Identify the crank before the pedal
Look for a model marking or documented pedal-thread specification on the crank or bicycle. Park Tool lists 9/16-inch x 20 TPI as common on three-piece cranks and 1/2-inch x 20 TPI on one-piece cranks. Other or proprietary systems require their own documentation.
Keep left and right separate
The right pedal uses a conventional right-hand thread. The left pedal uses a left-hand thread. Check the L and R markings and begin threading by hand so resistance is noticed before the crank threads are damaged.
Do not rely on a rough diameter check
A ruler, product photo, or visual comparison is not precise enough to confirm a thread. Use a verified part number, a suitable measuring method, or help from a bicycle shop when the crank specification is unknown.
Follow the maker's installation instructions
Use the specified thread preparation, tools, washers, and torque. Stop if the pedal does not turn in smoothly by hand. Cross-threading and forcing a mismatched pedal can permanently damage the crank.