A healthy fasting blood sugar level falls below 100 mg/dL (5.6 mmol/L) for most adults. After eating, blood sugar naturally rises and should stay below 140 mg/dL within two hours of a meal. These numbers apply to people without diabetes, and they shift slightly during pregnancy or when measured through different types of tests.
Fasting Blood Sugar
Fasting blood sugar is measured after at least eight hours without food, typically first thing in the morning. For a healthy adult, the target is under 100 mg/dL. A reading between 100 and 125 mg/dL falls into the prediabetes range, meaning your body is starting to struggle with blood sugar regulation. A fasting level of 126 mg/dL or higher, confirmed on two separate tests, meets the diagnostic threshold for diabetes.
These fasting numbers are the most commonly used screening tool because they reflect your baseline, the level your body maintains when it isn’t actively processing food. If you’ve been told your fasting glucose is “borderline,” that typically means you’re in the prediabetes window, and changes to diet, exercise, and weight can often bring it back down.
Blood Sugar After Meals
Your blood sugar starts rising within 15 to 30 minutes of eating and usually peaks around one hour after a meal. In a healthy person, it drops back to near-fasting levels within two to three hours. The CDC considers a reading below 180 mg/dL at the two-hour mark normal for people with diabetes who are managing their condition, but for someone without diabetes, a healthy post-meal reading is typically below 140 mg/dL.
The type of food matters. A meal heavy in refined carbohydrates (white bread, sugary drinks, white rice) will spike blood sugar faster and higher than one built around protein, fiber, and healthy fats. This is why two people can eat the same number of calories and see very different glucose responses.
How Your Body Controls Blood Sugar
Your pancreas acts as the control center. It contains two types of cells that produce opposing hormones: beta cells release insulin, and alpha cells release glucagon. These two hormones work like a thermostat, constantly adjusting to keep blood sugar in a tight range.
When you eat and blood sugar rises, beta cells release insulin. Insulin acts like a key, unlocking cells throughout your body so they can absorb glucose from the bloodstream and use it for energy. When blood sugar drops too low, between meals or during exercise, alpha cells release glucagon. Glucagon signals your liver to convert its stored glucose back into a usable form and release it into the blood. It also tells the liver to stop absorbing glucose so more stays in circulation. Your body can even manufacture new glucose from amino acids when stores run low.
In diabetes, this system breaks down. In type 1, the immune system destroys the beta cells, so insulin production stops. In type 2, cells gradually become resistant to insulin’s signal, forcing the pancreas to produce more and more until it can’t keep up.
A1C: The Longer View
A single blood sugar reading is a snapshot. The A1C test provides a wider picture by measuring what percentage of your red blood cells have glucose attached to them, reflecting your average blood sugar over roughly two to three months.
A healthy A1C is below 5.7%. Between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher on two separate tests indicates diabetes. To put those percentages in practical terms, the American Diabetes Association uses a conversion formula: an A1C of 6% translates to an estimated average glucose of about 126 mg/dL, while an A1C of 7% corresponds to roughly 154 mg/dL. Each half-percentage-point increase in A1C adds about 14 mg/dL to your estimated daily average.
A1C is useful because it isn’t thrown off by a single good or bad day. Someone who fasts before a blood draw might get a reassuring fasting number while their post-meal spikes over the past several weeks tell a different story. A1C captures that pattern.
Blood Sugar During Pregnancy
Pregnancy changes the thresholds. During gestational diabetes screening (usually between weeks 24 and 28), the targets are stricter because elevated blood sugar poses risks to both the mother and baby.
On the three-hour glucose tolerance test used for screening, healthy results are: fasting below 95 mg/dL, one-hour reading below 180 mg/dL, two-hour reading below 155 mg/dL, and three-hour reading below 140 mg/dL. Some providers use a shorter two-hour test instead, where fasting should be below 92 mg/dL and the two-hour reading below 153 mg/dL. Meeting or exceeding any one of these cutoffs can lead to a gestational diabetes diagnosis.
These lower thresholds aren’t arbitrary. Even mildly elevated blood sugar during pregnancy increases the risk of complications like high birth weight, early delivery, and preeclampsia. Most cases of gestational diabetes resolve after delivery, but they do signal a higher risk of developing type 2 diabetes later in life.
When Blood Sugar Drops Too Low
Low blood sugar, or hypoglycemia, is defined as a reading below 70 mg/dL. At this level, you may feel shaky, sweaty, dizzy, or suddenly irritable. These symptoms are your body’s alarm system signaling that your brain and muscles aren’t getting enough fuel.
A reading below 54 mg/dL is considered severe hypoglycemia. At that level, confusion and loss of consciousness become real risks, and you may need someone else to help you. Hypoglycemia is most common in people taking insulin or certain oral diabetes medications, but it can occasionally happen in people without diabetes after prolonged fasting, intense exercise, or heavy alcohol consumption.
If you feel symptoms of low blood sugar, the standard response is to consume 15 grams of fast-acting carbohydrates (four glucose tablets, half a cup of juice, or a tablespoon of honey), wait 15 minutes, and recheck. Repeat if you’re still below 70 mg/dL.
Home Monitors vs. Lab Tests
If you’re checking blood sugar at home with a finger-stick monitor, expect some variance from what a lab would report. Home glucometers are considered accurate if they fall within 15% of a lab result, according to Mayo Clinic. That means a lab reading of 100 mg/dL could show up as anywhere between 85 and 115 on your home device.
Several things affect accuracy: test strips exposed to heat or humidity, insufficient blood on the strip, dirty hands (residual sugar from food is a common culprit), and extreme temperatures. If a reading seems off, wash your hands, use a fresh strip, and test again. Consistent readings that seem unusually high or low are worth confirming with a lab draw.
Quick Reference: Healthy Ranges
- Fasting (no food for 8+ hours): below 100 mg/dL
- Two hours after eating: below 140 mg/dL
- A1C (3-month average): below 5.7%
- Low blood sugar warning: below 70 mg/dL
- Severe low blood sugar: below 54 mg/dL
- Pregnancy fasting (3-hour test): below 95 mg/dL