Normal blood sugar for a healthy adult is below 100 mg/dL when fasting and below 140 mg/dL two hours after eating. These two numbers are the standard benchmarks used to screen for diabetes and prediabetes, and they apply to most non-pregnant adults. But blood sugar isn’t a single fixed number. It shifts throughout the day based on what you eat, how active you are, how well you slept, and even how stressed you feel.
Fasting and Post-Meal Ranges
A fasting blood sugar test measures your glucose after at least eight hours without food, typically first thing in the morning. Below 100 mg/dL is considered normal. Between 100 and 125 mg/dL falls into the prediabetes range, meaning your body is starting to have trouble managing glucose. At 126 mg/dL or higher on two separate tests, the result meets the threshold for a diabetes diagnosis.
After you eat, blood sugar naturally rises as your body breaks down carbohydrates into glucose. In a healthy person, it peaks roughly 60 to 90 minutes after a meal and then drops back down. The standard cutoff is below 140 mg/dL at the two-hour mark. If your post-meal reading hits 140 to 199 mg/dL, that suggests prediabetes. A reading of 200 mg/dL or above points toward diabetes.
The A1C Test: Your Three-Month Average
While fasting and post-meal tests capture a snapshot, the A1C test reflects your average blood sugar over the past two to three months. It measures the percentage of hemoglobin (a protein in red blood cells) that has glucose attached to it. The higher your blood sugar has been running, the higher your A1C.
The thresholds are straightforward. Below 5.7% is normal. Between 5.7% and 6.4% indicates prediabetes. At 6.5% or above, the result meets the criteria for diabetes. An A1C of 5.7% corresponds roughly to an average blood sugar of about 117 mg/dL, while 6.5% corresponds to roughly 140 mg/dL. Because A1C captures a longer window, it’s less affected by a single bad day of eating or a stressful morning.
How Your Body Keeps Glucose in Check
Two hormones do most of the work. After you eat and blood sugar rises, your pancreas releases insulin. Insulin acts like a key, opening the door for glucose to enter muscle and fat cells. It also tells the liver to stop making new glucose and to store the excess as glycogen for later use. This combination pulls blood sugar back down.
When blood sugar drops too low, such as between meals or during exercise, the pancreas releases a second hormone called glucagon. Glucagon does the opposite: it signals the liver to break down its glycogen stores and release glucose back into the bloodstream. This push-pull system keeps your blood sugar in a narrow, healthy range without you having to think about it. Problems arise when insulin stops working efficiently (insulin resistance) or when the pancreas can no longer produce enough of it.
When Blood Sugar Goes Too Low
Blood sugar below 70 mg/dL is considered low, a condition called hypoglycemia. At this level, you may notice a fast heartbeat, shaking, sweating, dizziness, sudden hunger, or irritability. These symptoms are your body’s alarm system, driven by the stress hormones it releases to push glucose back up.
Below 54 mg/dL is classified as severely low. At that point, symptoms can escalate to weakness, trouble walking, blurred vision, confusion, and in rare cases, seizures. Severe hypoglycemia is most common in people taking insulin or certain diabetes medications, but it can also happen after prolonged fasting or intense exercise in people without diabetes.
When Blood Sugar Goes Too High
Occasional spikes after a large meal are normal and temporary. Persistent fasting levels above 100 mg/dL or post-meal levels above 140 mg/dL, however, signal that your body is struggling to clear glucose from the bloodstream efficiently. Early on, high blood sugar often causes no obvious symptoms. Over time, it can produce increased thirst, frequent urination, fatigue, and blurred vision. Chronically elevated blood sugar damages blood vessels and nerves, which is why catching prediabetes early matters.
Ranges During Pregnancy
Pregnancy shifts the goalposts. Hormones from the placenta can make cells more resistant to insulin, so the body has to work harder to keep blood sugar in range. The targets for pregnant women are tighter than the standard adult ranges: fasting blood sugar should stay below 95 mg/dL, readings one hour after eating should be below 140 mg/dL, and two-hour post-meal readings should stay below 120 mg/dL. Women who exceed these thresholds on a glucose tolerance test may be diagnosed with gestational diabetes, which typically resolves after delivery but increases the risk of developing type 2 diabetes later in life.
Ranges for Children and Teens
Children with diabetes have slightly different targets than adults, largely because very young children are more vulnerable to dangerously low blood sugar and less able to recognize or communicate symptoms. For toddlers and preschoolers (ages 0 to 6), blood sugar targets typically range from 100 to 200 mg/dL, with an A1C goal below 8.5%. School-age children (6 to 12) aim for 90 to 180 mg/dL and an A1C below 8%. Adolescents and young adults (13 to 19) have a narrower target of 90 to 150 mg/dL with an A1C below 7.5%. By adulthood, the general target tightens to 80 to 150 mg/dL and an A1C below 7%.
These wider ranges for younger children reflect a deliberate tradeoff: avoiding severe lows is prioritized over keeping numbers as tight as possible.
What Makes Blood Sugar Fluctuate Day to Day
Even if you eat the same meals two days in a row, your blood sugar readings can differ. Several everyday factors explain why.
- Stress: When you’re under prolonged stress, your body releases hormones like cortisol and adrenaline that prompt the liver to dump extra glucose into your bloodstream. This made sense when stress meant running from a predator. It’s less helpful when the source is a work deadline.
- Sleep: Poor or short sleep reduces insulin sensitivity, meaning your cells don’t respond to insulin as effectively the next day. Even one night of bad sleep can raise fasting blood sugar the following morning.
- Physical activity: Exercise pulls glucose into your muscles for energy, which lowers blood sugar during and after a workout. Intense or prolonged exercise can occasionally cause a drop that continues for hours afterward.
- Hydration: Dehydration concentrates glucose in your blood, which can lead to higher readings. Drinking enough water helps your kidneys flush excess glucose and keeps readings more stable.
- Meal composition: A meal heavy in refined carbohydrates spikes blood sugar faster and higher than one with the same calories but more fiber, protein, and fat. The speed of digestion matters as much as the total amount of carbohydrates.
Understanding these factors helps explain why a single blood sugar reading doesn’t tell the full story. Patterns over days and weeks are far more informative than any one number.