A continuous glucose monitor produces hundreds of numbers a day. Knowing which ones to look at keeps the data useful instead of stressful, and helps you bring something concrete to your clinician.
Quick answer: The main CGM measures are average glucose (and GMI), time in range, time above and below range, and variability. For most adults with type 1 or type 2 diabetes, international consensus targets are glucose between 70 and 180 mg/dL more than 70 percent of the time, below 70 mg/dL less than 4 percent, below 54 mg/dL less than 1 percent, above 180 mg/dL less than 25 percent, and above 250 mg/dL less than 5 percent. Your clinician may set different targets for you. For people without diabetes there is no standard clinical target.
The core CGM numbers
| Metric | What it shows | Consensus target for most adults with diabetes |
|---|---|---|
| Time in range (70 to 180 mg/dL) | Share of readings in the standard target band | More than 70 percent |
| Time below 70 mg/dL | Share of readings that are low | Less than 4 percent |
| Time below 54 mg/dL | Share of readings that are clinically significant lows | Less than 1 percent |
| Time above 180 mg/dL | Share of readings above the target band | Less than 25 percent |
| Time above 250 mg/dL | Share of readings that are very high | Less than 5 percent |
| Average glucose and GMI | Mean sensor glucose and the A1C-style estimate from it | No numeric target is set here; individualized |
These targets are published by the NIDDK summary of the international consensus report. The same source says a report should be based on 14 days of data with the sensor active at least 70 percent of the time, which is why short wear periods are less trustworthy. It also notes that every 5 percent increase in time in range is clinically important for people with type 1 or type 2 diabetes, so small improvements are worth noticing. Older or higher-risk adults and pregnant women with type 1 diabetes have their own targets, which your care team sets. You can read the primary summary on the NIDDK page on clinical targets for CGM data.
Average glucose and GMI
The glucose management indicator (GMI) converts your sensor’s mean glucose into an A1C-style percentage using GMI = 3.31 + 0.02392 × mean glucose (mg/dL). A mean of 135 mg/dL gives 6.54, reported as 6.5 percent. GMI and a lab A1C can differ by a few tenths because they measure different things over different periods; see the A1C to average glucose chart and continuous glucose monitor vs A1C.
A worked example: one day of sensor data
A sensor that reads every 5 minutes produces 288 readings a day. Suppose a day’s readings fall into these bands:
| Band | Readings | Share of the day |
|---|---|---|
| Below 54 mg/dL | 1 | 0.3% |
| 54 to 69 mg/dL | 8 | 2.8% |
| 70 to 180 mg/dL | 227 | 78.8% |
| 181 to 250 mg/dL | 48 | 16.7% |
| Above 250 mg/dL | 4 | 1.4% |
Time in range is 227 ÷ 288 = 78.8 percent. Time below 70 mg/dL is (1 + 8) ÷ 288 = 3.1 percent, and time above 180 mg/dL is (48 + 4) ÷ 288 = 18.1 percent. Each reading represents 5 minutes, so the 9 low readings add up to 45 minutes below 70 that day. Measured against the consensus targets for adults with diabetes, this day passes on all five measures: time in range is above 70 percent, time below 70 is under 4 percent, time below 54 is about 0.3 percent, time above 180 is under 25 percent, and time above 250 is about 1.4 percent. It still contains 45 minutes below 70, and a person who feels symptoms during those minutes would want to discuss the pattern, because the targets are about averages and not about how you feel. One day proves little. A pattern over 14 days is what the targets are about. For a 14-day period with 5-minute readings you would expect up to 4,032 readings, and a 70 percent wear threshold means at least 2,823 of them should be present for the report to be meaningful.
Reading a spike
Look at the shape, not the peak. A rise after a meal that returns toward baseline within a couple of hours is the expected pattern in most people. Context matters: what you ate, how much, whether you had protein or fat with it, how you slept, whether you were active, and whether you were ill or stressed. One spike is not a verdict.
- Fast, tall, fast to recover. Often a concentrated carbohydrate eaten alone.
- Slow, long plateau. Often a larger or mixed meal, or reduced insulin sensitivity that day.
- Early-morning rise with no food. A pattern clinicians sometimes ask about, which is worth bringing to an appointment rather than solving alone.
- Sudden overnight drop that recovers quickly. May be a compression artifact from lying on the sensor.
Use the Post-Meal Glucose Experiment Tracker to compare the same meal on different days, and the Glucose Load Meal Builder to plan variations. Our carb guide, how to read a nutrition label for carbs, explains the inputs.
