<p>The rapid technological advancements of today have ushered in a new era of impulse high-voltage testing equipment capable of reaching high voltage ranges. Traditional calibration methods fall short in calibrating such high voltage equipment. This study introduces an on-site calibration approach tailored for impulse high-voltage testing equipment with an 800 kV range. The method employs two standard dividers: one for measuring impulse voltage up to 500 kV peak and the other for DC charging voltage up to 100 kV. An efficiency factor technique is proposed for calibration beyond 500 kV peak. Results indicate achieved linearity across all calibrated points with a linearity error under 1%, validating the extension of calibration up to 800 kV peak. The unit under calibration aligns with specified accuracy standards. Uncertainty is evaluated for each range to ensure result reliability. A comparison and verification tool using a normalized error approach confirms the efficacy of the proposed technique, demonstrating satisfactory outcomes.</p>