Having a constantly running toilet is bad all the way around. It’s bad for the environment because of the waste of water. It’s bad for your waste system in general as it will cause accelerated corrosion or mineral buildup, and it’s bad for your budget because it’s wasting water that you pay for.
So, to start let’s see why it’s running.
Carefully remove the toilet tank lid and look into the tank. Usually one of two things will be happening. Either the float is set too high and the water is running into the overflow tube or the flush valve is not sealing and the water isn’t running into the overflow tube.
The overflow tube is a ½ - 1 inch diameter tube sticking up in the middle of the tank and usually has a small hose attached to the top pointing down into the tube. If water is running down this tube you need to adjust the float. There are several styles. One is the older float ball that resembles a balloon on a stick. The other is a cylinder that slides up and down the valve where the water comes out. Either way the adjustment is the same principal, to make the float close the valve at the appropriate height, lower. The ball and arm type is easy, either turn the screw in the top of the valve down a turn or two or bend the arm slightly so the ball is lower in the tank. Flush and see if this takes care of the problem, if not repeat the process and try it again. This should take care of it.
The other type float is a little more involved and requires that you shut the water off to the tank, empty the tank by holding the flush handle down, sponging out the remaining water and then removing the valve from the tank. Once you have the valve out you will see that the top of the valve will turn to adjust the level of the water. Make sure you turn it so the float is lower. Then reinstall the valve making sure the valve gasket in the bottom of the tank is securely seated so there are no external leaks. Once everything is back together again turn on the water slowly and refill the tank.
If after completing the described steps the valve still runs it has either failed or has debris in it that is not allowing the diaphragm to close.
Removing the top of these valves is fairly easy. Once again turn off the water to the tank and drain it. No need to sponge it out this time. The screws holding the top of the valve in place are obvious. Remove them very carefully and set them aside. Carefully lift the top of the valve. Watch for any small springs or other parts that might be there to fall out. With the valve top and diaphragm removed hold your hand over the top of the valve and slowly turn on the water just a small amount. Allow the water to flow through the valve for five seconds then shut the water off and reassemble the valve top. Fill the tank and check it. If the problem remains you will have to replace the valve.
If no water is running down the overflow tube when you first lift the tank lid, but the toilet is running, your flush valve is not seating. Sometimes they get dislodged slightly, but usually it is from age. They get less pliable and just simply no longer seal. If you can see right away that the vale is slightly askew and can reposition it to correct the problem great. If not, then replace it. They are very inexpensive.
In the end if any of these remedies begin to be more trouble than they are worth, they probably are, and my advice would be to replace everything. An entire valve system, complete with flush valve and float valve can be found at most hardware stores for between $15 - $30 and it would give you an opportunity to upgrade the system to a newer, more efficient style.
I hope this helps. Toilets are not that complex, but if in the end you decide you can’t fix it or don’t want to mess with it, call a plumber or handyman. Either one should be able to take care of it quickly at not too great an expense.
