Weight Diary is a well-loved iPhone trend tracker that costs $7.99 once, with no subscription (App Store listing, October 2026). Trendline differs by keeping every reading with its time, source, and unit, and by showing a trend that states its uncertainty. If price decides it, Weight Diary wins. We make Trendline; neither company is affiliated with the other.
Trendline is an iPhone app that reads Apple Health. Weight Diary, by CURLYBRACE APPS, is a different iPhone app with a similar goal. Neither company is affiliated with the other.
First, the honest part
Weight Diary has a 4.9-star average across about 2,600 App Store ratings, has been around since 2011, and was last updated in August 2026 (its App Store listing, checked October 2026). It costs $7.99 once, with no subscription and no in-app purchases. Its listing promises to help you "cut through daily fluctuations with a clear trend line", which is the same measurement literacy Trendline is built on. It syncs with Apple Health and connects directly to Withings and Fitbit scales, and its App Store privacy label says the developer collects no data from the app.
Where the two apps genuinely differ
| Weight Diary | Trendline | |
|---|---|---|
| Price | $7.99 once, no in-app purchases (App Store listing, October 2026) | $0.99/week · $3.99/month · $10.99/year · $34.99 lifetime |
| How weights get in | Manual entry, Apple Health, and direct Withings and Fitbit scale sync | Manual entry, plus weight, body fat %, and lean body mass from any app that writes them to Apple Health |
| Trend | A trend line its developer calls "unique"; the method is not published | Statistical trend with explicit uncertainty; declines to conclude when evidence is sparse |
| Readings per day | Not documented | Every reading kept, with its time, source, and unit |
| Goals | Estimated rate of loss and time to target; a goal range for maintenance; lose, maintain, or gain at setup | Lose Weight, Build Lean, and Maintain as distinct phases, with a goal forecast |
| Body composition | Records body fat %; shows BMI | Calculated fat and lean mass from paired readings, plus manual entry for values Apple Health has no type for |
| Calories | Estimates weekly loss and a daily calorie change | Not in the iPhone app; a free browser TDEE calculator is on this site |
| Storage | On the device or in an iCloud backup; no account | On the device; Apple Health is the only "account", and Trendline runs no iCloud sync service |
| Devices | iPhone and iPad (iOS 18 or later) | iPhone only |
Read the price row plainly: Weight Diary is a small one-time purchase, and Trendline is a subscription or a larger lifetime purchase. If price decides it, Weight Diary wins and we won't pretend otherwise. It also connects to Withings and Fitbit directly, which Trendline does not; Trendline reads the weight, body fat %, and lean body mass your scale's app writes to Apple Health. What Trendline offers instead is a trend that states its uncertainty, every reading kept with its provenance, losing, building, and maintaining treated as different jobs, and body-composition estimates kept in context. The full math is public on the methodology page.
Who should pick which
- Pick Weight Diary if you want a polished, long-running trend tracker for a one-time price, want direct Withings or Fitbit sync, or want to use it on an iPad too.
- Pick Trendline if you want the trend to admit uncertainty, keep several readings or sources a day, want phases beyond losing weight, or want your scale's composition estimates kept in context.
Try the idea
Paste a few weeks of weigh-ins into the free weight trend calculator to see a slower signal drawn against every reading, in your browser, before you install either app.
Questions people ask
How much does Weight Diary cost?
$7.99 once, with no subscription and no in-app purchases (its App Store listing, checked October 2026).
Does Weight Diary work with Apple Health?
Yes. It syncs with Apple Health and connects directly to Withings and Fitbit scales, and its App Store privacy label says the developer collects no data from the app.
Where does Weight Diary store my data?
On the device or in an iCloud backup, with no account (Weight Diary privacy policy). Trendline stores data on the device, uses Apple Health as its only account, and runs no iCloud sync service.
Does Weight Diary work on iPad?
Yes. Weight Diary runs on iPhone and iPad (iOS 18 or later), while Trendline is iPhone only.
Should I pick Weight Diary or Trendline?
Pick Weight Diary if you want a polished, long-running trend tracker for a one-time price, want direct Withings or Fitbit sync, or want to use it on an iPad too. Pick Trendline if you want the trend to admit uncertainty, keep several readings or sources a day, want phases beyond losing weight, or want your scale's composition estimates kept in context.
Sources
- Weight Diary on the App Store (version 15.1: price, ratings, devices, Apple Health and scale sync, privacy label)
- Weight Diary website (trend line, scale sync, calorie estimate)
- Weight Diary release notes (goal range, maintain and gain modes, body-fat display)
- Weight Diary privacy policy (local and iCloud backup storage)
Trendline is not affiliated with or endorsed by Weight Diary or CURLYBRACE APPS LTD. Facts above were verified against the linked pages on October 3, 2026; app stores change, so check the listing for current pricing.