Free blood sugar calculator
Make sense of any blood sugar reading in two seconds. Enter your reading in mg/dL or mmol/L, pick the reading type — fasting, after a meal, or random — and the calculator returns the same value in the other unit, its ADA category (normal, prediabetes, or diabetes), and a full blood sugar level chart, updated live, as you type.
On this page14 sections
| Reading | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| Fasting | < 100 (5.6) | 100–125 (5.6–6.9) | ≥ 126 (7.0) |
| After a meal | < 140 (7.8) | 140–199 (7.8–11.0) | ≥ 200 (11.1) |
| Random | — | no band | ≥ 200 w/ symptoms |
Blood sugar categories are informational, not a diagnosis. When to see a doctor
Results are estimates. Consult a professional.
What is blood sugar, and what is a normal level?
Blood sugar (blood glucose) is the amount of glucose circulating in your blood — your body's main fuel. It is reported in mg/dL in the United States and mmol/L almost everywhere else, and the level that counts as normal depends entirely on when the reading was taken. A fasting reading first thing in the morning, a reading two hours after a meal, and a random reading in the middle of the day are judged against different cut-offs. This blood sugar calculator converts your reading between the two units and tells you which category it falls into for the context you choose.
As a quick orientation, for someone without diabetes a normal fasting blood sugar is below 100 mg/dL (5.6 mmol/L), and a normal level two hours after eating is below 140 mg/dL (7.8 mmol/L). Anything higher does not automatically mean diabetes — it moves you into the prediabetes or diabetes bands, which are normally confirmed with a repeat test.
Blood sugar vs. A1c — two different questions
A blood sugar reading is a snapshot — your glucose at one moment. A1c, by contrast, is your average over the past two to three months. A single high reading can come from a big meal, stress, or illness; A1c smooths all of that into one trend. The two work together: this calculator reads the moment, and A1c reads the long run.
How the blood sugar calculator converts mg/dL and mmol/L
mg/dL and mmol/L measure the same thing in different ways: mg/dL is a weight of glucose per decilitre of blood, while mmol/L is the number of molecules per litre. Converting between them uses glucose's molar mass, which gives a fixed factor of 18.0182.
Enter your reading in whichever unit your meter shows, pick the reading type (fasting, after a meal, or random), and the calculator returns the same value in the other unit plus the diagnostic category. Classification always runs on the mg/dL value internally, so an mmol/L reading lands in exactly the same category as its mg/dL equivalent.
A worked example using the blood sugar calculator
Priya's meter reads 110 mg/dL before breakfast, after not eating overnight. She wants to know what that means and what it is in mmol/L. Here is the arithmetic the calculator runs.
Step 1 — Pick the reading type
Because she took the reading after an overnight fast, she chooses 'Fasting'. This tells the calculator to judge the number against the fasting cut-offs (normal below 100, prediabetes 100–125, diabetes 126 and up), not the higher after-meal ones.
Step 2 — Convert the units
Divide by 18.0182: 110 ÷ 18.0182 = 6.1 mmol/L. That is the same reading expressed in the unit most of the world uses.
Step 3 — Read the category
110 mg/dL sits in the fasting prediabetes band (100–125 mg/dL). It is above the normal ceiling of 100 but below the 126 diabetes threshold — a signal to act, not a diagnosis.
Blood sugar level chart: fasting, after meals, and random
Most people want the whole picture, not just their own number. The table below shows the ADA diagnostic bands for each context, in both mg/dL and mmol/L, on one page — the reference competitors usually split across several. Find your reading type in the left column and read across.
| Reading type | Normal | Prediabetes | Diabetes |
|---|---|---|---|
| Fasting (8h+ no food) | Below 100 mg/dL (5.6 mmol/L) | 100–125 mg/dL (5.6–6.9 mmol/L) | 126 mg/dL+ (7.0 mmol/L+) |
| 2 hours after a meal / OGTT | Below 140 mg/dL (7.8 mmol/L) | 140–199 mg/dL (7.8–11.0 mmol/L) | 200 mg/dL+ (11.1 mmol/L+) |
| Random (any time) | — | no defined band | 200 mg/dL+ with symptoms |
Source: American Diabetes Association diagnostic criteria. A random reading has no prediabetes band — only ≥ 200 mg/dL with classic symptoms is diagnostic of diabetes. Prediabetes and diabetes are normally confirmed with a repeat test.
mg/dL vs. mmol/L — which unit is which
The two units trip people up constantly, mostly because the numbers look so different: a healthy fasting reading is about 90 in mg/dL but only 5.0 in mmol/L. They describe the identical glucose level. The US, and a handful of other countries, use mg/dL; the UK, most of Europe, Canada, Australia, and China use mmol/L.
| mg/dL | mmol/L | What it is |
|---|---|---|
| 70 | 3.9 | Low threshold (hypoglycaemia below this) |
| 90 | 5.0 | Healthy fasting |
| 100 | 5.6 | Top of normal fasting |
| 126 | 7.0 | Fasting diabetes threshold |
| 140 | 7.8 | Top of normal post-meal |
| 180 | 10.0 | Post-meal target ceiling (with diabetes) |
| 200 | 11.1 | Post-meal / random diabetes threshold |
Each row is the same glucose level in both units (mmol/L = mg/dL ÷ 18.0182, rounded).
