A web app and an iPhone app
TrendWeight is a free, open-source web app by Erv Walter. Its FAQ says the site works at phone sizes and that a native mobile app isn’t planned, so iPhone owners use it in the browser. You sign in with an emailed code, a Google, Apple, or Microsoft account, or a passkey. Each person gets a private URL for sharing charts and stats with friends, and that URL can be changed or switched off. There’s also an HTTP API for reading your data and adding weights from scripts.
Balance Point runs natively on iPhone and iPad and has no web version. It’s still in development. Records sit in the app’s own storage on the device, with no account to create and no server on Rival Bear’s side.
The math behind each trend
TrendWeight publishes its formulas in full on a dedicated math page. The trend follows John Walker’s The Hacker’s Diet: each day it moves 10% of the way toward that day’s weight. Weekly rate comes from a least-squares line through two weeks of trend values, and calorie estimates use the 3,500-calories-per-pound rule, which the page calls a simplification. Advanced Settings add presets based on Holt’s linear trend method for a line that lags less during steady loss, and the page lays out what you give up in exchange.
Balance Point uses a statistical filter that expects weigh-ins on uneven days, then refines earlier history using readings that arrived afterward. The line carries a range, and the weekly rate on Today shows its own uncertainty, such as −0.5 lb/wk ±0.5.
Days with no weigh-in
TrendWeight’s smoothing expects a value for every day, so it draws a straight line between real weigh-ins and uses points along it for the missing days. Weigh 150 lb on Monday and 148 lb on Thursday, and Tuesday becomes 149.33. It uses only the first weigh-in of each day, and a weight you log yourself takes priority over a scale reading on the same date. Some comparisons stay hidden until there’s enough data, such as four weigh-ins in the last seven days for the weekly one.
Balance Point leaves missing days empty. Its estimates are worked out when a screen calls for them and are never stored as weigh-ins or written to Apple Health. Over a sparse stretch the trend’s range opens wider.
How a goal date gets calculated
TrendWeight divides the distance between your trend and your goal by the current daily rate and adds the result to today’s date. Its math page says the date assumes the rate holds and that loss often slows near a goal. Once the trend crosses the goal, a maintenance band of ±2.5 lb (±1.134 kg) shows whether you’re holding steady there.
Balance Point expresses its forecast as a spread of dates, and with too little recent data it shows no date at all. After 112 days and 20 weigh-ins with visible curvature, it moves to a curved model able to show the trend flattening out short of the goal. The plan, built from your chosen goal and pace, sits beside the forecast under its own name.
Getting weights in
TrendWeight connects to Withings scales, or to a Withings account if you type weights into the Health Mate app, and it has its own weight log with a Log Weight button on the dashboard. Fitbit syncing is winding down; the FAQ gave September 2026 as the expected end and says synced history stays put. The site doesn’t mention Apple Health.
Balance Point takes weigh-ins typed in the app, spoken to Siri, or sent through the Log Weight Shortcut. With your permission it reads weight, body fat, and waist from Apple Health.
Choosing TrendWeight or Balance Point
TrendWeight fits someone with a Withings scale who wants a free trend chart in any browser, including on a computer, along with published formulas and an API. Balance Point fits an iPhone user who weighs in irregularly and would rather see gaps left open and a forecast given as a range; it isn’t released yet, and nothing about its price is public. Balance Point’s missed weigh-ins guide covers how it handles the stretches between readings.