Scale Relay
Xiaomi Body Composition Scale S400 to Garmin Connect on iPhone
The Xiaomi Body Composition Scale S400 is Xiaomi's current 25-metric scale and, unlike its predecessor, it encrypts every Bluetooth measurement. This guide explains how Scale Relay gets the key from your Xiaomi account once, decodes measurements on your iPhone and sends weight and body composition to Garmin Connect, and what to do when a measurement does not arrive.
What you need
- A Xiaomi Body Composition Scale S400, model MJTZC01YM. The model number is printed on the label underneath the scale.
- The scale already added in the Xiaomi Home app under your Xiaomi account. Xiaomi issues the scale's encryption key to that account, and Scale Relay retrieves it from there.
- An iPhone with iOS 17 or later and Bluetooth on.
- A Garmin Connect account. Two-factor authentication is supported.
- Scale Relay from the App Store. The first two Garmin uploads are free.
Not a Xiaomi scale? If your scale's app saves weigh-ins to Apple Health (Withings, Renpho, Eufy, Etekcity, Zepp Life and most others), Scale Relay can relay them to Garmin Connect too: weight, plus body fat and BMI when the app provides them. See the Apple Health guide.
Why the S400 is different from the Scale 2
The older Mi Body Composition Scale 2 broadcasts its measurements in the open; any app can listen. The S400 uses Xiaomi's encrypted beacon format instead. Each measurement is sealed with a 16-byte key that only exists on the scale and in Xiaomi's cloud for the account the scale is paired with. Without that key the Bluetooth data is noise.
Raspberry Pi bridges solve this by asking you to extract the key and the scale's MAC address with token-extraction scripts. Scale Relay does it in the app: a one-time Xiaomi authorization retrieves the key, stores it in the iOS Keychain, and from then on the app decodes measurements locally. You never see or type the key. After that, the S400 works the same way as the Scale 2: open the app, step on, send.
Step by step
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Make sure the scale is in Xiaomi Home
Open Xiaomi Home, confirm the S400 is listed under your account and that a weighing shows up there. If you have not set the scale up yet, do it now; Scale Relay cannot authorize a scale that is not in your Xiaomi account. Note that the S400 pairs with Xiaomi Home, not with Zepp Life.
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Install Scale Relay and enter your profile
Download Scale Relay from the App Store, allow Bluetooth access, then open Settings and enter your sex, height and age. Body fat, muscle, water and bone mass are calculated from the scale's impedance reading and these values.
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Authorize with Xiaomi (once)
Step on the scale so it wakes up. Scale Relay recognises the S400 and explains that it needs a one-time Xiaomi authorization. Tap Connect with Xiaomi and sign in with your Xiaomi account. The app retrieves the scale's key from Xiaomi and saves it in the Keychain. Settings then shows Xiaomi S400 authorized. Your Xiaomi password is used for this sign-in only and is not stored.
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Close Xiaomi Home and weigh yourself
The S400 stops broadcasting measurements while an app is connected to it, so close Xiaomi Home first. With Scale Relay open, step on the scale barefoot and stand still. The scale measures weight, then impedance, then your heart rate; wait until its display finishes before you step off.
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Review and send to Garmin
Scale Relay shows weight, BMI, body fat, water, muscle mass, bone mass and BMR. Tap send. The first time, sign in to Garmin Connect with your email and password and enter the verification code if your account uses two-factor authentication. The app confirms when Garmin has accepted the measurement; find it under Health Stats › Weight in Garmin Connect.
Ready to try it? Get Scale Relay on the App Store; the first two Garmin syncs are free.
What is transferred to Garmin Connect
| Metric | Source | Sent to Garmin Connect |
|---|---|---|
| Weight | Measured by the scale | Yes |
| BMI | Calculated from weight and height | Yes |
| Body fat % | Calculated from impedance | Yes |
| Body water % | Calculated from impedance | Yes |
| Muscle mass | Calculated from impedance | Yes |
| Bone mass | Calculated from impedance | Yes |
| Visceral fat rating | Calculated from impedance | Yes |
| Metabolic age and physique rating | Calculated from impedance | Yes |
| BMR | Calculated from impedance | Shown in the app |
| Heart rate | Measured by the scale | Not sent |
The S400 measures impedance at two frequencies. Scale Relay uses the low-frequency reading, which matches the formulas validated on the Mi Body Composition Scale 2, so values are comparable between the two scales. Xiaomi Home's 25 indicators are its own derived numbers; Garmin Connect only accepts the metrics in the table.
Troubleshooting
"This scale is not in your Xiaomi account"
Scale Relay found an S400 but your Xiaomi account has no such scale. Add it in Xiaomi Home first, make sure you sign in to Scale Relay with the same Xiaomi account, then try again. If the scale was set up by another family member, use their account for the authorization.
Authorized, but no measurement arrives
- Close Xiaomi Home. While it is connected to the scale, the S400 does not broadcast measurements to anyone else.
- Keep Scale Relay in the foreground and step on the scale after the app is open.
- If you have already weighed yourself and nothing appeared, remove the scale in Xiaomi Home and add it again, check that a weighing shows up there, then use Settings › Disconnect Xiaomi Scale in Scale Relay and authorize the scale again.
Measurements stopped arriving after re-pairing in Xiaomi Home
Re-pairing the scale in Xiaomi Home regenerates its encryption key, so the key Scale Relay saved no longer works. Go to Settings › Disconnect Xiaomi Scale and run the Xiaomi authorization again. This takes a minute and does not affect your Garmin sign-in.
Weight arrives but body composition is missing
Measure barefoot with dry feet and wait for the scale to finish its full cycle. Check that height, age and sex are filled in under Settings.
Garmin sign-in or upload problems
- Use your Garmin account email and password, and enter the two-factor code when asked; codes expire quickly.
- Repeated failed sign-ins are temporarily blocked by Garmin. Wait a few minutes.
- A failed upload stays queued in the app: open Scale Relay and tap Retry. Nothing is sent in the background. The app confirms each accepted upload.
- In mainland China: Scale Relay signs in to connect.garmin.com only; Garmin China (garmin.cn) accounts are separate and not supported.
Frequently asked questions
Why does the S400 need a Xiaomi authorization when the Scale 2 does not?
The S400 encrypts its Bluetooth measurements with a key issued to the Xiaomi account the scale is paired with. Scale Relay signs in once to retrieve that key and then decodes measurements locally. The Mi Body Composition Scale 2 broadcasts unencrypted, so it needs nothing.
Do I have to type in a MAC address or bind key?
No. Scale Relay obtains the key during the one-time authorization and learns the scale's Bluetooth address on its own. Raspberry Pi tools and Android exporters require both to be entered manually.
Can I keep using Xiaomi Home?
Yes, just not at the same moment. Close Xiaomi Home before weighing with Scale Relay, and avoid removing and re-adding the scale there, because that changes its key.
Is the Xiaomi S400 Pro supported?
The tested and supported model is the S400 (MJTZC01YM). If you own another Xiaomi scale, tell us which one through the contact form.
What does it cost?
The app is free, the first two successful Garmin uploads are free, and unlimited uploads are a one-time $14.99 purchase. No subscription.
Where is my Xiaomi password stored?
It is not. The password is used for the one-time sign-in; the scale key and session data are stored in the iOS Keychain. Scale readings are processed on the phone and are not stored on any Scale Relay server.
Get Scale Relay
Authorize your S400 once, then send every measurement to Garmin Connect from your iPhone. Two Garmin uploads are free, then a one-time $14.99 purchase.
Download on the App StoreIndependent app. Not affiliated with Garmin or Xiaomi.
