked out.
On the calculator, hit “Add your whole recipe to one Amazon cart” — the base and three minerals, at a season's quantity.
A full first cart runs about $40–60 and lasts weeks to months; per session it lands near $1.50–2 versus $6.50 for pre-mixed.
Why we don't sell the powder: the personalization lives in the math and the vessel; the minerals stay sealed from the brands you trust. No markup on salt.
Payoff: a season of your exact mix, one cart.
Reference · exact amounts
The four ingredients, exactly
What to order, how much, how long it lasts — and how to swap anything for what you already have.
Two are dead simple (a salt and a salt-substitute). One's a flavor base. Magnesium is the only one where products genuinely differ — so here's the exact math and the swap rule for each.
Ingredient
What it does
Per session
Buy for a year*
The baseSkratch · 20 servings
Flavor + carbs + 400 mg sodium, and the touch of sugar that switches on absorption.
1 serving
~11 tubs (reorder monthly)
SodiumRedmond Real Salt · 16 oz
The electrolyte you lose most — 90%+ of the job.
~1.8 g (¼ tsp)
1 pouch (lasts ~a year)
PotassiumKCl salt-substitute · 1 kg
The lever most tabs skip. A pinch closes the gap.
~0.6 g (a pinch)
1 bag (lasts years)
MagnesiumMg malate · 500 g
A recovery top-up — you barely sweat it out, but most people run short in their diet.
~2.5 g (½ tsp)
1 bag (200 servings)
*At 4 sessions a week. The base is the only thing you reorder often — one bag of each mineral lasts months to years. Exact product links are in the loadout below, and the calculator sizes your whole cart automatically for how often you train.
Why magnesium is the big scoop. Magnesium malate is only about 12% elemental magnesium — so it takes ~2.5 g of powder to deliver 300 mg, more than the salt. That's why it out-bulks everything in the jar. Nothing's wrong; it's just a low-concentration mineral, and it's a supplementation dose (not sweat replacement), so it's the one you can dial down or swap for a more concentrated form.
Mix & match — swap any of it
Sodium Any unrefined salt works — Celtic, sea, Himalayan. All run ~385 mg sodium per gram (a level ¼ tsp ≈ 1.5 g ≈ 575 mg). Table salt works too; unrefined just carries trace minerals.
Potassium Any potassium chloride or “salt substitute” — NoSalt, Nu-Salt, LoSalt. All run ~525 mg potassium per gram. Same pinch, whatever the brand.
Magnesium This is the one to check. Read your label's “elemental magnesium per serving” and match it to ~300 mg. Malate ≈ 120 mg/g; glycinate ≈ 140; citrate ≈ 160 — a more concentrated form means a smaller scoop. Skip magnesium oxide (barely absorbed).
The base Already loyal to LMNT, Nuun or Liquid I.V.? Use it as your base — then the gap tool tells you the exact pinch of salt, potassium and magnesium to add on top.
Payoff: order once, dose anything, hit your exact number.
Move 5 · 5/6
Make it a habit
Mix a month once. Scoop it daily. No scale, no fuss.
Measuring three minerals every session is friction, and friction is why people quit. Blend a month at your ratio so one level scoop is accurate without a scale.
DIY: mix ~30 sessions in a jar — 3 tbsp salt · 1 tbsp potassium chloride · 6 tbsp magnesium — one teaspoon per session into your base.
Done for you: the Jar Kit ships the jar, a calibrated scoop, and a waterproof card printed with your formula. We ship the vessel and the math.
The shortcut, not a lock-in: the jar recipe is free forever on the recipe page. The Kit just saves you the sourcing and the scale.
Payoff: one scoop a session, dosed to you.
Move 6 · 6/6
Run your weekly review
The actual coach's job — done by an AI that reads your own week.
This is the part the $200 test never gives you: a look back. Once a week, have Claude (or Sage) read your training, weight trend and how you felt, and tell you the one thing to change. Works today — no app, no key.
Your options — pick one
Save your sessionsPaste the prompt into ClaudeConnect Apple Health (Sage)
Do this
Log as you go: tap “Save this session” after each workout — it builds a private history on your device.
Every Monday, paste the prompt below into Claude (or ask Sage on the site). It reads your week and hands you the one change worth making.
Paste this into Claude every MondayReview my last 7 days of training and hydration. Here's my data: [paste your saved SweatSciences sessions, plus your Apple Health sleep, HRV, resting heart rate and weight trend]. Tell me, in plain English: (1) how consistent my electrolyte replacement was vs. my sweat losses, (2) whether my morning weight and HRV suggest I'm going into sessions under-hydrated, and (3) the ONE change this week that would move the needle most. Keep it short and specific.
Keep it to one change. A review that dumps ten fixes gets ignored; one that says “pre-salt your Saturday long class” actually gets done. That restraint is the coaching.
Payoff: a hydration coach for ~$0 a month, instead of $200 + a markup.
Your loadout · 1 shortcut unlocked
The whole stack, in one rack
One solid pick per move — swap in whatever you already use. Everything free except the Kit, which just saves you the sourcing.