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Don't ignore it thinking the water bill won't spike - a running toilet can waste thousands of gallons monthly. Most of the time it's either a faulty flapper valve (the rubber seal at the tank bottom) or the fill valve acting up. Start by checking if the flapper is warped or cracked by lifting the tank lid and looking at it, and if it seems worn, replacing it costs like $10-20 and takes five minutes. If that doesn't work, the fill valve might need adjustment or replacement, which is a bit trickier but still doable if you're comfortable with basic plumbing - otherwise calling someone makes sense.
The flapper's usually the culprit, but not always the only thing. Sometimes it's actually the fill valve (the mechanism that refills the tank) getting stuck or worn out. The flapper gets blamed for everything because it's the most obvious part, but if you replace it and the toilet still runs, you're looking at that fill valve next.
Start by checking if the flapper's even the problem. Turn off the water, flush to empty the tank, then add food coloring to the water. If the color seeps into the bowl within 30 minutes without flushing, the flapper's shot and needs replacing. They're cheap - few bucks - and you just unhook the old one and snap a new one in. Takes five minutes. If that doesn't stop it, the fill valve's probably leaking internally, which means either adjusting it or swapping it out entirely. Some fill valves have an adjustment screw you can fiddle with, but honestly most people just replace the whole thing since they're not much more expensive than a flapper.
One thing people skip: clean the valve seat (the rubber ring where the flapper sits) before buying anything new. Sometimes mineral buildup or debris prevents a good seal even with a fresh flapper. Wipe it down with a cloth while the tank's dry. If the toilet still runs after that, then yeah, replace the flapper. Test before you assume you need the fill valve too.
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