Please do not choose a (wrong) part number. Belts stretch a LOT; old belts are usually 5-15% longer than when new!
Please provide COMPLETE information so we can find appropriate replacements for you.
We need to know:
1--Type of equipment: Cassette Tape Recorder; VCR; CD or DVD; Reel to Reel Tape; 8-Track Player; Phonograph; Telephone Answering Machine; Turntable Record Player; Movie Projector, Key Machine; or ?????
2--Brand: Make, Model Number, approximate Year made. so we can use cross reference books if it is listed.
3--Style of Belt: Square, Flat, or Round? Square belts fit V-groove pulleys! Many customers ask for round belts when they need SQUARE!
4--Function of the belt: Drives a tape capstan; Moves a load tray: Drives a take-up reel; Drives a counter; Rotates a turntable?
5--Condition of the old belt: Broke ten minutes ago; Broke six months ago; Stretched a lot & falling off; Moderately Stretched (Almost Working); Stretched very little still working great?
6--Measurements of the old belt: Width, Thickness, and Circumference (length from END to END when broken or cut?) Please tell us HOW you measured the belt or belt path along with the measurement results you obtained.
Measuring the CIRCUMFERENCE (Length) of Old Belts:
Broken belt--lay it next to a ruler and measure (inches) end to end.
Unbroken belts: mark with a ball point pen; then roll along a ruler until you reach the mark again!
Lost belts: wrap 20ga to 26ga hookup wire tightly around pulleys; twist it together, take the wire off, measure between the kinks on a ruler (inches).
Compare the thickness of square belts to the edge of a Coin; a Dime is about .047 inch thick; a Penny is about .055 inch thick; a Quarter is about .068 inch thick; a Nickel is about .077 inch thick. On a ruler: 1 MM is .039 inch thick; 1/16 inch is .062 inch thick; 2MM is .078 inch thick.
Width of a flat belt: 1/8 inch is .125" width; 3/16 inch is .187" width; 7/32 inch is .219" width; 1/4 inch is .250" width; 9/32 inch is .281" width; 5/16 inch is .312" width, 3/8 inch is .375" width.

Free Technical Repair Tip--Rubber Belt Life Expectancy: Flexible synthetic rubber compounds are affected by temperature, humidity and air contaminants. "Air Purifiers" that generate OZONE drastically shorten the life of "rubber" (neoprene) drive parts in electronic equipment!

belts for Fisher-Price Toys:
TOT43 fits Fisher Price 800 Talk To Me tape player (need measurements of belt-path)
TOT62 fits Fisher Price 826 & 3800 & 73800 cassette tape recorder: order SBS10.7 =SCX10.7
TOT63 fits Fisher Price 3700 & 3818 cassette tape recorder (use SBS11.0)
TOT65 fits Fisher Price 820 GramoPhone (need old belt or belt-path measurements)
TOT66 fits Fisher Price 3808 tape recorder (need old belt & belt-path measurements)
TOT68 fits Fisher Price 835 Portable GramoPhone (need measurements of belt-path)

Free Technical Repair Tip--CD and DVD players: Many Computer CD Drives and DVD players use a square rubber "load belt" to open and close the drawer. If the drawer doesn't open or close: you probably need to replace the "load" belt. Please measure the circumference (=length) & thickness of the old belt (.047" is the thickness of a dime); (.055" is the thickness of a penny); (.065" is the thickness of a quarter) (.077" is the thickness of a nickel)
Many CD "load belts" are between 2.5" and 4.5" circumference
Please tell us the actual measurements of the old belt?
and--tell us the condition of the old belt (is your old belt broken?)
or (is it stretched a little and still working?)
or (is it stretched a lot and slipping badly?)
so we can select an appropriate replacement for you.

Many Chrysler & Honda Autosound Cassette Players use SBS16.0 rubber belt.
Many Ford Autosound Cassette Players use SBS15.0 rubber belt.
'85 Mustang and '87 Lincoln Ford Autosound use SBT6.0 & SBS9.5 rubber belts
GM/Delco Autosound Cassette Players 1980 thru mid-1990's made in 4 styles.
(1) Non-auto reverse models use SBS7.4
(2) Auto reverse Fijitsu 3 (Mechanical) Button Mechanisms use SBS8.6
(3) Auto reverse Blaupunkt 4-button Mechanisms use 2 belts: AV-SBS4.9 & AV-SBS9.2
(4)Pioneer (88 up) and Alpine Mechanisms (91 up) have electronic switch buttons SBS9.2