Sheet conveyor and image forming system

A sheet conveyor comprising:

• a holder configured to detachably hold a sheet roll;
• a rotation measuring device configured to measure a rotation amount of the sheet roll;
• a conveyance roller configured to convey a sheet pulled out from the sheet roll held by the holder;
• a tensioner provided between the holder and the conveyance roller and configured to contact with the sheet pulled out from the sheet roll toward the conveyance roller;
• a position detector configured to detect a position of the tensioner;
• a motor configured to generate driving force for rotating the conveyance roller;
• and
• a controller configured to control the motor,
• wherein the controller is configured to estimate a diameter of the sheet roll based on a conveyance amount of the sheet caused by rotation of the conveyance roller driven by the motor, the rotation amount of the sheet roll measured by the rotation measuring device, and the position of the tensioner detected by the position detector.