Toilet Tank Won’t Fill Up Or Is Slow To Fill-Causes And Fixes

Since the first flushing toilet was invented, the immediate removal of waste has changed the paradigm of life and the population’s wellness. While variations allow for different customs and practices, the humble toilet has arguably done more for global health than any other device.

The causes of a slow or non-filling toilet are related to the water pressure and damaged or clogged components. Unless the problem is the water pressure, you must first identify the cause, and then the offending component must be unblocked, cleaned, fixed, or replaced.

If you have to wait longer and longer before you can flush the toilet after it has been used more than once in a row, it is time to find out the reasons and, where possible, rectify the situation.

The Reasons Why The Toilet won’t Fill Up Or Is Slow To Fill

Several possible causes will result in the toilet not filling up or only filling up very slowly.

Most of the problems can be rectified by anyone.

There Is Insufficient Water Pressure

outlet tap at the wall

The toilet filling too slowly may have nothing to do with the toilet.

It is possible that the house water pressure has reduced and therefore there is insufficient water pressure to fill the toilet up fast.

Have you noticed a pressure drop?

Perhaps it is taking longer to fill the bathtub up. If this is a possibility, check in with your neighbors to find out if they have noticed a change and resolve the problem with your local water utility.

If there has not been a pressure reduction, check that the outlet tap is fully open.

It is worthwhile conducting a small experiment to check the water pressure in the tank.

  1. Switch off the outlet tap at the wall.
  2. Remove the end of the inlet pipe which connects to the toilet inlet system.
  3. Place the end in a shallow bucket and turn the wall tap on.
  4. If the water that comes out is at full pressure, the problem is within the toilet.
  5. If the water comes out at reduced pressure, there is a problem within the inlet pipe or plumbing to the wall tap.

The Inlet Pipe Is Clogged Or Damaged

The inlet pipe is the tube that carries the water from the wall’s outlet to the toilet inlet pipe.

The most durable pipes are made from rubber and braided aluminum, and where possible, it is recommended that pipes not made from this material be replaced.

If the water from the main supply is hard (that means it contains a high mineral content), the minerals (possibly lime) will deposit a layer on the inside of the pipe, which slowly grows and restricts the water flow.

If this is the situation, while you can try to clean the residue out with a mixture of vinegar and baking soda, it is recommended that you replace it with a new one.

There Are Fill Valve Issues

Close up of man's hands hold lock jaw pliers and old broken fill valve to fix and replace with clean white toilet and bathroom in background

The fill valve controls the volume of water flowing into the tank.

If the fill valve is fully or partially blocked, it will cause the tank to either not gill or fill more slowly.

The steps needed to clean the fill valve are as follows.

  1. Close the outlet tap on the wall.
  2. Remove the fill valve and fill cap.
  3. Partly turn the outlet tap back on and let the water flow freely through the valve.
  4. It should clean out any debris caught in the valve.
  5. Turn the tap off.
  6. Turn the fill cap over, remove the washer with a screwdriver, and wash the mineral build-up off.
  7. Reassemble the valve into the toilet
  8. Turn the wall tap on and check that the problem is resolved.

Adjust The Water Flow Through The Fill Valve

The steps listed below will enable you to adjust the water flow out of the fill valve.

  1. Take the toilet tank lid off and place it on a flat surface where there is no risk of falling or someone standing on it.
  2. Find the position of the fill valve, which will be either on the right or left side of the tank.
  3. Check that the fill valve is securely attached and that the tube is not loose.
  4. On older toilets, the adjustment screw needs to be loosened to raise the fill valve, which will enable a higher volume of water to flow into the tank.
  5. On more modern units, turn the fill valve adjustment knob to control water flow into the tank.
  6. Ensure that the water level only fills to an inch below the top of the overflow tube.
  7. Flush the toilet and then monitor the speed at which the tank fills up so that it doesn’t overfill and pour into the overflow tube.

A Damaged Or Misadjusted Float Ball Or Cup

loose float ball could be the reason why the water level in toilet bowl keeps dropping
Image from Military Plumbing

If the float ball or the float cup is not adjusted correctly, it will not allow the filler valve to open properly, and so the volume of water filling the tank will be insufficient.

There are several possible reasons why the float ball is not achieving the required angle.

The Float Arm Is Bent At The Wrong Angle

Check to see the position of the float arm in the tank.

If the float ball is attached to a position too low in the tank, bend the arm into a shape that causes the ball to rise higher, enabling more water to enter the tank.

If this doesn’t resolve the issue, consider purchasing a new float ball and float arm.

The Float Ball Becomes Waterlogged

In some circumstances, particularly with older float balls, they become waterlogged and don’t float anymore.

It results in the inlet valve remaining completely or partially closed, and therefore the speed at which the tank fills is compromised.

The best solution is to replace the float ball with a new component.

The Flapper Assembly Is Not Functioning Properly

flapper valve replaced
Image from Cathy Pearce

The flapper assembly comprises the.

  1. The flapper handle or buttons (toilet handle).
  2. The flapper chain.
  3. The flapper valve.

If the flapper valve is not closing properly, it will cause a continual flow of water into the toilet bowl. If the leak is sufficiently large, the inflow of water through the inlet valve will be insufficient to fill the bowl.

There are two possible causes of the flapper valve not closing correctly, and these are.

  1. The flapper chain is incorrectly tensioned and is holding the flapper valve away from the outlet pipe.
  2. The flapper valve has become contaminated by dirt, or it has lost its flexibility and won’t seal over the outlet pipe.

If the chain is incorrectly tensioned, it is easy to adjust by changing the connecting point on the chain.

If the flapper valve is not closing because it is contaminated or misshapen, replacing it with a new one is generally far easier.


Toilet problems are never fun, and when the toilet doesn’t refill after use or takes ages to refill, the issue needs to be rectified. If you go about the diagnosis and clean or replace any parts as required, the problem is relatively easily fixed.

While a slow or non-filling toilet may be inconvenient, if the reason is due to water continually draining into the bowl, it affects your monthly utility bill and needs to be rectified.

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