How to read CGM glucose numbers.
To make sense of your continuous glucose monitor, you only need a few things: a sense of what is in range for you (often roughly 70 to 140 mg/dL after meals), what your fasting morning number looks like, and which direction the arrow next to your current reading is pointing. Everything else is noise until those three become familiar.
The numbers and units, in normal words
In the United States, glucose is reported in milligrams per deciliter, written mg/dL. In most of the rest of the world, it is reported in millimoles per liter, written mmol/L. Your CGM app uses whichever one is set to your region. Both describe the same thing: how much sugar is currently floating around in your blood.
For reference, 90 mg/dL is about 5.0 mmol/L. If you ever toggle the unit by accident, the rough conversion is "divide mg/dL by 18 to get mmol/L." For a quick instant conversion, use our A1C to average glucose and mg/dL to mmol/L converter.
Rough ranges most non-diabetic adults sit in, in mg/dL: [VERIFY: cite ADA reference ranges]
- Fasting morning glucose: roughly 70 to 99
- One to two hours after a meal: under about 140
- Three hours after a meal: back near your fasting baseline
In prediabetes, those numbers drift higher. In type 2 diabetes, they drift higher still. You will not need to memorize cutoffs to use your CGM well; you just need to know your own baseline.
Your fasting number
The first thing most people look at every morning is the fasting glucose: the reading on your CGM right when you wake up, before you have eaten or had coffee. It is your baseline.
A few honest things to know about the fasting number:
- It can vary by 10 to 20 mg/dL from one day to the next without anything being "wrong." Sleep, stress, and even the position you slept in can move it.
- It is the dawn phenomenon, not your breakfast, that often makes the morning number look higher than the rest of the day. Cortisol rises in the early morning and signals the liver to release glucose. That is by design.
- Tracking a weekly average is far more useful than reacting to a single morning number.
Post-meal readings: the most useful data
Honestly, this is the killer feature of a CGM. Until you put one on, you have no idea which of your usual meals send your glucose climbing fast and which ones leave it nearly flat. After two weeks of wearing a sensor, you will.
A few patterns to look for:
- The peak. How high did the reading go after the meal? For most adults not on insulin, staying under about 140 mg/dL after a normal meal is a reasonable target.
- The shape. A gentle hill that comes back to baseline by hour two is great. A sharp climb followed by a crash often signals a meal that was carb-heavy and protein-light.
- The recovery. How quickly did the line come back down? Slow recovery, especially staying elevated for several hours, is a useful signal that your body is working harder than it should to clear that meal.
Once you see two or three meals that consistently send you climbing, you have actionable information. You do not have to eliminate them; you just know what they are doing.
Our pillar Food, carbs, and your glucose goes deeper on what's happening, and Why blood sugar spikes matter covers the bigger-picture question of when peaks are worth caring about.
Time in range
Time in range, often shortened to TIR, is the percentage of the day your glucose spends in a target window. For non-insulin users, that window is typically 70 to 140 mg/dL during the day and a bit lower overnight. [VERIFY: ADA time-in-range standards]
Why this matters more than a daily average: two days can have the same average but very different stories. One steady day at 110 mg/dL all afternoon and one volatile day swinging between 70 and 200 average out the same. Time in range punishes the volatility your average hides.
For someone in prediabetes or early type 2 not on insulin, a useful starting goal is "more time in range this week than last week." That is it. You do not need to chase a number; you need to bend the trend.
Trend arrows
Most CGM apps show a small arrow next to your current reading. The arrow is doing real work, and people miss it on day one.
A reading of 130 mg/dL with a flat arrow is calmer than a reading of 130 mg/dL with a sharp up-arrow. The first one is staying put. The second one is climbing fast, and your sensor is telling you the peak is still ahead.
The arrows usually translate like this:
- Flat: glucose is roughly stable. Nothing to do.
- One arrow up or down: glucose is moving moderately. Worth noticing what you just ate or did.
- Two arrows up or down: glucose is changing fast. If you are climbing fast soon after a meal, that meal is a trigger worth remembering. If you are dropping fast and you are not on glucose-lowering medication, you may be due for water, a small snack, or a sensor check.
Weird readings and what they probably mean
A few things almost every CGM user runs into in the first month:
- The 3 a.m. low that did not wake you up. This is usually a "compression low" caused by sleeping on the sensor. The reading is not real. If you wake up feeling fine, it almost certainly was not real.
- A sudden spike with no obvious cause. Check whether you just took acetaminophen (Tylenol), which can affect some sensors, or whether you had something with hidden sugar.
- A reading that does not match how you feel. Trust how you feel first. If the sensor says you are at 60 mg/dL but you feel completely normal and you are not on insulin, the sensor may be wrong. Confirm with a fingerstick if you have a meter.
- The "warm-up flat line" on a new sensor. New sensors take 30 minutes to one hour before showing data. That is normal.
For deeper reading on what these numbers mean clinically: American Diabetes Association, CDC Diabetes, NIDDK (NIH).
A simple two-week starter routine
If you are brand new and feeling overwhelmed, a two-week starter routine:
- Days 1 to 3. Just look. Do not try to change anything. Note your morning number and your highest reading each day.
- Days 4 to 7. Pick one meal you eat regularly. Eat it once with no changes, once with a 10-minute walk afterward, and compare the peak.
- Days 8 to 14. Test one swap that interests you. A different breakfast, a smaller portion of pasta, a glass of water before a meal. Compare the curves.
That gives you four to six small experiments and a real sense of how your body responds to what you do.
If you have not chosen a sensor yet, our pillar What is a CGM? A plain English guide covers what to know before you buy.
This is educational only and not medical advice. If anything on your CGM concerns you, please share it with your healthcare provider before making changes to your medication, diet, or activity. OTC sensors are not designed for insulin dosing decisions.
Common questions
What is a "normal" CGM reading?
For most non-diabetic adults, glucose sits around 70 to 99 mg/dL fasting and under about 140 mg/dL one to two hours after a meal. In prediabetes those numbers drift higher; in type 2 diabetes higher still. Your own baseline is more useful than a textbook number.
Why does my fasting number look high if I have not eaten?
It is usually the dawn phenomenon: cortisol rises in the early morning and tells the liver to release glucose. A higher fasting number than you expected is often this, not your dinner from the night before.
What is a good time in range goal?
For adults not on insulin, a useful starting goal is simply "more time in range this week than last." Many CGM apps default a target window of 70 to 140 mg/dL, which is reasonable. The trend matters more than the number on any given day.
Why does my CGM show a different reading than my fingerstick?
CGMs read interstitial fluid, which lags your blood by roughly 5 to 15 minutes. After a fast meal or quick exercise, expect the two to diverge briefly. They should match again once your glucose has stabilized. For a deeper comparison see CGM vs fingerstick.
Should I act on every spike I see?
No. Single spikes are not the story. Patterns are. Look for meals or routines that consistently produce the same response across multiple days before deciding to change anything.
References
- American Diabetes Association. Continuous glucose monitors and time in range. diabetes.org. [VERIFY: link to current ADA TIR consensus]
- NIDDK. Continuous glucose monitoring overview. niddk.nih.gov. [VERIFY: confirm canonical URL]
- CDC. Diabetes basics. cdc.gov/diabetes. [VERIFY: link to overview page]
- [VERIFY: add a recent reference on the dawn phenomenon and morning glucose variability]
Last reviewed: .
Tell us what you're struggling with
Questions, corrections, or "this is exactly where I'm stuck": leave a note. We read every comment and reply when we can.