Blood sugar levels measure the amount of glucose circulating in your bloodstream at any given moment. For a healthy adult without diabetes, a normal fasting blood sugar falls between 70 and 99 mg/dL. That number shifts throughout the day based on what you eat, how you sleep, your stress levels, and how active you are. Understanding these ranges helps you interpret lab results, spot early warning signs, and make sense of what your body is doing with the food you consume.
How Your Body Regulates Blood Sugar
Two hormones produced by the pancreas work in opposition to keep blood sugar stable. Insulin lowers blood sugar by allowing cells in your muscles, fat, and liver to absorb glucose from the bloodstream and use it for energy or store it as fat. Glucagon does the opposite: when blood sugar drops too low, it signals the liver to convert stored glucose back into a usable form and release it into the blood. Glucagon also triggers the body to manufacture new glucose from other sources, like amino acids from protein.
This back-and-forth operates constantly. After a meal, insulin rises to manage the flood of incoming glucose. Between meals or overnight, glucagon keeps levels from falling too far. When this system works well, blood sugar stays within a narrow, healthy range without you ever noticing. Problems develop when the body stops producing enough insulin, stops responding to insulin properly, or both.
Normal Blood Sugar Ranges
A fasting blood sugar (measured after at least 8 hours without eating) of 70 to 99 mg/dL is considered healthy for adults without diabetes. Some people naturally run a bit lower, and values between 50 and 70 mg/dL can be normal as well if you feel fine and have no symptoms.
After eating, blood sugar naturally rises. In people without diabetes, it typically peaks within an hour or two and comes back down on its own. For people managing diabetes, the general target is below 180 mg/dL two hours after the start of a meal.
During pregnancy, the targets are tighter. Guidelines from the American College of Obstetricians and Gynecologists recommend a fasting level below 95 mg/dL, with readings below 140 mg/dL one hour after eating or below 120 mg/dL at two hours.
Prediabetes and Diabetes Thresholds
The American Diabetes Association uses specific cutoffs to distinguish normal blood sugar from prediabetes and diabetes. These are based on fasting glucose, A1c (a measure of your average blood sugar over roughly three months), or an oral glucose tolerance test.
- Normal: Fasting glucose below 100 mg/dL, A1c below 5.7%
- Prediabetes: Fasting glucose 100 to 125 mg/dL, A1c 5.7% to 6.4%
- Diabetes: Fasting glucose 126 mg/dL or higher, A1c 6.5% or higher
A random blood sugar of 200 mg/dL or higher, combined with classic symptoms like excessive thirst or frequent urination, also meets the diagnostic criteria for diabetes. In most cases, an abnormal result needs to be confirmed with a second test on a different day before a diagnosis is made.
What A1c Tells You
While a single blood sugar reading is a snapshot, the A1c test reflects the bigger picture. It measures the percentage of your red blood cells that have glucose attached to them, which gives an estimate of your average blood sugar over the past two to three months.
The conversion is straightforward. An A1c of 6% corresponds to an average blood sugar of about 126 mg/dL. At 7%, the average is roughly 154 mg/dL. At 8%, it’s around 183 mg/dL, and at 9%, approximately 212 mg/dL. Each half-percentage point increase in A1c translates to roughly a 14 mg/dL jump in average blood sugar. This makes A1c useful for tracking long-term trends rather than reacting to daily fluctuations.
When Blood Sugar Gets Too High
Hyperglycemia, or high blood sugar, doesn’t always produce obvious symptoms right away. Many people with diabetes don’t notice anything until their blood sugar exceeds 250 mg/dL. People who haven’t been diagnosed with diabetes tend to develop symptoms at lower levels because their bodies aren’t accustomed to elevated glucose.
The early signs include increased thirst, frequent urination, blurred vision, fatigue, and headaches. Over time, persistently high blood sugar damages blood vessels and nerves, which is why the long-term complications of uncontrolled diabetes affect the eyes, kidneys, heart, and feet. This damage accumulates gradually, often years before serious symptoms appear, which is why routine screening matters even if you feel fine.
When Blood Sugar Drops Too Low
Hypoglycemia is classified in three levels of severity. Level 1 (mild) occurs when blood sugar falls below 70 mg/dL but stays at or above 54 mg/dL. At this stage, you might feel shaky, sweaty, dizzy, or notice a pounding heartbeat and headache. These symptoms are your body’s alarm system, and they typically resolve quickly after eating or drinking something with sugar.
Level 2 (moderate) starts below 54 mg/dL. Confusion becomes more pronounced, and you may have trouble swallowing safely, experience vision changes like blurriness or double vision, or slur your words. At Level 3 (severe), a person can lose consciousness, have seizures, and needs help from someone else because they can no longer function independently. Severe hypoglycemia is a medical emergency.
Low blood sugar is most common in people taking insulin or certain diabetes medications, but it can occasionally happen in people without diabetes after prolonged fasting, intense exercise, or excessive alcohol intake.
Factors That Shift Blood Sugar Beyond Food
Diet is the most obvious influence on blood sugar, but several other factors can push your numbers up or down in ways that may surprise you.
Stress triggers the release of cortisol and other hormones that raise blood sugar as part of the body’s fight-or-flight response. This includes emotional stress, physical stress like illness or infection, and even pain from something as simple as a sunburn. Sleep deprivation has a similar effect: even one night of poor sleep reduces your body’s ability to use insulin effectively, leading to higher blood sugar the following day.
Exercise generally lowers blood sugar by increasing the rate at which muscles pull glucose from the bloodstream. However, the timing, intensity, and type of activity all play a role. Dehydration, certain medications (including steroids and some cold medicines), and hormonal fluctuations during menstrual cycles or menopause can also cause unexpected spikes.
How Blood Sugar Is Measured
The most common method is a finger-prick glucometer, which uses a small drop of blood on a test strip to give you a reading in seconds. These devices are widely available and generally accurate within about 5 to 10% of a lab result.
Continuous glucose monitors (CGMs) use a tiny sensor inserted just under the skin, typically on the arm or abdomen, that measures glucose in the fluid between cells every few minutes. Modern CGMs have accuracy within about 8 to 12% for most consumer models and provide trend data showing whether your blood sugar is rising, falling, or holding steady. This pattern information can be more useful than any single number because it reveals how your body responds to specific meals, activities, and sleep patterns.
Lab-drawn blood tests remain the gold standard for diagnosis. Fasting glucose and A1c tests ordered by your doctor use venous blood processed in a laboratory, which is more precise than any at-home device. These are the numbers used to formally diagnose prediabetes or diabetes.