A surface plate is a flat, smooth, and solid piece of metal or granite used as a reference plane in precision measurement and inspection. It is used as a reference point for setting other measuring tools such as micrometers, height gauges and dial indicators or for checking the flatness and parallelism of machine parts and other workpieces.

Surface plates are usually made of cast iron or granite, and the flatness and accuracy of the surface are carefully checked during the manufacturing process. Granite surface plates are considered more accurate and stable but are also more expensive, while cast iron surface plates are more affordable and commonly used in most machine shops and other industrial applications.

Surface plates are a critical tool for quality control, inspection, and machining tasks and play an important role in ensuring accurate and precise measurements.

Surface plates come in various grades (flatness accuracy) based on the DIN 876 standard. These categories are in order GG3 (lowest) GG3 - GG1 - GG0 - GG00 - GG000 (highest). They are usually used in combination with height gauges.