Kelvin
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kelvin k1

$199 onyx
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no subscription · free shipping · 30 day returns

there is no screen. that is the entire point.

everything a watch tracks. nothing to look at.

no display, one button

nothing lights up at you, nothing buzzes for a notification, nothing pulls your eyes down mid-conversation. one ridged button on the module starts your favourite activity. a status led and a vibration motor handle everything else the band needs to tell you.

80+ activities, auto-detected

press the button or let the band notice on its own. a limited set of sports is detected automatically and sent to the app to confirm or reassign, and the classifier gets better at your patterns each time you correct it.

k-optic 4 heart rate and pulse ox

continuous optical heart rate, heart rate variability, respiration and skin temperature. pulse ox is off by default because it costs battery. turn it on for sleep or all day, and give it about three nights to build a baseline.

up to ten days

ten days on typical use, so one charge every second week rather than one every night. a full charge takes up to ninety minutes on the four-pin contacts. battery saver dims the led and drops heart rate, pulse ox, on-device start and auto-detect when you need to stretch it.

5 atm, and the band washes

rated for swimming at fifty metres. pull the quick-release pins and the softloop band goes in the machine on a gentle cycle at forty degrees, then air dries. wash it before you first wear it and regularly after.

no subscription

sleep, heart rate, reserve, readiness, training status and vo₂ max are in the free app on the day you open the box. kelvin plus adds coaching and live views for people who want them. we do not put your own body's numbers behind a monthly fee.

the day it saw while you forgot it was there

with no display, this is where the product actually lives. one real day at one second resolution: an early session, a long flat stretch of desk work, an evening walk, four sleep cycles. you open the app once in the morning and close it again.

recovery<114 bpm
aerobic114–133
tempo134–152
threshold153–171
maximum172+
01 / readiness

a number that tells you to stop

every morning the app combines last night's sleep, your hrv against your own baseline, acute load and resting heart rate into training readiness. the point is not the score. the point is that on the mornings it reads low, something says so before you talk yourself into intervals.

reserve sits beside it as the day's energy budget, draining with stress and effort and refilling with rest. training status, acute load, load ratio, training effect and recovery time fill in the rest of the picture. vo₂ max updates from a timed fifteen minute brisk walk or run.

inputs
hrv, sleep, load, resting hr
vo₂ max needs
15 min brisk walk or run
02 / sleep

the real reason it has no screen

a display on your wrist at 2am is a light source you did not ask for. k1 stages light, deep and rem sleep, scores the night, tracks naps and sleep alignment, reports overnight respiration and breathing variations, and measures skin temperature against your own normal.

smart wake vibrates you up inside a lighter stage, never later than the time you set. skin temperature data also improves period prediction, and the app syncs with natural cycles for anyone using it, which carries its own separate subscription.

stages
light, deep, rem, awake
wake
haptic only, no sound
03 / gps

connected gps, said plainly

there is no gnss receiver in the module. outdoor pace, distance and route come from the phone already in your pocket. that single decision is what buys the ten days and the nine millimetre thickness, and being direct about it beats pretending otherwise.

it works on auto-detected sessions too, so a walk from the station you never started still comes back with a map. leave the phone behind and everything else still records: heart rate, effort, recovery, the full session minus the route.

route mapping
needs paired phone
everything else
standalone
04 / fit

wrist, or move it up your arm

the module pulls off on 14 mm quick-release pins and drops into an upper arm band, which is where the optical array reads most cleanly during lifting or anything that flexes the wrist. set the position in the app so the metrics know where it is sitting.

wear it above the wrist bone, lightly snug rather than tight, module on the outside of the arm by default. for wheelchair mode, wrist reads better than upper arm.

wrist
s/m 120–200, l/xl 145–240 mm
upper arm
s/m 200–320, l/xl 280–450 mm

one button and one light, doing all the talking

with no display, the whole on-device interface is a ridged side button and a multi-colour led. both are documented here rather than buried in a manual, because you will use them from day one.

button

press, idle
starts your favourite activity. led flashes green and the band vibrates.
press, in activity
stops it. led flashes red until you save, resume or discard in the app.
press, alarm buzzing
snooze
hold, alarm buzzing
stop the alarm
three quick presses
pairing mode. holding about six seconds does the same.
hold 3 seconds
toggles heart rate broadcast, if device control is on
double-tap the top
battery check, if enabled in power manager
pause
app only, not on the band

status led

flashing purple
pairing mode, about fifteen minutes
flashing green
recording an activity
solid green
activity paused, or battery full on a double-tap
flashing red
activity stopped and waiting, or a pairing error
flashing orange
broadcasting heart rate
flashing white
paired module powering on or rebooting
green / orange
software update running
green / yellow / red
factory reset running

seven softloop colours, $49

1 / 7

80+ activity profiles

start any of these from the button or the app. outdoor distance and pace use connected gps from your phone. some sports are also detected automatically for you to confirm afterwards.

