Fixing device, image forming apparatus incorporating same, and method of dimensioning fixing device

A fixing device for fixing a toner image on a recording medium, comprising:

• an endless fixing member to rotate in a predetermined direction of rotation, formed in a loop;
• a pressing member contacting an outer circumferential surface of the fixing member to form a fixing nip between the fixing member and the pressing member through which the recording medium bearing the toner image passes;
• a temperature detector facing the fixing member to detect a temperature of the fixing member;
• and
• a heater facing the fixing member to heat the fixing member to a predetermined temperature based on a detection result provided by the temperature detector,
• the heater disposed with respect to the temperature detector with a circumferential distance A between the temperature detector and the heater along a circumference of the fixing member defined by the following formula:
• A≧v×(T1+T2)
• where v is a circumferential velocity of the fixing member rotating in the predetermined direction of rotation, T1 is a response time of the temperature detector, and T2 is a response time of the heater.