If the drain is completely stopped, a plunger can create the hydraulic pressure needed to move the clog.
Bathtub drained in 5+ minutes, water pooled around ankles. bathtub clog remover
To help you find the right product for your specific drain type: (PVC, copper, or old galvanized steel) Current status (fully blocked or just draining slowly) Presence of standing water If the drain is completely stopped, a plunger
Not just for toilets! A cup plunger can clear a tub clog by using pressure. If the drain is completely stopped
You likely have some of these items already; others are cheap investments at a hardware store.