A 20-inch foldable bicycle missing its seat will require a replacement seat compatible with its seat post diameter and overall design. The seat post diameter is typically 31.6mm for folding bikes, but it's crucial to confirm this with the specific bike model or by measuring the existing seat post. Other important specifications to consider include the saddle type (e.g., comfort, sport), and whether the seat is designed for a specific riding position. 
Key Specifications to Consider:
Seat Post Diameter:
Most folding bikes use a 31.6mm seat post, but it's essential to verify this for the specific model. 
Saddle Type:
Choose a saddle that matches your riding style and comfort preferences (e.g., comfort, sport, etc.). 
Riding Position:
Consider whether the saddle needs to be adjusted for a more upright or leaned-forward position. 
Compatibility:
Ensure the saddle is compatible with the bike's seat post clamp or mounting mechanism. 
Additional Tips:
Measure Existing Seat Post:
If you have the original seat post, measure its diameter to ensure the replacement seat is compatible. 
Check Bike Model Specifications:
Consult the bike's manual or the manufacturer's website for specific recommendations on saddle types and dimensions. 
Consider Professional Help:
If you're unsure about the compatibility or installation, consider seeking assistance from a local bike shop.