Why Smart Scales Default to Kilograms
And Why It Makes Your Data Better
Smart scales commonly use kilograms because kg is the standard unit used by their apps and health integrations. The displayed unit is a preference: changing it to stone should change the format, not the weight.
Why do smart scales use kilograms?
Smart scales commonly display kilograms because kg is the standard unit used by health, fitness and data-integration systems. UK users may prefer stone, but the default is usually a product or app setting rather than a different measurement method. The exact menu and available units vary by manufacturer and model.
Kilogram, pound and stone displays may also round the same underlying reading differently. A decimal kg display can show smaller increments than a stone-and-pound display rounded to whole pounds, but the actual resolution depends on the scale and app. You can use our convert kilograms to stone tool to interpret 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, then converted to the display unit and rounded to the nearest available increment.
| 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. A scale transmitting in 0.05 kg increments can show a consistent downward slope of 200 g per day.
If the same data is displayed in stone and pounds rounded to 1 lb, the graph may stay flat for days and then jump. The underlying change is identical, but the display resolution tells a 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 SI 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.
A practical way to read your scale
Use the unit that helps you read your trend consistently. If you compare readings from different devices, check the unit and rounding first; small differences may reflect display formatting rather than a real change in body weight. For medical decisions, use the measurement requested by your healthcare professional.
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. In the Renpho app, open Profile, My Profile and Unit to choose your preferred display:
| 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
Dr Muhammad Usman graduated with his MBBS from Avicenna International Medical University in Bishkek, Kyrgyzstan (2019–2024).
He brings clinical training across general medicine, surgery, and public health, and writes the site's health information for UK readers in clear, practical language.
Disclaimer: This content is for informational and educational purposes only and does not constitute medical advice. If you have concerns about your weight or health, please speak to your GP.