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Your doctor says 7.0 mmol/L but your American friend says 126 mg/dL — are they the same? Most people see different numbers and worry they mean different things. The good news: it's usually the same measurement expressed in two units. Why two systems? And how do you convert, remember ranges, or explain values to someone using the other unit? This guide walks through the formula, practical tricks, normal and diabetic ranges in both units, and how HbA1c fits into the picture.
1What mmol/L and mg/dL actually mean
One number counts molecules per volume, the other counts mass per volume. mmol/L is a molar concentration: millimoles of glucose per liter of blood. mg/dL is a mass concentration: milligrams of glucose per deciliter of blood. Both tell you how much glucose is circulating, just in different ‘languages’.
Moles versus mass — the short version
A mole links to particle count (Avogadro's number). mmol/L measures how many millimoles — useful for chemistry and labs that follow SI units. mg/dL measures milligrams per deciliter, a legacy clinical unit used widely in the United States.
The conversion formula (and a quick example)
Convert mmol/L to mg/dL by multiplying by 18. Formula: mmol/L × 18 = mg/dL. Example: 7.0 mmol/L × 18 = 126 mg/dL. To go the other way, divide by 18: 126 mg/dL ÷ 18 = 7.0 mmol/L.
2Why different countries use different units
The split is historical and practical. Many countries adopted SI (molar) units for lab reporting. The US clinical system kept mass-based units for routine chemistry tests. Both systems are accepted by labs, but mixing them without conversion causes confusion.
History and lab practice
Clinical labs evolved locally. US labs used mg/dL for decades because clinical chemistry kits and reference ranges were established that way. Many other countries shifted to mmol/L to align with the International System of Units (SI).
Standards and international agreements
Organizations such as BIPM and national metrology institutes promote SI units; NIST documents SI practice for the US. The practical effect: international studies often report mmol/L, while US-centric guidelines often show mg/dL.
3Interpreting glucose: normal, pre-diabetic, diabetic
Here are commonly used thresholds. Labs and guidelines vary slightly, so always follow your clinician's local ranges. The numbers below are typical values used by major diabetes organizations.
Fasting blood glucose (before eating)
Normal: < 5.6 mmol/L (< 100 mg/dL). Prediabetes: 5.6–6.9 mmol/L (100–125 mg/dL). Diabetes: ≥ 7.0 mmol/L (≥ 126 mg/dL) on two separate tests.
2-hour post-meal / Oral glucose tolerance
Normal: < 7.8 mmol/L (< 140 mg/dL). Prediabetes (impaired glucose tolerance): 7.8–11.0 mmol/L (140–199 mg/dL). Diabetes: ≥ 11.1 mmol/L (≥ 200 mg/dL).
Random readings and when to worry
A random reading ≥ 11.1 mmol/L (≥ 200 mg/dL) with symptoms can indicate diabetes. For people with diabetes, target ranges differ by age, pregnancy, and health conditions—ask your clinician what target fits you.
4Practical tricks, meters, and common mistakes
Understanding and converting numbers quickly helps when traveling, comparing notes, or reading international research. Small mental shortcuts and awareness of meter settings avoid dosing mistakes.
Mental math shortcuts
Quick trick: multiply mmol/L by 18 for mg/dL. For rough estimates, multiply by 20 then subtract 10%: e.g., 7 × 20 = 140, minus 10% (14) ≈ 126. To go from mg/dL to mmol/L, divide by 18 — or approximate by dividing by 20 and adding 10%.
Meters, settings and human errors
Some glucose meters can display either unit — check settings. The most common real-world mistake is mixing units when reading results or instructing insulin doses. Always confirm the unit and, if unsure, convert before acting.
A famous units mistake (why this matters)
Unit mix-ups have caused major problems outside medicine. NASA's 1999 Mars Climate Orbiter crashed after metric and imperial units were mixed between teams. In medicine, a unit confusion can lead to wrong dosing or miscommunication between patients and providers.
5HbA1c — how it links to daily glucose numbers
HbA1c reports average blood glucose over roughly three months. Labs report HbA1c as a percentage. There are formulas to estimate average glucose (eAG) from A1c so you can compare it to your daily mg/dL or mmol/L readings.
The eAG conversion formulas
NGSP-derived formula: eAG (mg/dL) = 28.7 × A1c (%) − 46.7. Convert that eAG to mmol/L by dividing by 18: eAG (mmol/L) ≈ (28.7 × A1c − 46.7) ÷ 18. Example: A1c 7% → eAG ≈ 154 mg/dL → 8.6 mmol/L.
What common A1c values mean
Typical interpretations: A1c < 5.7% — non-diabetic. 5.7–6.4% — increased risk / prediabetes. ≥ 6.5% — diagnostic for diabetes (on separate tests). Targets for treatment often aim for ~7% or individualized goals set by your clinician.
Pro Tips
- 1Quick conversion: mmol/L × 18 = mg/dL; mg/dL ÷ 18 = mmol/L
- 2Fast estimate: ×20 then −10% gives a close mg/dL from mmol/L
- 3Confirm your meter’s unit setting before treating high or low glucose
- 4Convert A1c to eAG: eAG (mg/dL) = 28.7×A1c − 46.7, then ÷18 for mmol/L
- 5When traveling, keep a printed conversion chart or phone app handy
Different units can make the same blood sugar value look unfamiliar. Multiply mmol/L by 18 to get mg/dL, or divide mg/dL by 18 to get mmol/L — that's the practical takeaway. Use the mental tricks and conversion formulas here next time you compare readings from another country or a lab report.
