The math is worse than it sounds
A toilet that runs continuously, or even one that just runs briefly every few minutes, can waste well over 100 gallons a day — some worn flappers waste several hundred. Most of that is invisible: no puddle, no obvious sign, just a water bill that's higher than it should be month after month until someone finally checks.
How to actually check, right now
Add a few drops of food coloring to the tank (not the bowl) and wait 10-15 minutes without flushing. If color shows up in the bowl, water's leaking past the flapper into the bowl continuously — which is exactly the running-toilet waste described above, even if you can't hear it running.
What's usually actually wrong
Most running toilets come down to one of three things: a worn flapper that no longer seals fully, a fill valve that's not shutting off completely, or a float that's set too high so the tank keeps trying to overfill. All three are common, inexpensive fixes — this is rarely a 'replace the whole toilet' situation.
When it actually is time to replace
If you're dealing with repeat flapper failures, mineral buildup that keeps fouling the fill valve (common with hard water), or a toilet old enough that parts are hard to source, replacement can end up cheaper over a few years than repeated repairs. We'll tell you honestly which situation you're in instead of defaulting to the bigger job.