How to read a water level gauge on a tank
A water level gauge shows the depth of water in a tank to someone who is there to look at it. To know the level from somewhere else, or to be told when it changes, you need a monitor.
By Luke Patterson, co-founder of Trivertix. Last checked October 7, 2026.
Three gauges, and how to read each
Float and marker
A float in the tank is tied by a cable to a marker on the outside wall. Read the marker against the scale. On most of these the marker hangs low when the tank is full and climbs as the water falls, so check which way your scale runs.
Sight tube
A clear tube runs up the outside of the tank from a fitting near the bottom. The water in the tube stands level with the water in the tank, so its height is the reading.
Pressure gauge
A gauge at the outlet reads the weight of the water above it. Each foot of fresh water adds about 0.433 psi, so divide the reading by 0.433 to get the feet of water above the gauge.
Source, read October 7, 2026: Waterworks Mathematics by Alvord and Blasberg, section 1.7.1.
From a reading to gallons
A gauge gives a depth. To turn it into gallons, you need to know how much water each inch of your tank holds. For an upright round tank, that depends only on how wide the tank is.
- 4 feet across
- 7.8 gallons an inch
- 5 feet across
- 12.2 gallons an inch
- 6 feet across
- 17.6 gallons an inch
- 8 feet across
- 31.3 gallons an inch
- 10 feet across
- 49.0 gallons an inch
- 12 feet across
- 70.5 gallons an inch
Multiply by the inches of water in the tank. A tank 8 feet across with 5 feet of water in it holds 60 inches times 31.3, or about 1,880 gallons.
For a tank of another width, multiply its width in inches by itself, then by 0.0034. These figures take a U.S. gallon as 231 cubic inches.
This works for a tank with straight, upright walls. A tank that lies on its side, or one with a cone at the bottom, fills unevenly, and its maker's chart is the thing to go by.
Source, read October 7, 2026: NIST Handbook 44 (2026), Appendix C (PDF).
What a gauge cannot tell you
- It tells you nothing unless someone is looking at it. A tank you pass once a week is checked once a week.
- It keeps no record. It cannot show how fast the level is falling, or how much water you use in a day.
- It cannot warn you. A stuck float valve, a split pipe or a pump that has stopped shows on a gauge only when someone next reads it.
How much that matters depends on the tank. The first Trivertix pilot units went onto stock tanks that sit miles apart, on the Patterson ranch outside Karnes City, Texas, where checking the water meant a drive to each tank.
A gauge or a monitor
A gauge is enough when
- You pass the tank most days.
- Running low would be a nuisance and not an emergency.
- You have no need to know how much water you use.
A monitor is the better tool when
- The tank is out of sight of the house, or you are often away.
- Livestock or a household depend on it.
- You order hauled water and want to time the next load.
- You want an alert when the level crosses a line you set.
How a monitor reads the same tank
A monitor takes the reading for you, on a schedule. Trivertix is one. An ultrasonic sensor at the top of the tank measures down to the water about every 15 minutes, in tanks up to 16 feet deep, and reports a change as small as a quarter of an inch.
The reading goes by LoRa radio to a receiver in the house, which shows the level on its display. In the app you enter the tank's type and size once, and it does the sum above for you: gallons and percent full, the level over time, and an alert at a high or a low line you set.
A monitor has more to go wrong than a float on a cable. This one needs sun on its solar panel, a radio link to the house and, for the app, Wi-Fi. There is no harm in leaving the gauge on the tank as a second opinion.
The four kinds of tank level indicator, side by sideQuestions about reading a tank gauge
How do I read a water tank level with a pressure gauge?
Divide the reading in psi by 0.433 to get the feet of water above the gauge. Each foot of fresh water adds about 0.433 psi, so a gauge at the bottom of a tank holding 6 feet of water reads about 2.6 psi.
How many gallons is one inch of water in my tank?
For an upright round tank, multiply the width of the tank in inches by itself, then by 0.0034. A tank 8 feet across is 96 inches wide, which comes to about 31 gallons for each inch of depth.
Why does the marker on my float gauge sit low when the tank is full?
Because the marker hangs from the other end of the float's cable. When the float rises with the water, the marker on the outside of the tank goes down, so a low marker means a full tank. Check which way the scale on your tank runs before you trust a reading.
Can a water level gauge send an alert?
Not a plain one: someone has to look at it. An alert takes an electronic sensor and a link to a phone or an alarm, and that is what a tank level monitor is.
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