FIT 9to5

FIT 9to5 is my coaching business, and I'm the coach. I started it to share what I learned on my own weight-loss and fitness journey. The pitch is "Get Fit on Your Time": fitness, nutrition and mindset coaching for people with day jobs.

The site has two jobs: take subscription payments without ever touching a card number, and host a free macro calculator that clients and anyone else can use. It started as a copy of Flashcards, my first project with working Stripe subscriptions, and the calculator moved in from its own subdomain later.

The macro formula

The calculator runs entirely in the browser and needs no account. It starts from a maintenance estimate of 14 kcal per pound of current weight, then nudges it toward your goal: one kcal/lb for every 5 lb you want to change, after a 1 lb dead zone.

Every input is mirrored into the query string, so a link reproduces the exact result with no database. The total on screen is recomputed from the rounded grams, so it can land a few kcal above the target. That's on purpose: it matches what you'd actually eat.

multiplier = 14 kcal per lb
  ± (|target − current| − 1) / 5
    toward the goal
  + 1 hard gainer, − 1 easy gainer
calories = ceil(current × multiplier)
protein  = 1 g per lb of target
fat      = 25% of calories ÷ 9
carbs    = whatever's left ÷ 4

180 lb → 170 lb, neutral:
multiplier = 14 − 9/5 = 12.2
calories   = ceil(180 × 12.2) = 2,196
protein 170 g, fat 61 g, carbs 242 g
(2,197 kcal on screen)

Billing I didn't have to build

Stripe hosts checkout and billing. createCheckoutSession verifies the Firebase ID token, finds or creates the Stripe customer, and passes the uid along as client_reference_id. After that, cancelling, reactivating, updating a card and viewing invoices all happen in Stripe's Customer Portal.

Every one of those changes arrives at stripeWebhook, which checks the signature and writes a single field per user: subscriptionStatus. The site reads that field to decide which button to show.

Signing in on phones

Google sign-in broke twice on mobile before it worked:

1. signInWithPopup failed silently on mobile Chrome, so phones now use the redirect flow and desktop keeps the popup.
2. The redirect then broke on third-party storage blocking, because the OAuth handler lived on Firebase's own domain.
3. vercel.json now proxies /__/auth/* to that handler, so the whole handshake happens on fit9to5.com.