A normal fasting blood sugar level falls between 70 and 99 mg/dL (3.9 to 5.5 mmol/L) for most adults. This is measured after at least 8 hours without eating or drinking anything other than water, typically first thing in the morning before breakfast. If your result lands between 100 and 125 mg/dL, that’s considered prediabetes. A reading of 126 mg/dL or higher on two separate tests indicates diabetes.
The Three Ranges That Matter
Fasting blood sugar is one of the simplest and most common tests for screening diabetes risk. The ranges break down into three categories:
- Normal: 70 to 99 mg/dL (3.9 to 5.5 mmol/L)
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL or higher (7.0 mmol/L or higher)
A single high reading doesn’t automatically mean you have diabetes. The test needs to be repeated on a separate day to confirm the result. Your doctor may also order additional tests, like an A1C (which reflects your average blood sugar over the past two to three months) or an oral glucose tolerance test, to get a fuller picture.
If you’re outside the United States, your results are likely reported in mmol/L rather than mg/dL. To convert, divide the mg/dL number by 18. So 99 mg/dL becomes about 5.5 mmol/L, and 126 mg/dL becomes 7.0 mmol/L.
How the Test Works
Fasting for at least 8 hours before the blood draw is essential. Food and most drinks trigger your body to release insulin and shift glucose levels, which makes the results unreliable. Water is fine and won’t affect the test. Most people schedule the test for early morning so the fasting window overlaps with sleep, making it easier to go without food.
If you’re taking medications, ask your provider whether to take them before the test. Some prescriptions can influence blood sugar readings, and skipping or taking them at the wrong time could skew results in either direction.
Why Morning Blood Sugar Can Run High
Some people notice their fasting blood sugar is higher than expected even when they haven’t eaten anything unusual the night before. This is often caused by something called the dawn phenomenon. Between roughly 3 a.m. and 8 a.m., your body releases a surge of hormones, including cortisol and growth hormone, that signal your liver to push more glucose into your bloodstream. This natural process provides energy to help you wake up, but it can bump fasting readings above where you’d expect them.
The dawn phenomenon affects people with and without diabetes, but it’s more pronounced if your body doesn’t produce enough insulin or doesn’t use it efficiently. If your fasting numbers consistently seem higher than your readings at other times of day, this hormonal surge is a likely explanation. Poor sleep, late-night eating, and stress can amplify the effect.
Ranges for Children and Infants
Normal fasting blood sugar for adults doesn’t apply across all age groups. Infants and very young children have lower reference ranges. Newborns typically fall between 30 and 60 mg/dL, which would be considered dangerously low for an adult but is normal for the first days of life. Infants generally range from 40 to 90 mg/dL, and children under two from 60 to 100 mg/dL. By later childhood and adolescence, the ranges align more closely with adult values.
Different Thresholds During Pregnancy
Pregnancy changes how your body processes glucose, and the cutoffs for gestational diabetes are stricter than the standard adult thresholds. The most widely used one-step screening approach sets the fasting threshold at just 92 mg/dL, a level that would be considered normal outside of pregnancy. In the two-step approach, the fasting cutoff is 95 mg/dL.
These tighter limits exist because even mildly elevated blood sugar during pregnancy increases risks for both the mother and baby, including higher birth weight, delivery complications, and the baby’s own blood sugar dropping too low after birth. Screening for gestational diabetes typically happens between weeks 24 and 28 of pregnancy, though women with higher risk factors may be tested earlier.
What Prediabetes Actually Means
Landing in the 100 to 125 mg/dL range doesn’t mean diabetes is inevitable. Prediabetes is a signal that your body is starting to struggle with blood sugar regulation, but it’s also the stage where lifestyle changes have the most impact. Losing 5 to 7 percent of your body weight (roughly 10 to 14 pounds for someone who weighs 200) and getting about 150 minutes of moderate physical activity per week has been shown to cut the risk of progressing to type 2 diabetes by more than half.
The tricky part is that prediabetes rarely causes noticeable symptoms. Most people find out through routine bloodwork. If your fasting glucose sits in the upper 90s, it’s worth keeping an eye on it even though it technically falls within the normal range. Blood sugar trends over time matter more than any single reading.
Factors That Affect Your Results
Several things beyond food can shift your fasting blood sugar. Stress raises cortisol, which in turn raises glucose. A poor night of sleep can reduce your body’s sensitivity to insulin, pushing morning numbers higher. Illness and infections also temporarily increase blood sugar as part of the body’s immune response. Even the timing of your last meal matters: a heavy, carb-rich dinner close to bedtime can still influence your reading the next morning, especially if you’re insulin resistant.
Dehydration is another subtle factor. When you’re low on fluids, the glucose in your blood becomes more concentrated, which can make your reading appear higher than it would be if you were well hydrated. Drinking water in the hours before your test is both allowed and encouraged.