A fasting blood sugar level of 100 mg/dL or higher is considered above normal, and 126 mg/dL or higher on two separate tests meets the diagnostic threshold for diabetes. But “too high” depends on context: when you last ate, whether you’re pregnant, and whether you already have a diabetes diagnosis all shift the numbers that matter.
Fasting Blood Sugar Ranges
Fasting blood sugar is measured after at least eight hours without food, typically first thing in the morning. A result below 100 mg/dL is normal. Between 100 and 125 mg/dL falls into the prediabetes range, meaning your body is starting to struggle with blood sugar regulation but hasn’t crossed into diabetes territory yet. At 126 mg/dL or above on two separate fasting tests, the diagnosis is diabetes.
A random blood sugar test, taken at any point regardless of when you last ate, uses a different cutoff. A reading of 200 mg/dL or higher suggests diabetes, especially when accompanied by symptoms like excessive thirst or frequent urination.
After-Meal Targets
Blood sugar naturally rises after eating and typically peaks about one to two hours after your first bite. For people with diabetes, the general target is to stay below 180 mg/dL at the two-hour mark. In people without diabetes, blood sugar rarely climbs above 140 mg/dL after a meal and usually returns to baseline within a couple of hours.
Consistently exceeding 180 mg/dL after meals, even if your fasting numbers look reasonable, can signal that your body isn’t producing enough insulin or isn’t using it efficiently. Post-meal spikes are one of the earliest signs that blood sugar regulation is breaking down, and they often show up before fasting levels become abnormal.
Pregnancy Changes the Numbers
Blood sugar targets are significantly tighter during pregnancy. The American College of Obstetricians and Gynecologists recommends a fasting blood sugar below 95 mg/dL for women with gestational diabetes. One hour after the start of a meal, the target is below 140 mg/dL, and at two hours, below 120 mg/dL.
These stricter limits exist because even moderately elevated blood sugar during pregnancy increases the risk of complications for both mother and baby, including excessive birth weight and delivery complications. What would be considered a normal post-meal reading outside of pregnancy can be too high during it.
Where Symptoms Start
Many people with diabetes don’t notice physical symptoms until their blood sugar reaches 250 mg/dL or higher. People who haven’t been diagnosed with diabetes tend to feel symptoms at lower levels, since their bodies aren’t accustomed to elevated glucose. The classic signs include increased thirst, frequent urination, blurred vision, fatigue, and headaches.
This gap between “diagnostically too high” and “noticeably too high” is one of the reasons diabetes often goes undetected. You can walk around with a fasting blood sugar of 140 mg/dL and feel perfectly fine, even though sustained levels in that range are quietly damaging blood vessels, nerves, and organs over months and years.
A1c: The Bigger Picture
A single blood sugar reading is a snapshot. The A1c test captures a broader view by measuring the percentage of your red blood cells that have sugar molecules attached to them, reflecting your average blood sugar over roughly three months. A normal A1c is below 5.7%. Between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
For people already managing diabetes, the American Diabetes Association suggests aiming for an A1c of 7%, which corresponds to an average blood sugar of about 154 mg/dL. Your doctor may set a more or less aggressive target depending on your age, how long you’ve had diabetes, and other health factors. The A1c is often more useful than individual readings because it captures the day-to-day reality of blood sugar control, including the overnight dips and post-meal spikes that a single fasting test would miss.
Dangerously High Levels
Blood sugar above 250 mg/dL warrants close attention, especially if it doesn’t come down with your usual management steps. At this level, people with type 1 diabetes (and sometimes type 2) are at risk for diabetic ketoacidosis, a condition where the body starts breaking down fat for fuel too rapidly, producing acids called ketones that can make the blood dangerously acidic. A blood sugar of 200 mg/dL or higher combined with high ketone levels and other metabolic changes is the diagnostic criteria for this condition. Symptoms include nausea, vomiting, abdominal pain, fruity-smelling breath, and confusion.
At the extreme end, blood sugar above 600 mg/dL points to hyperosmolar hyperglycemic state, a life-threatening emergency that occurs mostly in people with type 2 diabetes. At these levels, blood becomes thick and syrupy as the body loses massive amounts of fluid through urination. Confusion, seizures, and loss of consciousness can follow. This condition develops over days or weeks, often triggered by an illness or infection that drives blood sugar progressively higher.
What Sustained High Blood Sugar Does
The danger of chronically elevated blood sugar isn’t just the number on the meter. It’s what happens to your body when that number stays elevated over time. Excess glucose in the bloodstream damages the walls of small blood vessels, particularly in the eyes, kidneys, and extremities. This is why poorly controlled diabetes leads to vision loss (retinopathy), kidney disease (nephropathy), and nerve damage (neuropathy) that often starts as tingling or numbness in the feet.
Larger blood vessels are affected too, which is why people with diabetes face a significantly higher risk of heart attack and stroke. The damage is cumulative and largely silent in its early stages. By the time symptoms like blurred vision or foot numbness appear, the underlying vascular changes have often been progressing for years. This is precisely why the diagnostic thresholds matter: a fasting blood sugar of 110 mg/dL won’t make you feel sick today, but staying at that level year after year increases your risk of complications well before you reach a formal diabetes diagnosis.