BGL stands for blood glucose level, the concentration of sugar circulating in your bloodstream at any given moment. A normal fasting BGL is below 100 mg/dL (5.6 mmol/L). This sugar, called glucose, is your body’s primary fuel source. It comes from the food you eat: your body breaks down most of that food into glucose and releases it into your bloodstream, where insulin helps shuttle it into cells for energy.
Normal BGL Ranges
Blood glucose is measured in milligrams per deciliter (mg/dL) in the United States and millimoles per liter (mmol/L) in most other countries. Here’s what the numbers mean for someone without diabetes:
- Fasting (no food for 8+ hours): Below 100 mg/dL (5.6 mmol/L)
- Two hours after eating: Below 140 mg/dL (7.8 mmol/L)
If you have diabetes, the targets shift slightly. The CDC recommends aiming for 80 to 130 mg/dL before meals and below 180 mg/dL two hours after the start of a meal, though your specific targets may vary based on age and other health conditions.
Prediabetes and Diabetes Thresholds
BGL is one of the key measurements used to diagnose prediabetes and diabetes. The American Diabetes Association uses these cutoffs:
- Prediabetes: Fasting BGL of 100 to 125 mg/dL, or a two-hour glucose tolerance test result of 140 to 199 mg/dL
- Diabetes: Fasting BGL of 126 mg/dL or higher, a two-hour tolerance test of 200 mg/dL or higher, or a random reading of 200 mg/dL or higher with symptoms
There’s also the A1C test, which reflects your average blood glucose over roughly three months rather than a single snapshot. An A1C of 5.7% to 6.4% indicates prediabetes, while 6.5% or higher means diabetes. You can roughly convert A1C to an average daily BGL: an A1C of 7% corresponds to about 154 mg/dL, while an A1C of 9% translates to around 212 mg/dL.
When BGL Drops Too Low
Low blood glucose, called hypoglycemia, happens when sugar levels fall below what your body needs to function properly. For people with diabetes, the threshold is below 70 mg/dL (3.9 mmol/L). For people without diabetes, it’s generally below 55 mg/dL (3.1 mmol/L), because a healthy pancreas can usually prevent levels from dropping that far.
Early signs include shakiness, sweating, a racing heartbeat, and irritability. As levels continue to drop, confusion, blurred vision, and difficulty concentrating set in. Severe hypoglycemia can cause seizures, loss of consciousness, and requires emergency treatment. This is one reason people on insulin or certain diabetes medications check their BGL frequently throughout the day.
When BGL Climbs Too High
Persistently elevated blood glucose, called hyperglycemia, damages blood vessels and nerves over time. But acutely high BGL can trigger medical emergencies. Diabetic ketoacidosis (DKA) can develop when glucose reaches 200 mg/dL or higher alongside a buildup of acids in the blood. DKA is most common in people with type 1 diabetes and requires hospital treatment. Another emergency, hyperosmolar hyperglycemic state, involves extremely high blood sugar with severe dehydration and typically affects people with type 2 diabetes.
Day-to-day hyperglycemia often shows up as increased thirst, frequent urination, fatigue, and blurry vision. Over months and years, elevated BGL contributes to complications in the eyes, kidneys, nerves, and cardiovascular system.
How BGL Is Measured
There are two main ways to track blood glucose. The traditional method is a fingerstick test using a small handheld meter. You prick your finger, place a drop of blood on a test strip, and get a reading in seconds. This gives you a single number at a single point in time, which is useful but can miss fluctuations that happen between checks, especially overnight or after meals.
Continuous glucose monitors (CGMs) take a different approach. A tiny sensor sits just under the skin, usually on the arm or abdomen, and measures glucose levels in the surrounding fluid every five minutes. That means up to 288 readings per day compared to the handful you’d get from fingersticks. CGMs also show trends, letting you see whether your glucose is rising, falling, or holding steady, and they can alert you before you hit a dangerous high or low. They’re increasingly used not only by people with type 1 diabetes but also by those with type 2 and gestational diabetes.
What Affects BGL Besides Food
Food is the most obvious factor, but several things influence your blood glucose that have nothing to do with what you ate. Your body follows a natural circadian rhythm that raises blood sugar in the early morning hours. Hormones like cortisol and adrenaline signal the liver to release stored glucose, which is why some people see higher fasting readings first thing in the morning even if they haven’t eaten for 10 or 12 hours.
Stress uses the same hormonal pathway. When your body perceives a threat, whether that’s a near-miss in traffic or a tense day at work, it floods the bloodstream with extra glucose to fuel a potential fight-or-flight response. If you’re stressed chronically, your blood sugar can stay elevated much of the time, even with a consistent diet. Sleep deprivation compounds this by increasing cortisol output and reducing insulin sensitivity, making it harder for your cells to absorb glucose efficiently. Physical activity, illness, certain medications, and hormonal changes during menstruation or menopause can all shift your BGL as well.
Understanding these factors helps explain why two identical meals can produce different glucose readings on different days. BGL is a dynamic measurement that reflects far more than just your last snack.