Toilet won't stop running

Good news first: nothing inside a toilet tank is dangerous — it's clean water and a handful of inexpensive parts designed to be swapped by hand. A running toilet is money down the drain, though, so it's worth fixing this week rather than someday.

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Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.

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In Canadian homes, start with the dye test. Drop food colouring or a dye tablet into the tank, wait 20 minutes without flushing, and look in the bowl: colour creeping in means water is getting past the flapper. It's the most common cause by far, and the fix is a $5–15 part that pushes on by hand — just match the brand and size (2-inch or 3-inch) to your old one.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check — the dye test

Take the lid off and add enough food colouring to tint the tank water clearly (or a dye tablet from the hardware store).
Wait 15–20 minutes without flushing.
Look in the bowl. Colour creeping in means water is getting past the flapper — replace it. No colour means the leak is elsewhere: check the fill valve and float next.

buildNo tools schedule20 min

handymanHow to fix it

Shut off the water at the wall valve, flush to empty the tank, unclip the old flapper, and push the matched replacement on by hand. Five minutes, no tools — matching the part is the whole game.

Close the shutoff valve at the wall behind the toilet, then flush and hold the handle down to drain the tank.
Unhook the chain from the flush arm and pop the old flapper off its ears on the overflow tube.
Match the new one: 2-inch is the long-time standard, 3-inch on many newer high-efficiency toilets — and some canister-style tanks (Toto and Kohler among them) take brand-specific seals, so bring the old part or your toilet's model number to the store.
Push the new flapper on, reattach the chain with about two links of slack, open the shutoff, and re-run the dye test to confirm the seal.

schedule15 min savings$5–15

Toilet flapperMatch brand and size — 2" is standard, 3" on many newer high-efficiency toilets; canister-style tanks take brand-specific seals, so bring the old part or the toilet's model number

visibilityHow to check

Lift the lid and find the waterline. Below the top of the overflow tube but hissing anyway? Stay here. At or over the top of the tube? Check the float setting first instead.
Gently lift the float — the cup around the valve stem, or the arm — as high as it goes. If the hiss continues with the float fully up, the valve isn't shutting off. It's done.
One rescue worth trying: on many fill valves the top cap twists off so you can rinse grit from the rubber seal underneath — a speck of debris can hold the valve open. If the hiss survives a cleaning, replace the valve.

buildNo tools schedule10 min

handymanHow to fix it

warning The one place to be gentleThe connections under the tank are plastic against porcelain: hand-tight plus a quarter turn is the rule, for the valve locknut and the supply line both. Overtightening cracks the tank or strips the coupling — the two ways this easy job turns into a wet one. Keep a towel and a small bucket under the work. And if the wall shutoff valve itself won't close fully (old ones seize), don't force it — that valve is a sensible reason to have a plumber in.

Replace the fill valve: water off at the wall, tank drained and sponged dry, one plastic nut under the tank swaps the old valve for a universal replacement set to your tank's height.

Close the wall shutoff, flush and hold to drain, and sponge the last of the water out of the tank into the bowl.
With a small bucket underneath, unscrew the supply line from the tank's underside, then the plastic locknut holding the fill valve — the whole tower lifts out.
Set the new valve's height so its marked waterline sits 2–3 cm below the top of the overflow tube, drop it in, and snug the locknut hand-tight plus a quarter turn.
Reconnect the supply line — same gentle rule — clip the refill hose to the overflow tube, open the shutoff, and fine-tune the water level.

schedule45 min savings$15–30

Universal toilet fill valveAdjustable-height universal valves fit nearly all two-piece tanks; check your tank's height before buying (most adjust roughly 23–35 cm)

visibilityHow to check

Lid off: if water is running over the top of the open tube in the middle of the tank, the level is set too high — the tank can never finish filling, so the valve never rests.
Find the adjustment: a screw on top of the fill valve, or a pinch-clip on the side of the float cup. (Older ball-on-an-arm floats: the arm bends gently downward.)

buildNo tools schedule5 min

handymanHow to fix it

Adjust the float so the water settles 2–3 cm below the top of the overflow tube — many fill valves mark the ideal line on the tube itself. Turn the screw or slide the clip, flush, and watch where it settles.

schedule5 min savingsusually free

visibilityHow to check

Flush with the lid off and watch: does the flapper drop fully closed, or does the chain snag underneath it or pull it short?
With the flapper seated, the chain should have about two links of slack. Taut is too tight; a long loose tail is a trap waiting to slide under the seal.

buildNo tools schedule2 min

handymanHow to fix it

Move the clip up or down the chain until there's just a little slack with the flapper seated — about two links — and trim or hook up the extra chain so it can't swim underneath the flapper. Flush twice to confirm.

schedule5 min savingsusually free

visibilityHow to check

This is the check after the checks: fresh flapper, dye test still fails, float and fill valve behave — something else below the waterline is passing water into the bowl.
Run a flashlight and a fingertip around the overflow tube, especially near its base, feeling for hairline cracks. Then look at the seat rim the flapper closes against: mineral pitting there breaks a seal that even a new flapper can't make.
A worn seat can sometimes take a glue-on repair-seat kit without pulling the tank; a cracked tube means replacing the flush valve assembly.

buildflashlight schedule15 min

handymanHow to fix it

warning Porcelain rulesEverything in this job threads against porcelain, and porcelain doesn't bend — it cracks. Snug is the maximum: tank bolts, spud nut, supply line. Corroded tank bolts that won't budge get cut with a mini hacksaw, never forced. If any of that reads as more project than you signed up for, this is genuinely a plumber's bread and butter — no shame in the call.

Replace the flush valve: unbolt the tank from the bowl, swap the assembly through the big spud nut, and rebuild with fresh gaskets. The part is $20–40; the job is one to two careful hours — or a routine visit for a plumber, which is the right call if the tank bolts are corroded or wrestling porcelain isn't your idea of a Saturday.

Water off at the wall, tank drained and sponged dry, supply line disconnected.
Unbolt the tank from the bowl — hold the bolt heads inside the tank with a screwdriver while turning the nuts below — and lift it onto an old towel.
Remove the large spud nut on the tank's underside, fit the new flush valve with its gasket, and set the overflow tube so its top sits below the tank's handle opening but above the fill line.
Rebolt the tank with the new rubber washers — snug, not tight, alternating sides — reconnect the supply, refill, and check every joint stays dry over the next hour.

schedule90 min savings$20–40

Flush valve assemblyMatch the drain size (2" or 3") and your tank's style; kits include the spud nut and tank-to-bowl gasket

sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.