Frequently asked questions

Can't find the answer you're looking for? Email me at{" "} erv@ewal.net .

No. TrendWeight has a built-in{" "} weight log : just type in your weight each day, right on the site, and you get the same trend charts and stats as someone with a connected scale. There's a Log Weight button on your dashboard to make this quick.

That said, a smart scale makes daily weigh-ins effortless — you just step on the scale and your weight shows up in TrendWeight automatically. TrendWeight works with Withings scales, which you can find on Amazon:{" "} Withings Scales. You can also connect a Withings account without a scale and enter weights in the{" "} Withings Health Mate app if you prefer.

Google is retiring the open-to-everyone Fitbit API that TrendWeight uses to read weight data from Fitbit accounts. There is a newer replacement API, but getting access to it requires an approval process that isn't practical for a free hobby project like TrendWeight. You can still connect a Fitbit account for now, but heads-up that Fitbit syncing is winding down and is expected to stop in September 2026.{" "} Read the full story of what's happening

If you already have a Fitbit connection, it will keep syncing until Google shuts down the old API, which is expected in September 2026. After that, your Fitbit weight history stays in TrendWeight and keeps appearing in your charts — nothing you've already synced goes away.

To keep new weigh-ins flowing after the shutdown, you can connect a Withings account or use the built-in{" "} weight log . New data is automatically combined with your existing Fitbit history into one continuous view.

Most users don't experience this issue, but some do—particularly on iPhone devices. TrendWeight uses Clerk for authentication, which normally keeps you logged in for up to one year. If you're being asked to re-login more frequently, your browser is likely clearing your session data due to privacy settings or other browser behavior, and unfortunately there's nothing I can do to prevent that.

However, there are better ways to log in that can help you avoid having to enter email codes repeatedly. You can connect your account to Google, Apple, or Microsoft for faster social login. Even better, you can add a{" "} passkey to your account.

Passkeys are a modern, secure way to log in that uses your device's built-in security (like Face ID, Touch ID, or your phone's lock screen) instead of passwords or email codes. They're faster, more secure, and work seamlessly across your devices.

Once you've set up a passkey, you can use the "Use passkey" link on the login page to sign in instantly—no email address or code required since the passkey identifies you automatically.

To set these up, go to your{" "} settings page {" "} and look for the "Account Security" section. From there, you can click "Open Account Profile" to add social logins (Google, Apple, Microsoft) or "Open Security Settings" to add a passkey.

No. The TrendWeight website works great on mobile sizes, and there are no plans for a native mobile app.

If you have data from one source (say, Fitbit) and start using another (a Withings account, or the built-in weight log), there's nothing special to do — add the new connection via the{" "} settings page {" "} or just start logging weights. TrendWeight keeps all your existing weight data and combines it with data from the new source, giving you a unified view of your weight over time.

TrendWeight will only use one weigh-in per day—the first weigh-in of the day is used. To get the most reliable weight readings day after day, professionals recommend you step on your scale first thing in the morning after you wake up (and use the restroom). As such, TrendWeight uses only the first reading of the day.

Feel free to weigh yourself as many times as you want in the day, as those subsequent readings won't throw off TrendWeight even if you weigh yourself right after a large meal.

If your first weigh-in of the day is incorrect for some reason and you don't want TrendWeight to use that data, you can log into the Fitbit or Withings apps and edit or delete that incorrect data point. TrendWeight will see the corrected reading the next time it syncs your data. Alternatively, you can log a weight yourself for that day — an entry in your{" "} weight log {" "} always takes priority over scale readings on the same date.

Yep! Losing weight is hard, and sharing your progress with the people who support you can help improve your odds of success. Each user gets a private URL that can be given to friends and family. That URL will allow anyone to see your trend graphs and statistics (so only give it to people you want to see your charts and stats).

Go to your{" "} Settings {" "} page to find your private URL. You can also change your private URL at any time in case you gave your URL to someone and later decide you don't want them to see your stats anymore. You can also disable sharing entirely if you prefer.

Yes. TrendWeight has a simple HTTP API that lets your own scripts and tools work with your data: you can read your daily weight data with trend values, download the raw readings from your connected scales, and add, edit, or delete weight log entries. It's handy for things like backing up your data or automatically logging weights from another app or device.

To use it, you first need an API key. Go to the API Key section of your{" "} Settings {" "} page and click "Generate API Key". The key is shown only once, so copy it somewhere safe — and treat it like a password, since anyone who has it can read and change your data. You send the key with each request, either as an Authorization: Bearer header or an{" "} X-Api-Key header.

Full documentation for every endpoint lives in the interactive API reference. You can even paste in your API key there and try requests right from the docs page.

The techniques used on TrendWeight come from John Walker's The Hacker's Diet. I found it to be an interesting read (and it's free!). In particular, I recommend reading the "Signal and Noise" chapter for a better understanding of the weight tracking methodology used on this site.

Additionally, we have a{" "} detailed explanation of the math behind TrendWeight {" "} that covers everything from the basic concepts to the advanced calculations.

TrendWeight is an app that I created in my free time out of a passion for tech gadgets and software development. And because I wanted this functionality for myself.

TrendWeight is free. It will always be free, but if you'd like to know how to help support TrendWeight, you can read more{" "} here .

Yes. You can find the project on GitHub here. However, it's essentially a one-man show (me), and I'm pretty protective of the project—probably too overprotective. That said, if you have something you'd like to contribute, please reach out.

No problem. If you have a question you don't see answered here, feel free to email{" "} erv@ewal.net {" "} and let me know. If you have a suggestion, you can also email that to me, or you can post your idea{" "} here.