Why Smart Scales Default to Kilograms
And Why It Makes Your Data Better
Step on a Renpho, Withings or Garmin Index smart scale for the first time and it will almost certainly display your weight in kilograms. There is a precise engineering reason behind this default, and it directly affects your data quality.
Introduction
Step on a Renpho, Withings or Garmin Index smart scale for the first time and it will almost certainly display your weight in kilograms. For most UK users this produces immediate frustration — we weigh ourselves in stone, not kilograms, and the number on the screen means nothing without a conversion. But there is a precise engineering reason behind this default, and it directly affects the quality of the data your scale sends to Apple Health, MyFitnessPal and Garmin Connect.
The short version: kilograms allow smart scales to report weight changes in increments of 50 grams. Ounces — the imperial equivalent — produce increments of 28.35 grams per unit, which sounds more precise, but scales that track in pounds can only display to 0.1 lb (45.4 grams). Stone-and-pound displays, meanwhile, show weight to the nearest pound — a resolution of 454 grams. For data-driven fitness tracking, coarse resolution means you can gain or lose 400 grams and your scale's app will not register the change at all. You can use our convert kilograms to stone tool to interpret your readings in familiar terms.
The Precision Argument — Grams vs Ounces vs Stone
Every smart scale measures weight in grams internally. The engineering sensors — load cells — have no concept of stones or pounds. The reading is taken in metric, converted to whatever the display setting requests, and the result is rounded to the nearest displayable increment. That rounding step is where precision is lost.
| Unit System | Smallest Increment | Equiv. in Grams | App Data Quality |
|---|---|---|---|
| Kilograms (kg) | 0.05 kg | 50 g | Very high — daily fluctuations visible |
| Pounds (lbs) | 0.1 lb | 45 g | High — similar to kg |
| Stone & Pounds | 1 lb | 454 g | Low — sub-pound changes invisible |
| Stone (decimal) | 0.1 st | 635 g | Very low — weekly trend only |
★ Why This Matters for Fitness App Graphs
Fitness apps like Apple Health and MyFitnessPal plot weight trends over time. When a scale transmits data in kg increments of 0.05 kg, the trend line has fine granularity — it can detect a consistent downward slope of 200 g per day. When the same data is transmitted in stone-and-pound increments rounded to 1 lb, the graph flatlines for days then jumps. The underlying weight change is identical; the data resolution tells a very different story.
How Smart Scales Actually Store Weight
Smart scales — including the Renpho range — do not store data in the unit you select on the device. The display setting is a preference layer. The raw weight is recorded in grams (or decimal kilograms) by the app, regardless of what appears on the scale's screen. This is confirmed by the fact that you can switch unit preferences freely in the Renpho app or Apple Health without corrupting or changing your historical data — the numbers simply re-render in the new unit.
This architecture means that if you have a Renpho scale set to display stone, the scale's bluetooth transmission to the app still contains a decimal kilogram value. The app then converts it for display. If you later switch the app to kg, every historical reading that appeared as, say, '14 st 3 lb' will re-render as its precise decimal kg equivalent — not a rounded one.
Why Kg Is the Global Default
The International System of Units (SI) — the global scientific standard — defines the kilogram as the base unit of mass. Every clinical setting in the UK, and every country with modern healthcare infrastructure, records patient weight in kilograms. Drug dosing, anaesthetic calculations, BMI computation, and nutritional guidelines all use kilograms as their reference unit.
Smart scale manufacturers ship a single global firmware. The path of least resistance is to default to the unit used by medical systems worldwide — kilograms — and let users in the UK, USA and other imperial-preference countries change the display setting manually. Building in a stone default would require region-specific firmware versions for a market (UK adults) that represents a small fraction of global scale sales.
Doctor's Note — Dr Muhammad Usman, MBBS
"From a clinical standpoint, the precision advantage of kilogram-based tracking is well established. In hospital settings, patient weight is recorded to the nearest 0.5 kg or better, because accurate weight underpins correct drug dosing. I would encourage patients who weigh daily for health management purposes to read the kg figure rather than the stone conversion, precisely because it captures smaller changes."
Renpho Scales — Switching from kg to Stone
Renpho is one of the UK's most popular smart scale brands. Its scales can display in kilograms, pounds, or stone — but the setting differs by model:
| Step | Action | Detail |
|---|---|---|
| Hardware button | Older Renpho models | A physical button on the underside of the scale cycles through kg → lbs → stone. Press and hold for 3 seconds. |
| App setting | Renpho app (all models) | Open Renpho app → Profile → My Profile → Unit → select your preference. |
| Note | Data integrity | The underlying data is stored in kg regardless. Switching to stone changes the display only — all previous readings remain accurate. |
If your Renpho scale displays in kg but you prefer stone, you can follow our step-by-step guide to changing weight units for more details on app synchronization.
Smart Scale Accuracy — What the Research Shows
Smart scales use bioelectrical impedance analysis (BIA) to estimate body composition metrics such as fat percentage and muscle mass. For raw weight measurement, the accuracy is considerably higher. A peer-reviewed study published in PMC (National Institutes of Health) found that three leading smart scales had a median absolute error of under one kilogram compared with DEXA scan weights — with one scale recording a median error of exactly 0 kg.
For body composition metrics like fat percentage, accuracy is lower. The same study found fat mass errors ranging from −2.2 kg to −4.4 kg across devices, with body fat consistently underestimated. The takeaway for UK users: trust your smart scale's weight reading, treat the body composition data as directional rather than precise, and never compare fat percentage readings between different scale models.
Converting Your Smart Scale Reading to Stone
If your smart scale shows a reading you want to convert to stone, the formula is:
To extract whole stones and pounds: take the decimal portion and multiply by 14 to get pounds.
Example:
82.4 kg ÷ 6.35029318 = 12.976 stone
= 12 stone (0.976 × 14) = 12 stone 13.7 pounds.
→ Use the instant kg to stone converter for instant, verified results.
Frequently Asked Questions
Why does my Renpho scale show kg instead of stone?
Renpho scales default to kg as the global measurement standard. To switch: press and hold the underside button to cycle units, or change in Renpho app under Profile > My Profile > Unit.
Why do smart scales use kilograms?
Kg is the SI base unit of mass and provides a display resolution of 0.05 kg — finer than the 1 lb (454 g) resolution in stone-and-pound display mode, making trend tracking more accurate.
Is a smart scale accurate in stone mode?
The underlying measurement is the same in any unit, but stone-and-pound display rounds to the nearest pound (454g). Small daily changes may not register on screen. For tracking, kg display is more granular.
Does changing Renpho from kg to stone change my data?
No. Your weight data is stored in decimal kg internally. Changing the display unit simply re-renders the same values in a different format.
How accurate is a smart scale vs doctor's scales?
For weight, smart scales are typically within 0.3–0.5 kg of medical-grade scales. A PMC-published study found median absolute errors below 1 kg vs DEXA for three smart scales tested.
Dr Muhammad Usman
MBBSDr Muhammad Usman graduated with his MBBS from Avicenna International Medical University in Bishkek, Kyrgyzstan (2019–2024) and brings international clinical training across general medicine, surgery, and public health. He writes all health content on this site to current NHS/WHO guidelines, ensuring clinical accuracy and real-world relevance for UK readers.
View LinkedIn ProfileMedical Disclaimer: This content is written by Dr Muhammad Usman, MBBS, for informational and educational purposes only and does not constitute medical advice. Content is written to current NHS guidelines. If you have concerns about your weight or health, please speak to your GP.