Fasting Glucose and A1C: What Your Numbers Mean

By Ferdie Correa · Published October 5, 2026 · Last updated October 5, 2026 · 5 min read

Not medical advice. This article is general education, not a diagnosis or treatment plan. Talk to your doctor or a registered dietitian before making major changes to your diet, training or medication, especially if you have a health condition, are pregnant, or take prescription drugs.

A routine blood test often comes back with two blood-sugar numbers: fasting glucose and A1C. A result flagged as slightly high can be alarming, and a normal one can be misleading if you do not know what it covers. This article explains each test, the ranges the American Diabetes Association uses, and the questions worth bringing to your next appointment. It does not replace that appointment.

Three tests, three different questions

Fasting plasma glucose (FPG) measures the glucose in your blood after at least 8 hours without food or drinks other than water. It is a single snapshot of one morning.
A1C (hemoglobin A1C, HbA1c) measures the percentage of hemoglobin in red blood cells that has glucose attached. Because red blood cells live for a few months, it reflects your average blood sugar over roughly the past 2 to 3 months. You do not need to fast.
Oral glucose tolerance test (OGTT) measures glucose 2 hours after you drink a standard 75 g glucose solution. It shows how well your body clears a sugar load, and it can pick up problems the other two miss.

The ADA ranges

These are the diagnostic categories in the American Diabetes Association's criteria, also used by the NIDDK and the CDC.

Test	Normal	Prediabetes	Diabetes
Fasting glucose	Below 100 mg/dL (below 5.6 mmol/L)	100 to 125 mg/dL (5.6 to 6.9 mmol/L)	126 mg/dL or higher (7.0 mmol/L or higher)
A1C	Below 5.7%	5.7% to 6.4%	6.5% or higher
2-hour OGTT	Below 140 mg/dL (below 7.8 mmol/L)	140 to 199 mg/dL (7.8 to 11.0 mmol/L)	200 mg/dL or higher (11.1 mmol/L or higher)

A random glucose of 200 mg/dL or more in someone with classic symptoms (excessive thirst, frequent urination, unexplained weight loss) is also diagnostic.

Converting between mg/dL and mmol/L

The US reports glucose in mg/dL; most other countries use mmol/L. To convert, divide mg/dL by 18 (or multiply mmol/L by 18).

mg/dL	mmol/L	Context
70	3.9	Common lower limit for a normal fasting value
99	5.5	Top of the normal fasting range
100	5.6	Fasting prediabetes threshold
126	7.0	Fasting diabetes threshold
140	7.8	2-hour OGTT prediabetes threshold
200	11.1	2-hour OGTT and random-glucose diabetes threshold

A1C can also be expressed as an estimated average glucose (eAG) using the formula from the ADAG study: eAG (mg/dL) = 28.7 × A1C − 46.7. An A1C of 5.7% corresponds to about 117 mg/dL, 6.5% to about 140 mg/dL, and 7.0% to about 154 mg/dL.

Why one reading is not a diagnosis

Unless blood sugar is clearly very high with symptoms, the ADA requires two abnormal results to diagnose diabetes, either two different tests from the same blood sample or the same test repeated on another day. Single readings vary for ordinary reasons:

Fasting glucose can run higher after a poor night's sleep, during an illness or infection, under acute stress, after a late heavy meal if the fast was short, or with medicines such as corticosteroids.
A1C can read falsely high or low with conditions that change red blood cells: iron-deficiency and other anemias, hemoglobin variants such as sickle cell trait, recent blood loss or transfusion, advanced kidney disease, and pregnancy.

It is also common for the tests to disagree, for example a normal A1C with a fasting glucose of 108. That is a reason for follow-up testing, not a contradiction.

Lifestyle levers with evidence

The best evidence comes from the Diabetes Prevention Program, a US trial published in 2002 that followed more than 3,000 adults with elevated glucose. The lifestyle group aimed to lose 7% of body weight and do 150 minutes of physical activity a week. Over an average of about three years, their risk of developing type 2 diabetes was 58% lower than the placebo group (71% lower in participants aged 60 and over). The group taking metformin saw a 31% reduction.

What this means in practice:

Modest weight loss counts. For a 200 lb person, 7% is 14 lb. See how to size a calorie deficit.
Activity counts on its own. Muscle uses glucose during and after exercise. Brisk walking adds up to 150 minutes quickly, and strength training helps too (beginner plan).
Fiber. The National Academies' adequate intake is 38 g a day for men and 25 g for women under 51, which is also what the FitStatsHub report shows. Beans, lentils, oats, vegetables and whole grains slow how quickly carbohydrate reaches the blood.

The CDC estimates about 1 in 3 US adults has prediabetes, and most do not know it. Its National Diabetes Prevention Program offers year-long lifestyle programs based on the DPP, many covered by insurance.

Questions to ask your doctor

Which test was used, and should it be repeated or confirmed with a different test?
Could anything have affected this result (anemia, a recent illness, a medicine I take)?
Given my other risk factors, how often should I be retested?
Would I benefit from a CDC-recognized lifestyle change program?
Is there a weight-loss or activity target you would suggest for me specifically?
If I already have diabetes: what A1C target is right for me? (Targets for people being treated are set individually and differ from the screening ranges above.)

How FitStatsHub's reference targets are sourced

The Health Report on the planner lists glucose goals for discussion with your clinician. It does not take your lab results as input and cannot assess them.

A1C under 5.7% matches the ADA's upper limit of the normal range.
Post-meal glucose under 140 mg/dL mirrors the ADA's normal cut-off for the 2-hour glucose tolerance test.
Fasting glucose of 80 to 95 mg/dL is a narrower planning range than the ADA diagnostic category. The ADA's normal range is anything below 100 mg/dL, so a fasting result of 96 to 99 is still classed as normal.

If you take insulin or other glucose-lowering medicine, your targets are personal and come from your care team, not from this or any calculator. Calorie changes can also affect your doses, so speak to your doctor before starting a diet plan.