When replacing a worn or limp turntable belt, you do not want to replace the belt with the same diameter belt you are removing. The belt you are removing has probably stretched and is no longer the same size it was when new.
Most turntable manufacturers recommend their own brand of belt because it has the exact tension on the motor shaft for minimum slippage and pressure on the motor bearing. A belt that is too tight will greatly reduce the life of the motor bearing.
Not all turntables are belt driven. Some are Direct Drive, which means the platter is turned by an electric current through a Stator that magnetically pushes the turntable at the correct speed chosen because it is locked to the frequency of the magnetic field. This type of turntable is very desirable because there is no direct connection to a motor so there is no motor noise transferred to the platter to be picked up by the needle and amplified.
The belt drive turntable is generally less expensive because it has a very small motor and less expensive electronics. However, “High End” belt driven turntables are every bit as good as Direct Drive because they have electronics and a large drive motor to turn the platter at the exact speed chosen. Some even have a “Pitch” control that allows the platter to be adjusted 5 or 10% faster or slower. Under most conditions, the Quartz lock turntable is the least fussy. When you push the button, it locks automatically at the chosen speed with no adjustments necessary. The motor is rubber mounted and isolated from the platter by a long rubber belt. No motor noise is transferred to the needle.
To check the belt function, place your finger lightly on the platter. If it slows or stops easily, the belt has stretched with age or use. It is time to replace it. Unplug the unit.
Lift off the rubber mat on the platter. You will see at least two large holes in the platter. Turn the platter with your finger until you see the belt running over the motor shaft through one of the holes. Use a small screwdriver to lift the belt off the motor shaft. Lift the platter straight up. If the belt is broken, it may be lying in the bottom of the turntable. If not broken, it will be clinging to the platter. Remove it and measure the diameter of the part of the platter that the belt runs on. If you are not ordering by part number, the diameter of the belt run is the size of belt you want to order. The width of the belt is important to keep it on the motor shaft. The drive pulley has a slight curve which keeps the belt centered on the shaft. Order a belt the same width and diameter as the old belt. The right size belt to match the belt run will give the proper tension when stretched over the motor shaft. Simply stretch the new belt onto the belt run and replace it on the center spindle. Then rotate the platter so you can see the motor. The belt will also be visible. Lift the belt and put it over the motor shaft. Before putting the mat back on, spin the platter with your finger and observe that the new belt stays on the motor shaft.
Video of how to replace a Crosley Record Player Belt.