You can calculate your A1C at home in two ways: plug your average blood glucose into the eAG formula – A1C (%) = (average glucose in mg/dL + 46.7) divided by 28.7 – or use an at-home A1C test kit that reads a fingerstick sample. Both methods deliver an estimate within roughly 0.3 to 0.5% of a laboratory A1C when done correctly.
Method 1: The eAG Formula
The estimated average glucose (eAG) formula was developed from the ADAG study in 2008, which compared 2,700 glucose readings per participant to lab A1C. The relationship is close to linear:
A1C (%) = (average glucose in mg/dL + 46.7) / 28.7
Or if you use mmol/L: A1C (%) = (18.02 x average glucose in mmol/L + 46.7) / 28.7
Step by Step
- Collect at least 14 days of glucose data – ideally from a continuous glucose monitor (CGM), or 4 to 7 fingerstick readings per day covering fasting, pre-meal, post-meal, and bedtime.
- Calculate the simple arithmetic mean of all values in mg/dL.
- Add 46.7 to the mean.
- Divide the sum by 28.7.
- The result is your estimated A1C in percent.
Quick Reference Table
| Average Glucose (mg/dL) | Estimated A1C (%) | Range |
|---|---|---|
| 97 | 5.0 | Normal |
| 114 | 5.6 | Normal / borderline |
| 126 | 6.0 | Prediabetes |
| 140 | 6.5 | Diabetes threshold |
| 154 | 7.0 | Diabetes |
| 183 | 8.0 | Diabetes, elevated |
| 212 | 9.0 | Diabetes, high |
| 240 | 10.0 | Diabetes, very high |
Example Calculation
Say your CGM shows an average glucose of 132 mg/dL over the past 30 days:
A1C = (132 + 46.7) / 28.7 = 178.7 / 28.7 = 6.23%
That result sits in the middle of the prediabetes range.
Method 2: Home A1C Test Kits
Several consumer kits measure A1C directly from a fingerstick blood sample. The two main types are:
- Instant-read kits (A1CNow, Bayer A1CNow+): you prick your finger, place blood in a cartridge, and get a result in 5 minutes. Typical error is 0.3-0.5% versus a lab.
- Mail-in kits (everlywell, LetsGetChecked, some pharmacy brands): you collect a small blood sample with a lancet, mail it to a certified lab, and receive results in 3-5 days. Accuracy matches standard lab A1C.
Cost ranges from $20 to $60 per test. Mail-in kits are more accurate; instant kits are faster and useful for trend tracking.
Method 3: Glucose Management Indicator (GMI)
GMI is a CGM-specific estimate designed to replace the older eAG formula for continuous monitor users:
GMI (%) = 3.31 + (0.02392 x mean glucose in mg/dL)
Most CGM apps (Dexcom Clarity, FreeStyle Libre, Stelo) calculate GMI automatically if you upload at least 12-14 days of data with 70% or more wear time. GMI is more accurate than eAG for CGM users because it is based on a larger, CGM-specific dataset.
When Home Calculations Are Accurate
Estimated A1C is reasonably accurate if:
- You have 14 or more days of data with at least 70% coverage
- You do not have anemia or hemoglobin variants (sickle cell trait, thalassemia)
- You are not pregnant or recently post-partum
- Your red blood cell lifespan is normal
- You have not had a recent blood transfusion or large blood loss
When It Is Not Reliable
- Limited data: fewer than 14 days, or only a few readings per day
- Hemoglobinopathies: can skew both lab A1C and estimates, but affect them differently
- Anemia: iron-deficiency anemia can falsely raise A1C; hemolytic anemia can falsely lower it
- Chronic kidney disease: affects red blood cell turnover
- Recent illness or steroids: can cause short-term glucose spikes that skew the average
Should You Use Home A1C Instead of the Lab?
No. Home estimates are useful for tracking between clinic visits, but diagnosis and medication decisions should rely on lab testing. The ADA recommends lab A1C every 3 to 6 months for people with diabetes and every 6 to 12 months for prediabetes. Use home numbers to see whether your diet and activity changes are working, then confirm with your doctor.
Tips for Better Home Estimates
- Use a CGM for at least 14 continuous days whenever possible
- If using fingersticks, test at varied times: fasting, 1-2 hours after each meal, bedtime, and occasionally overnight
- Calibrate your meter with its control solution every 1-2 weeks
- Record readings in an app or spreadsheet so you can average them quickly
- Compare your home estimate to your next lab A1C and note any consistent bias
The Bottom Line
Knowing how to calculate A1C at home gives you a simple way to track your glucose control between lab visits. Use the eAG formula with 14 days of CGM data, try the GMI formula if your CGM app does not do it automatically, or buy an at-home kit for a direct reading. Just remember: home numbers are estimates, and your doctor’s lab A1C remains the standard for diagnosis and treatment changes.
