The tank refills itself every few minutes — flapper or valve?

Usually the flapper. Water leaking past it lowers the tank level until the fill valve kicks in, so the refill you hear is a symptom. The dye test — 10 drops, 15 minutes — confirms it: colour in the bowl means the flapper.

How do I know whether I need a 2-inch or 3-inch flapper?

Measure the flush valve opening at the bottom of the tank with a ruler. The opening is either 2-inch (50 mm) or 3-inch (75 mm), and the flapper must match it — the two sizes are not interchangeable.

Do I really not need a pipe cutter to change the fill valve?

Correct — a fill-valve swap is a wrench job only. The valve connects through the 3/8-inch supply coupling and a shank nut, both undone with a 10 mm wrench and an adjustable spanner.

What if the dye test shows no leak but the toilet still runs?

Then suspect the fill valve or the chain. Check the flapper chain has slight slack, and time the refill: a healthy fill valve finishes a 3.8 L tank in 90-150 seconds, while refilling past 3 minutes means water is still escaping.

200-400 Lwater a running toilet can waste in a single day

3 toolsa 10 mm wrench, an adjustable spanner and a flat screwdriver

90-150 shealthy refill time for a 3.8 L (1 gallon) tank

10 dropsfood colouring for the dye test

Fix a Running Toilet Yourself

Dunitt — Step-by-step DIY home repair procedures: fixing | dunitt.com

A running toilet is almost always a worn flapper or a tired fill valve, and both are swapped with three hand tools. The dye test below confirms which part is at fault in 15 minutes, before you spend anything. Every size and minute quoted here comes from standard part specifications.

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Dunitt How-To Guides — home repair measured in minutes and millimetres
Plain-number repair guides for common household faults.

Diagnose, replace, verify

The fix in numbered order

10 dropsfood colouring for the dye testColour appearing in the bowl within 15 minutes confirms the flapper is leaking.

2 or 3 inthe only two flapper diametersFlappers come in 2-inch (50 mm) and 3-inch (75 mm) sizes, so measure the flush valve opening before buying.

5 yearsservice life of a braided supply lineReplace a 3/8-inch (9.5 mm) braided stainless line every five years or at the first visible bulge in the braid.

Sizes and timings worth writing down

The numbers this repair runs on: sizes, times and service lives for every part in the tank.
Check or part	Standard figure	Why it matters
Flapper diameter	2 in (50 mm) or 3 in (75 mm)	Measure the flush valve opening before buying a replacement
Tank refill time	90-150 s for 3.8 L (1 gal)	Refilling past 3 minutes points to a leaking flapper or valve
Dye test	10 drops, verdict in 15 min	Colour in the bowl confirms the flapper is leaking
Water wasted by a leak	200-400 L/day; 6,000-12,000 L/month	What an unfixed running toilet adds to the water bill
Tank-to-bowl bolts	1/4 in (6 mm); corrode in 8-10 years	A full reseal takes 45-60 minutes
Braided supply line	3/8 in (9.5 mm) stainless	Replace every 5 years or at the first visible bulge
Thread seal	3-5 turns of PTFE on 1/2 in (12.7 mm) threads	Seals the supply connection without paste

The numbers this repair runs on: sizes, times and service lives for every part in the tank.

Step one: run the dye test before touching a tool

Ten drops of food colouring in the tank tell you within 15 minutes whether the flapper is leaking.

Lift the tank lid, add 10 drops of food colouring to the tank water and wait 15 minutes without flushing. If colour appears in the bowl, water is slipping past the flapper and the fix is a new flapper, not a new valve.

If the bowl stays clear but the tank keeps topping itself up, suspect the fill valve instead. A healthy fill valve refills a 3.8 L (1 gallon) tank in 90-150 seconds; a tank that is still refilling after 3 minutes has a leaking flapper or a faulty valve.

This order matters because the two parts fail differently and cost different amounts of time. The dye test takes one minute of hands-on work and prevents the classic mistake of replacing a working valve.

Tank parts in plain words

Flapper: The rubber seal that caps the flush valve at the bottom of the tank. When it warps, water slips into the bowl and the tank refills itself.
Fill valve: The tall assembly that meters fresh water into the tank after a flush. A healthy unit refills a 3.8 L (1 gallon) tank in 90-150 seconds, then shuts off completely.
Flush valve: The fixed opening at the bottom of the tank that the flapper seals. Its diameter — 2-inch (50 mm) or 3-inch (75 mm) — dictates the replacement flapper size.
Dye test: A leak check using 10 drops of food colouring in the tank. Colour in the bowl within 15 minutes, with no flushing, proves the flapper is passing water.
Tank-to-bowl bolts: The 1/4-inch (6 mm) bolts clamping the tank to the bowl with a gasket between. They corrode after 8-10 years and cause leaks on the floor.
Braided supply line: The 3/8-inch (9.5 mm) flexible hose from wall valve to tank: stainless braid over a rubber inner hose. Replace every 5 years or at the first bulge.

Asked before the first wrench turn

The tank refills itself every few minutes — flapper or valve?: Usually the flapper. Water leaking past it lowers the tank level until the fill valve kicks in, so the refill you hear is a symptom. The dye test — 10 drops, 15 minutes — confirms it: colour in the bowl means the flapper.
How do I know whether I need a 2-inch or 3-inch flapper?: Measure the flush valve opening at the bottom of the tank with a ruler. The opening is either 2-inch (50 mm) or 3-inch (75 mm), and the flapper must match it — the two sizes are not interchangeable.
Do I really not need a pipe cutter to change the fill valve?: Correct — a fill-valve swap is a wrench job only. The valve connects through the 3/8-inch supply coupling and a shank nut, both undone with a 10 mm wrench and an adjustable spanner.
What if the dye test shows no leak but the toilet still runs?: Then suspect the fill valve or the chain. Check the flapper chain has slight slack, and time the refill: a healthy fill valve finishes a 3.8 L tank in 90-150 seconds, while refilling past 3 minutes means water is still escaping.

How this guide was assembled

Every figure on this page comes from standard part specifications — flapper diameters, bolt sizes, line sizes — and from timing checks any reader can repeat with a stopwatch. No prices are quoted because they vary by region and retailer; sizes and minutes do not. Where a range is given, such as 90-150 seconds for a refill, both ends of the range are stated.

Where the numbers come from

The measurements reflect widely published manufacturer specifications and utility guidance on household water waste.