Variability
Variability describes how much glucose swings. It is often reported as a standard deviation or as a coefficient of variation (CV), which is the standard deviation divided by the mean. In one multicenter study of healthy people without diabetes, the average within-person CV was about 17 percent (plus or minus 3). That result describes that study’s volunteers, not a personal goal, but it shows that healthy glucose is not a flat line. See normal CGM glucose range without diabetes.
Turning patterns into questions
The most useful thing you can do with a CGM report is turn what you see into specific questions for your care team. A few common patterns and the questions they raise:
| What you see | Possible contributors to discuss | Question to ask |
|---|---|---|
| Rise after most breakfasts | Meal composition, timing of medicine, morning hormone changes | Would changing the meal, its timing, or my plan make sense? |
| Lows in the late afternoon | Gap between meals, activity, medicine timing | Should my medicine or meal schedule be reviewed? |
| Flat line with brief overnight dips | Compression from sleeping position, sensor placement | Is this real or a sensor artifact? |
| Wide swings day to day | Variable meals, sleep, stress, illness, activity | What routine changes might make my days more comparable? |
| Sensor average much higher than A1C implies | Sensor error, red blood cell factors affecting A1C | Could my A1C be unreliable, or my sensor inaccurate? |
How to prepare for a CGM review visit
Most CGM systems generate a one-page summary, often called an ambulatory glucose profile, that shows time in range, a median glucose curve with its spread, and daily overlays. Bring the report, or share it with your clinic ahead of time if they accept it. Add a short note about anything unusual in the period, such as a new medicine, an illness, travel, or a large change in activity, because those explain patterns that the graph alone cannot. Writing down two or three questions beforehand makes the visit more productive than scrolling through the app together.
Limits to remember
- Lag. Sensors measure glucose in the fluid under the skin, which lags blood by roughly 5 to 15 minutes, especially when glucose is changing fast.
- Compression lows. Overnight lows can be false when you lie on the sensor.
- Accuracy. Sensors can be off by a meaningful margin compared with a lab test. If you feel unwell, or a reading does not match your symptoms, confirm with a fingerstick.
- Over-the-counter sensors are not diagnostic tools. They are cleared for general wellness use. See Stelo vs Lingo, do you need a CGM with prediabetes, and prescription vs over-the-counter CGMs.
Common mistakes
- Reacting to single readings instead of patterns over days.
- Judging a sensor in its first day or two, when readings can be less settled.
- Comparing your numbers with someone else’s report without noting the different targets and goals.
- Making large food or medicine changes in response to one alert.
- Skipping the clinic review, where the report’s patterns can be interpreted with your health history.
Low blood sugar warning: if you take insulin, a sulfonylurea, or another glucose-lowering medicine, changing what you eat or adding a product can push glucose too low. Symptoms include shakiness, sweating, confusion, and a fast heartbeat. Talk to your prescriber before changing your eating or your medicine, and follow your own low-blood-sugar plan.
A CGM low alert is a safety signal, and you should have a plan from your care team for what to do when it sounds. For the broader picture, see the complete guide to A1C and glucose and the A1C and average glucose calculator.
Questions to bring to your appointment
- What time-in-range target is right for me?
- How should I respond to a low or high alert?
- Which patterns in my data should we review together?
- Does my sensor average agree with my A1C?
- How many days of data do you want before we make a decision?
Frequently asked questions
How many days of CGM data do I need before the numbers mean anything?
The international consensus summary on the NIDDK site refers to 14 days of data with the sensor active at least 70 percent of the time. Shorter periods can still be informative for a particular meal experiment but are not enough to judge overall time in range.
Is 70 percent time in range a goal for people without diabetes?
No. The 70 to 180 mg/dL, more-than-70-percent target was written for most adults with diabetes. People without diabetes typically spend much more time between 70 and 140 mg/dL, but there is no clinical target for them.
What is a good coefficient of variation?
The consensus documents describe variability metrics, and your clinician can tell you how they use them. In one study of healthy people the mean CV was about 17 percent. That is a description of that group, not a pass-or-fail number.
Why does my CGM show a low when I feel fine?
Possible causes include compression from lying on the sensor, a sensor that is failing, or lag during a fast change. If you feel unwell or are unsure, check a fingerstick.
Is GMI the same as my A1C?
No. GMI estimates A1C from sensor data. It can differ from your lab A1C by a few tenths for biological and measurement reasons.
Can I change my medicine based on my CGM report?
No. Bring the report to your prescriber, who can decide whether any treatment change makes sense.
About the author: Charles Kirkland is the founder of Metabolic Clarity Labs and author of The Journey Back. He is not a physician. This is general education, not a medically reviewed treatment plan.
Sources checked October 31, 2026. Discuss personal decisions with a qualified clinician. Do not start, stop, or change prescribed treatment based on this article.
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