The four usual suspects
Suspect one, and the winner most of the time: a running toilet. A worn flapper leaks silently from tank to bowl around the clock, no sound, no puddle, hundreds of gallons a day. Suspect two: irrigation, where an underground zone leak or a stuck valve runs invisibly, often at night. Suspect three: a hidden supply leak in a wall, under the slab, or in the yard. Suspect four is paperwork, a rate change, an estimated read, or a misread meter, which is worth ruling out with one call to the utility.
Compare the bill against the same month last year, not last month, summer irrigation alone explains a lot of "sudden" jumps.
Find it in 20 minutes
You need food coloring and your water meter. That’s the whole toolkit.
Shut everything off and watch the meter
Every faucet, appliance, and irrigation zone off. If the meter’s leak indicator (usually a small triangle or dial) keeps moving, water is leaving the system somewhere.
Dye-test every toilet
A few drops of food coloring in each tank; wait 15 minutes without flushing. Color in any bowl means that flapper is your water bill.
Isolate the irrigation
If your irrigation has its own shutoff, close it and re-check the meter. Meter stops, the leak is in the sprinkler system. Look for soggy spots and suspiciously green patches.
Split hot from cold
Still moving? Shut the cold valve feeding the water heater. If the meter stops, the leak is on the hot side, useful intel that narrows detection work.
Call in detection
If the meter moves with everything isolated, the leak is hidden, wall, slab, or service line, and acoustic/thermal detection finds it without exploratory demolition.
“Your water meter already knows the answer. The 20-minute test is just asking it the right questions.”
The bottom line
Don’t pay a second inflated bill before running the meter test, and don’t ignore a jump, either. Every one of the four suspects gets more expensive with time, and three of them are actively putting water where your house doesn’t want it.