outdoor running

running, outdoor track, trail running, obstacle racing

indoor running

treadmill, indoor track

cycling

bike, road, mtb, gravel, commute, tour, ebike, emtb, indoor, bmx, handcycle, handcycle indoor

swim and water

pool swim, sup, row, kayak, fish, boat, snorkel

outdoor

walk, hike, bouldering, mountaineering, horseback, golf, disc golf, archery, inline skating

gym

strength, hiit, cardio, elliptical, stair stepper, floor climb, indoor climb, indoor row, jump rope, indoor walk, boxing, mma, mobility, pilates, yoga

racket

tennis, pickleball, badminton, squash, table tennis, padel, racquetball

team

basketball, volleyball, field hockey, ice hockey, soccer, american football, lacrosse, rugby, ultimate disc, cricket, softball, baseball, gaming

snow and winter

ski, snowboard, xc classic, xc skate, ice skating, snowshoe

wellness

breathwork, meditation, dance fitness, other

in the box

  • kelvin k1 sensor module
  • softloop wrist band, s/m or l/xl
  • charging cable, four-pin
  • start guide and warranty card
  • upper arm band sold separately, $59

specifications

physical
display
none, by design
module size
27 × 47 × 9 mm
module weight
14.8 g
total weight
20.7 g s/m · 21.2 g l/xl
case
fibre reinforced polymer
band
softloop fabric, 24 mm wide, machine washable
attachment
14 mm quick-release pins, official bands only
water rating
swim, 5 atm
memory
128 mb
battery and charging
battery life
up to 10 days, typical use
full charge
up to 90 minutes
charging
four-pin contacts, kelvin plug charger
battery saver
dims led, disables heart rate, pulse ox, on-device start, auto-detect
reduces battery
all-day pulse ox, heart rate broadcast
operating temp
−20 °c to 60 °c
charging temp
0 °c to 45 °c
sensors
optical array
k-optic 4, heart rate and pulse ox
accelerometer
yes, activity detection and wheelchair push tracking
skin temperature
yes, via the health stack
pulse ox default
off, enable in the app
ecg
not included
onboard gnss
none, connected gps only
health and wellness
baseline
overnight typical ranges for your key biometrics
heart rate
continuous, resting hr, high and low abnormal alerts
hrv status
yes, personal baseline
respiration
overnight and during workouts
stress
all day
reserve
yes, plus fitness age
sleep
score, coach, stages, naps, alignment, breathing variations, skin temp
women's health
cycle tracking, past ovulation estimates, natural cycles sync
other
jet lag adviser, hydration, calories, steps, intensity minutes, journaling
accessibility
wheelchair mode with push tracking
training and performance
training readiness
yes
training status
yes
vo₂ max
yes, from a timed 15 minute brisk walk or run
load
acute load, load ratio, load focus, training effect
recovery
recovery time, primary benefit per session
strength
optional rep counting
multi-device
metrics merge with other kelvin wearables
connectivity and smart features
bluetooth
yes, pairing and sync through the app
ant+
yes
heart rate broadcast
ant+ and bluetooth, to watches, bike computers, zwift, peloton
connected gps
yes, manual and auto-detected activities
live tracking
yes, with a paired phone
find my band
yes, vibrates until you press the button
alarms
smart wake and quiet start, vibration only
activity reminder
buzzes hourly while a session you started is running
notifications
none, by design
music storage
none
contactless pay
none
wi-fi
none
sizing and accessories
wrist s/m
120–200 mm · band 24.5 × 2.4 × 0.2 cm
wrist l/xl
145–240 mm · band 28.5 × 2.4 × 0.2 cm
upper arm s/m
200–320 mm · band 36.0 × 2.4 × 0.2 cm
upper arm l/xl
280–450 mm · band 50.0 × 2.4 × 0.2 cm
device colourways
onyx, fog gray, berry, navy
wrist bands
$49, seven colours, both sizes
upper arm bands
$59, onyx, both sizes
band care
module off, gentle cycle, water under 40 °c, air dry
app and subscription
free in the kelvin app
sleep, heart rate, reserve, readiness, training status, vo₂ max, activity tracking
kelvin plus adds
live session views with a heart rate graph, coaching plans, structured workouts, nutrition, guided breathwork, extended live tracking, 3d maps
required
no, core tracking never sits behind it
compatibility
ios and android
desktop sync
supported

what it does not do

stated up front, because finding this out after the box is open is worse for both of us.

kelvin k1 · $199 add to bag