An A1C Calculator estimates your A1C or average blood glucose using a standard medical conversion formula based on the information you provide. An A1C lab test, however, measures glycated hemoglobin directly from a blood sample, making it the most accurate method for evaluating average blood sugar over the previous two to three months. While an A1C Calculator is useful for monitoring trends and understanding results between appointments, it cannot diagnose diabetes or replace a laboratory test performed by a healthcare professional.