A CGM, or continuous glucose monitor, is a small wearable device that tracks your blood sugar levels around the clock without finger pricks. It works by reading glucose in the fluid just beneath your skin and sending a new reading to your phone or a separate display every few minutes. Originally designed for people with diabetes, CGMs are now available over the counter for anyone curious about how their body responds to food and exercise.
How a CGM Works
A CGM system has three parts: a tiny sensor, a transmitter, and a display (usually your smartphone). The sensor is a hair-thin filament that sits just under the skin, typically on the back of your upper arm or your abdomen. It measures glucose in your interstitial fluid, the liquid that surrounds your cells beneath the skin’s surface, rather than directly in your blood.
Because the sensor reads interstitial fluid instead of blood, there’s a natural delay. Glucose moves from your bloodstream into this fluid over time, so CGM readings lag behind a traditional finger-prick test by roughly 5 to 25 minutes. In practice, this means a CGM might show your sugar is still climbing a few minutes after it has already peaked in your blood. The gap is smallest when your glucose is stable and widest when it’s rising or falling quickly.
The transmitter, attached to the sensor on the surface of your skin, wirelessly sends data to your phone app or a dedicated receiver. Most current sensors deliver a new reading every 1 to 5 minutes, giving you a continuous line graph of your glucose rather than a single snapshot.
What You Actually Wear
The visible part of a CGM is about the size of two stacked coins. It sticks to your skin with a medical adhesive patch and stays in place through showers, exercise, and sleep. Depending on the model, each sensor lasts 10 to 15 days before you peel it off and apply a fresh one.
Inserting a sensor involves a spring-loaded applicator that pushes the filament under the skin in less than a second. Most people describe it as a brief pinch. After insertion, the sensor goes through a warm-up period, typically 30 to 60 minutes, during which it calibrates before delivering its first reading. Older models required finger-prick calibrations once or twice a day, but current-generation devices are factory calibrated and generally don’t need them.
Reading the Data
A CGM gives you three pieces of information at a glance: your current glucose number, a line graph showing where you’ve been over the past several hours, and a trend arrow showing where you’re headed. The trend arrows are especially useful. A flat arrow means your glucose is holding steady, changing less than 1 mg/dL per minute. Angled arrows indicate a moderate rise or fall, and straight vertical arrows signal a rapid change of more than 3 mg/dL per minute, which for someone on insulin can mean it’s time to act.
Over days and weeks, the data builds into patterns. You can see exactly how a bowl of oatmeal, a stressful meeting, or a morning run affects your glucose. This kind of continuous feedback is fundamentally different from the isolated snapshots that finger-prick testing provides.
Time in Range: The Key Metric
The most important number a CGM tracks isn’t any single reading. It’s “time in range,” or the percentage of the day your glucose stays between 70 and 180 mg/dL. An international consensus panel recommends that most people with type 1 or type 2 diabetes aim to spend at least 70% of their day in that window, which works out to about 16 hours and 48 minutes. For older adults or people at higher risk of low blood sugar, the target is more relaxed: at least 50% of the day, or 12 hours.
Time in range correlates directly with long-term blood sugar control. Every 10% increase in time in range corresponds to roughly a 0.5% to 0.8% drop in A1c, the lab value doctors use to assess average blood sugar over three months. Each additional 5% improvement is associated with meaningful clinical benefits. For many people, watching time in range climb week over week is more motivating than chasing a single A1c number twice a year.
Equally important is time below range. The consensus targets recommend spending less than 4% of the day (under one hour) below 70 mg/dL and less than 1% (under 15 minutes) below 54 mg/dL, the threshold for serious low blood sugar.
How Accurate Are Current CGMs
Device accuracy is measured by something called MARD, which stands for mean absolute relative difference. It’s essentially the average percentage by which the sensor’s reading differs from a lab-grade blood test. Lower is better, and anything under about 10% is considered very accurate for clinical decision-making.
A 2025 head-to-head comparison of three major systems in adults with type 1 diabetes found them closely matched. The FreeStyle Libre 3 and Dexcom G7 both came in around 9.5% to 10% MARD when compared to a venous lab analyzer, while the Medtronic Simplera scored around 12% to 14% depending on the reference method. All three were accurate enough for day-to-day glucose management, but the Libre 3 and Dexcom G7 had a slight edge in consistency across different testing conditions.
Who Uses a CGM
CGMs were originally prescribed for people with type 1 diabetes who need to dose insulin multiple times a day. They’re now widely used by people with type 2 diabetes as well, including those on oral medications alone. The real-time feedback helps people see the direct impact of their meals and activity, which often leads to better dietary choices without requiring more medication.
In 2024, the FDA cleared the first over-the-counter CGM: the Dexcom Stelo. It’s designed for adults 18 and older who don’t use insulin. That includes people with type 2 diabetes managed by oral medications and people without diabetes who simply want to understand how their body handles glucose. The Stelo uses a 15-day sensor and delivers readings every 15 minutes through a smartphone app. It does not include low blood sugar alerts, since it’s not intended for people at risk of dangerous hypoglycemia.
This over-the-counter availability has opened the door to a growing community of non-diabetic users: athletes optimizing fueling strategies, people experimenting with low-carb diets, and those with a family history of diabetes who want early insight into their metabolic health.
Limitations Worth Knowing
CGMs are not perfect replacements for blood glucose meters in every situation. The 5-to-25-minute lag means that during rapid glucose swings, the number on your screen may not match a finger prick taken at the same moment. If you’re making an urgent decision about insulin dosing during a steep drop, some clinicians still recommend confirming with a finger-prick test.
Sensor accuracy can also vary on the first day of wear while the sensor settles into the tissue. Compression lows are another quirk: if you sleep on the arm where the sensor is placed, the pressure can temporarily restrict fluid flow and produce a false low reading. Most experienced users learn to recognize these artifacts quickly.
Cost remains a barrier for some. Prescription CGMs for people with diabetes are increasingly covered by insurance, but out-of-pocket prices for the over-the-counter Stelo and similar devices can run $50 to $100 per month. Sensor adhesive can also irritate sensitive skin, though barrier wipes and patches designed to sit between the adhesive and skin help most people manage this.