If you've been managing diabetes for any length of time, you've heard about A1C. It's been the gold-standard number for decades. But if you use a CGM, you've probably also encountered "time in range," and you may be wondering how the two relate, which one matters more, and why your doctor seems to care about both. The short answer is that they measure different things, and understanding the difference helps you get a fuller picture of your glucose management. This guide breaks down what each number tells you and how to use them together.
A1C vs. Time in Range: The Key Difference at a Glance
According to the American Diabetes Association, A1C measures your average blood glucose over the previous three months, while time in range measures the amount of time you spend within your target glucose range. Here's the core distinction:
- A1C: A single blood test reflecting your average glucose over roughly three months. Expressed as a percentage (for example, 7.0%). The long-established standard for predicting and preventing diabetes complications.
- Time in Range (TIR): The percentage of time your glucose stays within your target range, typically 70 to 180 mg/dL for most people. Calculated from CGM data. Shows the daily highs and lows that an average can hide.
- The key insight: A1C tells you the average. Time in range tells you the story behind the average, including the variability that the average alone can't reveal.
Both numbers are useful, and neither fully replaces the other. The most complete picture comes from looking at them together.
What A1C Tells You and Where It Falls Short
A1C has been the cornerstone of diabetes management for good reason, and understanding both its strengths and its limitations helps you interpret it correctly.
A1C works by measuring the percentage of your hemoglobin (a protein in red blood cells) that has glucose attached to it. The ADA explains that the more glucose in your blood over time, the more hemoglobin becomes glycated, so the A1C percentage reflects your average glucose exposure over the roughly three-month lifespan of red blood cells. An A1C of 7.0% corresponds to an estimated average glucose of about 154 mg/dL. The ADA suggests a target of below 7% for many nonpregnant adults, though individual goals vary.
The strength of A1C is that it's well-validated as a predictor of long-term complications. Decades of research link A1C levels to the risk of complications affecting the eyes, kidneys, nerves, and cardiovascular system, which is why it remains the clinical standard.
The limitation is that an average can hide a lot. The ADA notes that A1C doesn't document the daily highs and lows a person may experience. Consider two people who both have an A1C of 7.0%. One might have steady glucose that stays close to 154 mg/dL most of the time. The other might be swinging between frequent severe lows and frequent highs that average out to the same number. Those two people have very different day-to-day experiences and very different risks, especially around hypoglycemia, but their A1C looks identical. A study published in PubMed Central analyzing data from over 500 adults with type 1 diabetes confirmed this, finding that for any given time in range percentage, there was a wide range of possible A1C levels, underscoring that the two metrics capture different information.
What Time in Range Adds to the Picture
Time in range emerged directly from the rise of continuous glucose monitoring, and it fills exactly the gap that A1C leaves open.
Because a CGM measures glucose every few minutes around the clock, it can calculate what percentage of the day your glucose spends within your target range, above it, and below it. For most people, the target range is 70 to 180 mg/dL, and the general goal recommended by the ADA is spending at least 70% of the day in range, which works out to roughly 17 out of every 24 hours. Your personal target may differ based on your situation, and your care team will help set it.
The power of time in range is that it reveals the daily patterns A1C can't. It separates your glucose into time spent high, time spent in range, and time spent low, giving you and your care team a much more actionable picture. If you're spending too much time low overnight, or spiking high after a particular meal, TIR and the detailed CGM reports behind it make those patterns visible in a way a single quarterly average never could. A review published in PubMed Central noted that time in range provides deeper insights into glucose variability and the detection of hypoglycemia, and that individualized TIR targets can be applied for special situations such as pregnancy.
To get a reliable time in range picture, research published in PubMed Central found that 10 to 14 days of CGM data generally provides a good estimate of your glucose metrics for a three-month period, as long as the sensor was active for at least 70% of that time. If you use a CGM like the Dexcom G7 or FreeStyle Libre 2 Plus, your companion app calculates time in range automatically, and you can review it daily or weekly.
