Tool selection
Start with the source of your number
The safest way to select a calculator is to identify where the input came from. A percentage printed on a laboratory report is A1C. A glucose meter may display individual readings and a stored average in mg/dL. A CGM summary usually provides mean glucose, Glucose Management Indicator, Time in Range, time below range, and data coverage. Similar-looking values are not interchangeable.
If you have laboratory A1C and want familiar glucose units, choose A1C to Blood Sugar. If an overseas report or clinical note requires IFCC units, choose the A1C Unit Converter. If you have an average from finger-sticks, Blood Sugar to A1C can provide a rough estimate, but the quality of the estimate depends heavily on whether the readings represent nights, meals, activity, and ordinary days.
For CGM, use the mean glucose shown by the device report in the GMI Calculator. Use Time in Range separately to understand how much of the day was spent within the selected boundaries. Neither input should be copied from a single current sensor reading.
What each calculator is—and is not—calculating
| Output | Starts with | What it represents | Do not call it |
|---|---|---|---|
| eAG | Laboratory A1C | An A1C-derived glucose estimate | Measured daily average |
| Estimated A1C | Meter mean glucose | Reverse ADAG estimate | Laboratory A1C |
| GMI | CGM mean glucose | CGM-specific glucose indicator | Lab A1C or eAG |
| Time per day | TIR percentage | Duration within stated CGM limits | A1C or GMI |
| IFCC mmol/mol | A1C percentage | Same HbA1c in another reporting unit | Glucose mmol/L |
The equations behind the tools
A1C-to-eAG uses the relationship developed in the A1C-Derived Average Glucose study: eAG in mg/dL equals 28.7 times A1C minus 46.7. Reversing that relationship estimates A1C from a mean glucose, but mathematical inversion does not turn home data into a laboratory test.
GMI uses a CGM-specific published relationship: 3.31 plus 0.02392 times mean glucose in mg/dL. IFCC conversion uses the NGSP master equation linking standardized HbA1c percentage and mmol/mol. Time in Range duration is direct arithmetic: multiply the percentage by 24 hours.
We round outputs to match their practical use. Small differences from another calculator may result from rounding or a different equation. Check the stated formula and units before concluding that either result is wrong.
Five input checks that prevent most mistakes
Read the label
Copy the metric name, not only the number. “Average glucose,” “GMI,” “A1C,” and “Time in Range” answer different questions.
Confirm the unit
US glucose commonly uses mg/dL; many international reports use mmol/L. HbA1c mmol/mol is a different unit again.
Check the date range
A CGM or meter mean needs enough representative days. A short illness or unusually sparse record can distort an estimate.
Preserve the original
Keep the measured value and report alongside the calculated result. A conversion should never erase the source number.
Match the range definition
For TIR, confirm the report uses the same lower and upper limits as the interpretation. Pregnancy may use a different range.
Review unexpected results
If a calculation conflicts with symptoms, laboratory testing, or well-collected device data, involve a clinician rather than forcing the numbers to agree.
Why calculated and measured results can disagree
A formula describes an average relationship across a study population; it does not reproduce every person’s biology. Laboratory A1C depends partly on red-blood-cell lifespan. Anemia, altered cell turnover, kidney disease, pregnancy, recent blood loss, transfusion, erythropoietin therapy, and some hemoglobin variants can affect interpretation.
Device data introduce other limitations. Meter averages reflect when tests were taken and can miss overnight or after-meal patterns. CGM summaries can be affected by incomplete wear, gaps, compression, sensor performance, and a period that does not resemble usual life. Recent improvement or worsening may appear promptly in device readings while laboratory A1C still incorporates earlier weeks.
When results diverge, label every metric and date range, review data coverage, and bring the original reports to a qualified clinician. Do not average A1C and GMI together or adjust medication until the cause and safety implications have been reviewed.
Calculators support—not replace—clinical decisions
These tools are educational. They cannot diagnose diabetes, determine diabetes type, identify an emergency, set a personal target, or select a medication dose. A diagnosis requires appropriate laboratory testing and, in many circumstances, confirmation. Treatment decisions also require current glucose patterns, low-glucose events, kidney and liver function, medication timing, pregnancy status, other health conditions, and personal priorities.
Use a result to ask a better question: Does my A1C-based eAG resemble my device average? Is my CGM report long enough to interpret? Why is GMI different from laboratory A1C? How many hours per day am I below range? A calculation is most helpful when it leads back to the measured data and a specific discussion.
For a quick visual reference rather than an interactive calculation, visit the A1C and blood sugar charts. Review our editorial policy for sourcing and correction standards and the medical disclaimer for tool limitations.
Calculator questions
Which calculator should I use for a laboratory A1C?
Use A1C to Blood Sugar when you want eAG in mg/dL or mmol/L. Use the A1C Unit Converter when you need the IFCC HbA1c unit mmol/mol.
Which calculator should I use for CGM data?
Use GMI when you have mean CGM glucose. Use Time in Range when you have the percentage of readings within a defined range. Do not enter TIR into the GMI field.
Can a calculator diagnose diabetes?
No. Diagnosis requires appropriate laboratory testing and clinical interpretation. A formula cannot confirm diabetes or determine its type.
Are eAG, estimated A1C, and GMI the same?
No. eAG is calculated from laboratory A1C, estimated A1C reverses an average-glucose relationship, and GMI uses a CGM-specific equation. Label the output you use.
Why does my calculated result differ from my lab report?
Recent glucose changes, incomplete device data, meter timing, red-blood-cell lifespan, anemia, kidney disease, pregnancy, hemoglobin variants, blood loss, or transfusion can contribute.
Can I use these results to change medication?
No. Medication decisions require current glucose patterns, low-glucose risk, health history, and a prescribed plan. Discuss calculated results with your care team.
Formula sources and review date
A1C/eAG: ADAG study, Diabetes Care. GMI: Bergenstal et al., Diabetes Care. HbA1c units: NGSP converter and master equation. Reviewed August 15, 2026. Each tool page documents its own inputs, assumptions, and limitations.