Blood Glucose Unit Converter
Convert blood glucose between mg/dL (USA) and mmol/L (international) instantly.
Includes fasting, pre-diabetes, and diabetes reference ranges in both units.
Why the world can’t agree on glucose units
The United States uses milligrams per deciliter (mg/dL) for blood glucose. Most of the rest of the world uses millimoles per liter (mmol/L). This isn’t a small inconvenience — diabetics traveling internationally, reading research papers, or using foreign glucometers constantly need to convert.
The historical reason: the US never fully adopted SI units in medicine, while Europe and most former British Commonwealth countries did. The UK switched in 1985, Australia in 1974, Canada partially around 1980. Today, the holdouts are the US, parts of the Middle East, and a few Latin American countries.
The conversion formula
The factor 18.0182 comes from the molecular weight of glucose:
C₆H₁₂O₆ = 12.01 × 6 + 1.008 × 12 + 16.00 × 6 = 180.16 g/mol
mmol/L = mg/dL ÷ 18.0182 mg/dL = mmol/L × 18.0182
For quick mental math, dividing by 18 (or multiplying by 18) is accurate to within 1% — fine for everyday use.
Reference ranges (fasting glucose, ADA 2024)
These are after 8+ hours without eating, typically measured first thing in the morning:
| Category | mg/dL | mmol/L | What it means |
|---|---|---|---|
| Severe hypoglycemia | <54 | <3.0 | Emergency — needs immediate sugar |
| Hypoglycemia | 54-69 | 3.0-3.9 | Low — eat something now |
| Normal fasting | 70-99 | 3.9-5.5 | Healthy range |
| Pre-diabetes | 100-125 | 5.6-6.9 | Elevated; intervene now |
| Diabetes (diagnostic) | ≥126 | ≥7.0 | Two readings required for diagnosis |
| Severe hyperglycemia | >240 | >13.3 | Significantly elevated; immediate attention |
| Diabetic ketoacidosis risk | >300 | >16.7 | Dangerous; check ketones |
| Hyperglycemic crisis | >600 | >33.3 | Medical emergency |
Post-meal (2-hour postprandial) ranges
Measured 2 hours after the start of a meal:
| Category | mg/dL | mmol/L |
|---|---|---|
| Normal | <140 | <7.8 |
| Pre-diabetes | 140-199 | 7.8-11.0 |
| Diabetes | ≥200 | ≥11.1 |
HbA1c — the longer-term measure
HbA1c (glycated hemoglobin) measures average blood glucose over the past 8-12 weeks, since red blood cells live ~120 days and glucose binds permanently to hemoglobin during their lifespan. It’s the gold standard for monitoring diabetes management:
| HbA1c (%) | Estimated Average Glucose (mg/dL) | Status |
|---|---|---|
| <5.7 | <117 | Normal |
| 5.7-6.4 | 117-137 | Pre-diabetes |
| 6.5+ | 140+ | Diabetes diagnostic |
| 7.0 (target for most diabetics) | 154 | Acceptable |
| 8.0 | 183 | Above target; adjust treatment |
| 9.0 | 212 | Significantly above target |
| 10.0 | 240 | Very poorly controlled |
The conversion from HbA1c to eAG (estimated average glucose) uses the formula: eAG (mg/dL) = (28.7 × HbA1c) − 46.7. The relationship was established by the ADAG study (Nathan et al., 2008).
What raises blood glucose
- Carbohydrate intake — direct sugar source, biggest single factor
- Stress (cortisol mobilizes liver glycogen)
- Illness or infection (inflammation raises insulin resistance)
- Dawn phenomenon — natural cortisol rise pre-waking, can spike fasting glucose
- Steroid medications (prednisone, dexamethasone)
- Thiazide diuretics, beta-blockers (mild effect)
- Sleep deprivation (insulin resistance increases)
- Dehydration (more concentrated blood)
- Caffeine (modest effect, can spike post-meal)
What lowers blood glucose
- Exercise — muscles take up glucose without insulin
- Sulfonylureas, insulin, GLP-1 agonists (medications)
- Skipped or delayed meals (can cause dangerous hypos in insulin users)
- Alcohol — initially raises, then drops dramatically; nighttime drinking can cause overnight hypos
- Fasting (after 12-24 hours; glycogen depletes, ketones rise)
Sensor vs blood — there’s a lag
Continuous Glucose Monitors (CGMs) like Dexcom and FreeStyle Libre measure interstitial fluid glucose, not blood. There’s a 5-15 minute lag between blood and interstitial values, more during rapid changes.
If your CGM reads 80 mg/dL and you feel hypoglycemic, your actual blood may be 70 mg/dL and falling — treat it. If you feel fine and CGM reads 250, recent food may be still being absorbed and the value may be falling. Trend arrows on CGMs matter more than the absolute number for action decisions.
HbA1c limitations
HbA1c is not perfect for everyone:
- Hemoglobinopathies (sickle cell, thalassemia) — affects measurement
- Recent blood loss or transfusion — younger red blood cells skew low
- Severe anemia — measurement unreliable
- Pregnancy — uses different criteria
- Children with diabetes — HbA1c may correlate less well with average glucose
In these cases, fructosamine (2-3 week average) or continuous glucose monitoring is preferred.
The “blood sugar feels low even when high” problem
People with poorly-controlled diabetes can experience hypoglycemic symptoms (shakiness, sweating, hunger) at normal or even elevated blood glucose levels. This is because the brain becomes accustomed to chronic high levels and reacts to rapid drops, even drops within the normal range. The treatment: gradual normalization over weeks under medical supervision.
Practical conversion shortcuts
Memorize these common conversions:
| mg/dL | mmol/L |
|---|---|
| 54 | 3.0 |
| 70 | 3.9 |
| 90 | 5.0 |
| 100 | 5.5 |
| 110 | 6.1 |
| 126 | 7.0 (diabetes threshold) |
| 140 | 7.8 (post-meal pre-diabetes threshold) |
| 180 | 10.0 |
| 200 | 11.1 (post-meal diabetes threshold) |
| 250 | 13.9 |
| 300 | 16.7 |
A useful mental shortcut: at 5.0 mmol/L you’re at 90; each whole mmol/L up adds about 18 mg/dL.
Bottom line
Two units, one molecule. Conversion factor is 18.0182 (essentially 18 for mental math). Pre-diabetes fasting: 100-125 mg/dL or 5.6-6.9 mmol/L. Diabetes diagnostic threshold: 126 mg/dL or 7.0 mmol/L. HbA1c is the better long-term measure than any single fingerstick reading. Personal targets vary by age, comorbidities, and treatment — work with your doctor.