TrendWeight is a free, open-source website that publishes its trend math, but its Fitbit connection is ending: Google's notice says Fitbit Web APIs stop being supported on October 30, 2026. History already in TrendWeight stays.
Trendline is an iPhone app that reads Apple Health. TrendWeight is a free website by Erv Walter. Neither is affiliated with the other.
First, the honest part
TrendWeight is free, and its developer says it "will always be free" (TrendWeight FAQ). It is open source, runs in any browser, and publishes its math in full: an exponentially weighted moving average in the style of John Walker's Hacker's Diet, moving the trend 10% of the way toward each day's weight, with gaps filled by linear interpolation (TrendWeight: the math). It predicts when you'll reach your goal at your current rate and estimates your daily calorie deficit from it. It is actively maintained, with a release as recent as September 2026. Few tools in this space are this transparent.
What's changing: Fitbit
Google is retiring the open Fitbit API that TrendWeight uses to read Fitbit weigh-ins. TrendWeight's FAQ says existing Fitbit connections keep syncing until Google shuts that API down, and Google's own notice says Fitbit Web APIs stop being supported on October 30, 2026 (Google Health Help). History already in TrendWeight stays. After the change, its supported inputs are Withings scales and its built-in weight log, where you type your weight each day. It also has an HTTP API for people who want to script their own data in.
If you weigh on a Fitbit Aria and use an iPhone, there is now another route: since August 2026 the Google Health iPhone app can write Fitbit weight and body fat % into Apple Health. Our Fitbit Aria and Apple Health guide covers the setup and what isn't documented yet. Once your weigh-ins are in Apple Health, any app that reads Health, including Trendline, can use them.
Where the two differ
| TrendWeight | Trendline | |
|---|---|---|
| Where it runs | Website in any browser; no native app planned | iPhone app |
| How weights get in | Withings scales, its built-in weight log, or its API; Fitbit ending | Manual entry, plus weight, body fat %, and lean body mass from any app that writes them to Apple Health |
| Readings per day | One: the first weigh-in of the day | Every reading kept, with its time, source, and unit |
| Trend | Published exponentially weighted moving average (10% per day) | Statistical trend with explicit uncertainty; declines to conclude when evidence is sparse |
| Body composition | Weight and body-fat trends from scale readings | Calculated fat and lean mass from paired readings, plus manual entry for values Apple Health has no type for |
| Goals and calories | Goal-date prediction, goal range, and a calorie-deficit estimate from your rate | Lose Weight, Build Lean, and Maintain as distinct phases, with a goal forecast; no calorie estimate in the app |
| Account and storage | Account required; data on TrendWeight's servers, with a private share link and data download | No account beyond Apple Health; data on the device, and Trendline runs no iCloud sync service |
| Price | Free, with optional tips | $0.99/week · $3.99/month · $10.99/year · $34.99 lifetime |
Who should pick which
- Stay with TrendWeight if you weigh on Withings or don't mind typing your weight, want it free and open source, like reading the exact formula, or use a browser rather than an iPhone.
- Pick Trendline if you're on iPhone and your weigh-ins can reach Apple Health, you want body-composition readings in context, you keep several readings a day, or you want the trend to admit uncertainty.
Try the idea
Our free weight trend calculator runs in your browser like TrendWeight does: paste a few weeks of weigh-ins and see the slower signal under the daily noise. Nothing you enter leaves the tab.
Questions people ask
Is TrendWeight free?
Yes. TrendWeight's developer says it "will always be free" (TrendWeight FAQ). It is open source and runs in any browser.
What is happening to TrendWeight's Fitbit connection?
Google is retiring the open Fitbit API that TrendWeight uses. TrendWeight's FAQ says existing Fitbit connections keep syncing until Google shuts that API down, and Google's notice says Fitbit Web APIs stop being supported on October 30, 2026 (Google Health Help). History already in TrendWeight stays.
How can I use a Fitbit Aria with an iPhone after the change?
Our Fitbit Aria and Apple Health guide covers the setup and what isn't documented yet. Once your weigh-ins are in Apple Health, any app that reads Health, including Trendline, can use them.
How does TrendWeight calculate its trend?
It uses a published exponentially weighted moving average in the style of John Walker's Hacker's Diet, moving the trend 10% of the way toward each day's weight, with gaps filled by linear interpolation (TrendWeight: the math).
Does TrendWeight have an iPhone app?
No. TrendWeight is a website that runs in any browser, and no native app is planned (TrendWeight FAQ).
Sources
- TrendWeight FAQ (free, Fitbit wind-down, weight log, API, one weigh-in per day, no native app)
- Google Health Help: Fitbit Web APIs end October 30, 2026
- TrendWeight developer post on its API (weight and body-fat trends)
- TrendWeight: the math (moving-average formula, interpolation, goal prediction, calorie estimate)
- About TrendWeight (Hacker's Diet method, goal range)
- TrendWeight source code and releases
Trendline is not affiliated with or endorsed by TrendWeight or Erv Walter. Facts above were verified against the linked pages on October 3, 2026.