One related term worth knowing: many CGM apps also display a GMI, or Glucose Management Indicator. This is an estimate of what your lab A1C might be based on your CGM average. It's a helpful between-appointment reference, but it isn't identical to a lab-measured A1C, and the two can differ. It's completely normal for your GMI and your lab A1C to not match exactly.
Why Looking at Both Numbers Gives You the Fullest Picture
A1C and time in range aren't competing metrics; they're complementary ones. A1C remains the validated standard for predicting long-term complication risk, while time in range gives you the day-to-day detail to understand and improve your management in real time. Used together, they let you and your care team see both the long-term trend and the daily patterns driving it. For people using insulin and a CGM, watching time in range provides faster, more actionable feedback than waiting three months for the next A1C, while A1C keeps the long-view risk picture in focus. The combination is genuinely more useful than either number alone. At Adapt Health Diabetes, we help make sure you have the CGM and supplies needed to track your time in range consistently. Visit our Learning Center and Education page for more guides on understanding and using your glucose data.
Frequently Asked Questions About A1C and Time in Range
Can my A1C be good but my time in range still be poor?
Yes, and this is one of the most important reasons both numbers matter. Because A1C is an average, frequent highs and frequent lows can cancel each other out to produce a seemingly acceptable A1C while masking significant glucose instability. A person with a "good" A1C who is experiencing frequent hypoglycemia and frequent spikes may actually be at higher risk than the number suggests. Time in range, and the breakdown of time spent high versus low, reveals that instability so it can be addressed. This is exactly why many care teams now review both.
What is a good time in range goal?
For most people with diabetes, the general goal is to spend at least 70% of the day within the target range of 70 to 180 mg/dL, which equals roughly 17 out of 24 hours. However, targets are individualized. Older adults, people with hypoglycemia unawareness, or those with certain other conditions may have different goals, and pregnancy uses a tighter target range. Your healthcare provider will help set a time in range goal that fits your specific situation, age, and management needs.
How often should I check my time in range?
Research has not established a single recommended frequency, but the ADA notes that many people find daily and weekly summaries helpful. Reviewing your time in range every week or two tends to strike a good balance: frequent enough to spot meaningful patterns, but not so frequent that normal daily variability becomes a source of stress. Your care team will typically review your time in range at appointments alongside your A1C to guide any adjustments to your management plan.
Will time in range replace A1C?
Not in the foreseeable future. The ADA has been clear that A1C has been and likely will remain the standard measure of diabetes management because it is well-established for predicting and helping prevent complications. Time in range is a powerful complement that adds detail A1C can't provide, but it doesn't replace the decades of research linking A1C to long-term outcomes. The trend in modern diabetes care is to use both together rather than choosing one over the other.
Why doesn't my CGM's GMI match my lab A1C?
The GMI (Glucose Management Indicator) is an estimate of your A1C based on your average CGM glucose, while a lab A1C measures glycated hemoglobin directly. Because these are two different measurements, they often differ, sometimes by a meaningful amount. Factors like individual variation in red blood cell lifespan, certain medical conditions, and the time window each metric covers can all contribute to the difference. A mismatch doesn't mean either number is wrong. Your care team can help interpret both in the context of your overall management.
Track Your Time in Range With the Right CGM
Seeing your time in range starts with a reliable CGM and consistent sensor wear. Adapt Health Diabetes delivers Dexcom and Abbott FreeStyle Libre CGM supplies through your insurance, shipped on a schedule that keeps you stocked. Visit our Resources page for more information, or contact our team with questions about coverage.
This article is for educational purposes only and does not replace professional medical advice. Always consult your healthcare provider regarding your A1C goals, time in range targets, and diabetes management decisions.
Sources
- American Diabetes Association. CGM and Time in Range. ADA.
- American Diabetes Association. The Difference Between A1C and eAG. ADA.
- Beck RW, et al. The Relationships Between Time in Range, Hyperglycemia Metrics, and HbA1c. PubMed Central. 2019.
- Bergenstal RM, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. PubMed Central. 2018.
