Blood sugar levels are measured in milligrams per deciliter (mg/dL), and the numbers fall into clear categories: normal, prediabetes, or diabetes. The specific ranges depend on which test you’re looking at, whether it’s a fasting reading, a post-meal check, a fingerstick from a home meter, or a continuous glucose monitor. Once you know the key thresholds, reading your results becomes straightforward.
Fasting Blood Sugar Ranges
A fasting blood sugar test is taken after at least eight hours without eating, typically first thing in the morning. The three categories break down like this:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher on two separate tests
That 100 mg/dL line is the first number worth memorizing. If your fasting readings consistently land between 100 and 125, your body is starting to struggle with blood sugar regulation, but the situation is still reversible with lifestyle changes. A single reading of 126 or above doesn’t confirm diabetes on its own. It takes two separate fasting tests at that level to establish a diagnosis.
Post-Meal Blood Sugar Ranges
Blood sugar naturally rises after eating, so a different scale applies. In a clinical setting, this is measured with a glucose tolerance test: you drink a sugary solution, then your blood is drawn two hours later. The cutoffs:
- Normal: below 140 mg/dL
- Prediabetes: 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
If you’re checking at home with a meter, the same general logic applies. A reading under 140 about two hours after your first bite is a healthy response. Numbers that regularly climb above 180 suggest your body isn’t clearing sugar from the bloodstream efficiently.
What A1c Tells You
While a fingerstick captures a single moment, the A1c test reflects your average blood sugar over the previous two to three months. It’s reported as a percentage. An A1c below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher points to diabetes.
Those percentages translate to concrete daily averages. Here’s how common A1c values map to estimated average glucose:
- 6% = roughly 126 mg/dL average
- 7% = roughly 154 mg/dL average
- 8% = roughly 183 mg/dL average
- 9% = roughly 212 mg/dL average
- 10% = roughly 240 mg/dL average
This conversion helps bridge the gap between the percentage your lab reports and the numbers you see on your home meter every day. If your A1c is 7%, your blood sugar has been averaging around 154 mg/dL, which means you’re spending a significant portion of each day above the ideal range.
Low Blood Sugar Thresholds
Low blood sugar (hypoglycemia) has its own tiered system, and the numbers matter because they signal how urgently you need to act.
- Level 1: 54 to 69 mg/dL. You may feel shaky, sweaty, or hungry. This is the “treat it now” zone, typically with 15 grams of fast-acting carbohydrates like glucose tablets or juice.
- Level 2: below 54 mg/dL. This is clinically significant hypoglycemia. Symptoms intensify, and confusion or difficulty concentrating can set in.
- Level 3: any reading where you’re too impaired to treat yourself. This is defined not by a specific number but by needing someone else’s help, and it’s a medical emergency.
For people on insulin or certain diabetes medications, recognizing these levels quickly prevents dangerous drops from getting worse.
High Blood Sugar Warning Signs
On the opposite end, persistently high readings carry their own escalation. If your blood sugar hits 250 mg/dL or above and you have diabetes, checking for ketones (using a urine strip or blood ketone meter) every four to six hours is important. Ketones are acids your body produces when it starts burning fat instead of sugar for energy, and high levels signal a dangerous condition called diabetic ketoacidosis.
The CDC recommends going to the emergency room or calling 911 if your blood sugar stays at 300 mg/dL or above, your breath smells fruity, you’re vomiting and can’t keep fluids down, or you’re having trouble breathing. These are signs that the body’s chemistry is shifting into a crisis that needs immediate intervention.
Reading a Continuous Glucose Monitor
If you wear a continuous glucose monitor (CGM), you’re not just seeing a single number. You’re seeing a trend line and an arrow that tells you which direction your blood sugar is heading and how fast.
A flat or horizontal arrow means your glucose is stable, changing by no more than 1 mg/dL per minute. A single arrow pointing up or down means a moderate shift of 1 to 2 mg/dL per minute. Two arrows (or an angled arrow, depending on the device) pointing up or down means a rapid change of 2 to 3 mg/dL per minute. A vertical arrow with a fast indicator means your glucose is moving more than 3 mg/dL per minute.
Those rates may sound small, but they compound quickly. A reading of 150 mg/dL with a double-up arrow could mean you’ll be above 250 within 30 to 45 minutes if nothing changes. The trend arrow is often more actionable than the number itself, because it tells you where you’re headed rather than just where you are.
Time in Range
CGMs also track a metric called “time in range,” which measures what percentage of the day your blood sugar stays between 70 and 180 mg/dL. An international consensus panel set the target at more than 70% of the day in that range, which works out to roughly 17 hours. For older adults or people at higher risk of low blood sugar, a more relaxed goal of 50% (about 12 hours) is appropriate.
Equally important is time below range. The target is to spend less than 4% of the day (under one hour) below 70 mg/dL, and less than 1% (under 15 minutes) below 54 mg/dL. On the high end, the goal is less than 25% of the day above 180 mg/dL, and less than 5% above 250 mg/dL. These percentages give you a much more complete picture of your blood sugar control than any single fasting number or even an A1c can provide.
Getting Accurate Readings at Home
Your numbers are only useful if they’re accurate, and several common factors can throw off a fingerstick reading. Extreme temperatures affect test strips, so store your meter and strips at room temperature rather than leaving them in a hot car or cold garage. Dehydration concentrates your blood and can skew results higher. Anemia (low red blood cell count) can also reduce accuracy.
Technique matters too. Wash your hands with warm water and soap before testing, since residue from food or lotion on your fingertips can contaminate the sample. When you lance your finger, apply gentle pressure to get blood flowing but avoid squeezing hard, which dilutes the blood drop with fluid from surrounding tissue and can produce a falsely low reading. Some guidelines recommend wiping away the first drop of blood and testing with the second, which tends to give a cleaner sample.
Home meters are allowed to be within 15% of a lab result, which means a true glucose of 100 mg/dL could show up as anything from 85 to 115 on your device. That’s accurate enough for daily management, but it’s worth keeping in mind if a single reading seems surprisingly high or low. Testing a second time with a fresh strip can help confirm an unexpected number.