A fast mental shortcut: to go from mmol/L to mg/dL, multiply by 18 (close enough for a quick check); to go the other way, divide by 18. The calculator uses the precise 18.0182 factor so the conversion is exact in either direction.
Normal vs. prediabetes vs. diabetes blood sugar
The American Diabetes Association sorts blood sugar into three diagnostic bands. The boundaries differ by reading type, but the meaning of each band is the same.
- Normal — your body is keeping glucose in a healthy range on its own. Fasting below 100 mg/dL; below 140 mg/dL two hours after eating.
- Prediabetes — glucose is higher than normal but not yet in the diabetes range (fasting 100–125, or 140–199 after a meal). It is a warning band, and it is often reversible with diet, activity, and weight loss.
- Diabetes — fasting at or above 126 mg/dL, or 200 mg/dL or higher two hours after eating. A single high reading is not a diagnosis; it is normally confirmed with a repeat test on another day.
What about a low blood sugar reading?
Below 70 mg/dL (3.9 mmol/L) is hypoglycaemia — low blood sugar — in any context. It is most common in people who take insulin or certain diabetes medications, and it can cause shakiness, sweating, confusion, and, if severe, loss of consciousness. A low reading needs prompt treatment with fast-acting carbohydrate, not a wait-and-see. The calculator flags any reading under 70 as low regardless of the reading type you pick.
What affects your blood sugar reading
A single number is only as meaningful as the context around it. The same person can read 95 mg/dL fasting and 160 mg/dL an hour after a pasta lunch — both can be normal for the moment. These are the factors that move a reading:
- Food — especially carbohydrates. Carbs raise glucose the most, and the peak usually lands about one hour after eating before settling by two hours.
- Time since your last meal. This is why reading type matters: 130 mg/dL is high fasting but unremarkable an hour after a meal.
- Activity. Exercise pulls glucose into muscle and usually lowers a reading, sometimes for hours afterwards.
- Stress and illness. Stress hormones and infections push glucose up, which is why a cold or a bad day can spike an otherwise normal reading.
- Sleep and the dawn phenomenon. A surge of morning hormones can lift the fasting reading even though you haven't eaten.
- Medications. Steroids raise glucose; insulin and many diabetes drugs lower it.
Because so much moves a single reading, no one number is a diagnosis. The same person can take two readings an hour apart and see them differ by 40 mg/dL or more purely from a meal, a walk, or a stressful phone call. Patterns over several readings — and an A1c — tell the real story.
When to see a doctor about your blood sugar
This calculator is for information, not diagnosis. Talk to a clinician — and in some cases seek urgent care — in these situations:
- A fasting reading of 126 mg/dL or more, or a post-meal reading of 200 mg/dL or more. These meet the diabetes thresholds and should be confirmed with a proper test.
- A reading in the prediabetes band. It is the cue to ask about a confirmatory A1c or fasting glucose test and a prevention plan.
- Repeated lows below 70 mg/dL, or any low with confusion, fainting, or seizures — treat immediately and review your medications with your clinician.
- Classic symptoms of high blood sugar — excessive thirst, frequent urination, unexplained weight loss, blurred vision, or persistent fatigue — even if a single reading looks borderline.
Data sources and methodology
Unit conversion uses glucose's molar-mass factor of 18.0182 (mmol/L = mg/dL ÷ 18.0182), the standard clinical conversion. Diagnostic categories — fasting normal < 100, prediabetes 100–125, diabetes ≥ 126 mg/dL; 2-hour normal < 140, prediabetes 140–199, diabetes ≥ 200 mg/dL; random ≥ 200 with symptoms — follow the American Diabetes Association's Standards of Care diagnostic criteria. The 70 mg/dL low threshold is the ADA's level-1 hypoglycaemia cut-off. Conversions are computed exactly and rounded for display.
American Diabetes Association — Diagnosis and Standards of Care in Diabetes.Frequently asked questions about the free blood sugar calculator
About this blood sugar calculator
This blood sugar calculator runs entirely in your browser. The reading you type never leaves your device — nothing is sent to a server, logged, or shared. It converts between mg/dL and mmol/L with glucose's molar-mass factor (18.0182) and classifies the result against the ADA diagnostic cut-offs for the reading type you choose, updating instantly on every change.
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