Blood Glucose Unit Converter

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 is not a small inconvenience. Diabetics travelling internationally, reading research papers, or borrowing a foreign glucometer have to convert constantly, and the two scales are far enough apart that a misread is obvious in one direction and dangerously subtle in the other.

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 or multiplying by 18 is accurate to within 1%, which is fine for everyday use.

Reference ranges (fasting glucose, ADA 2024)

These are after 8+ hours without eating, typically measured first thing in the morning:

Post-meal (2-hour postprandial) ranges

Measured 2 hours after the start of a meal:

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:

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, the direct sugar source and the biggest single factor
• Stress (cortisol mobilizes liver glycogen)
• Illness or infection (inflammation raises insulin resistance)
• Dawn phenomenon, the natural cortisol rise before waking, which can spike a fasting reading
• 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, because working muscle takes up glucose without needing insulin
• Sulfonylureas, insulin, GLP-1 agonists (medications)
• Skipped or delayed meals (can cause dangerous hypos in insulin users)
• Alcohol, which raises glucose first and then drops it hard. Drinking at night is a classic cause of an overnight hypo
• Fasting (after 12-24 hours; glycogen depletes, ketones rise)

Sensor versus blood, and the lag between them

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 already be 70 and falling. Treat it. If you feel fine and the CGM reads 250, recent food may still be absorbing and the value may be on its way down. For deciding what to do, the trend arrow matters more than the number.

HbA1c limitations

HbA1c is not perfect for everyone:

• Hemoglobinopathies such as sickle cell and thalassemia, which affect the measurement itself
• Recent blood loss or transfusion, since younger red cells have had less time to glycate and skew the result low
• Severe anemia, where the measurement is unreliable
• Pregnancy, which uses different criteria entirely
• Children with diabetes, in whom HbA1c correlates 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:

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. The conversion factor is 18.0182, and 18 is close enough in your head. Normal fasting is below 100 mg/dL or 5.6 mmol/L. Pre-diabetes runs 100 to 125, or 5.6 to 6.9. The diabetes diagnostic threshold is 126 mg/dL or 7.0 mmol/L, on two separate readings. HbA1c beats any single fingerstick for the long view. Personal targets vary with age, other conditions and treatment, so set yours with your doctor.