Use digital calipers for accurate and easy-to-read results. Use the top scale for inner diameter & the bottom scale for outer & wire diameters. Convert mm to inches with the touch of a button. Measures objects up to 4 inches/100mm. Large, easy-to-read display with automatic shut-off feature. Made of extra-strong carbon fiber composites. Minimum scale to read is 0.1mm/0.01". Battery powered ( One SR44, 1.55V, included ). Accuracy: +/- 0.2mm/0.0078".