How to Read a Manometer: Pressure Made Clear

Reading a manometer means determining the pressure represented by a fluid column or an instrument display. The exact method depends on whether you have a U-tube, inclined-tube, or digital manometer. Before starting, identify the model, its scale, and whether it measures gauge, differential, or absolute pressure. These details determine what the number means.

What You’ll Need

A manometer with a readable scale or display
The instrument’s instructions, if available
A stable, level surface for a liquid-column model
Safety glasses and any protective equipment required for the system being measured
A suitable pressure source and compatible tubing or fittings, if taking a measurement

Do not connect an instrument to a system unless its pressure range and materials are suitable. For gas appliances, combustion systems, medical equipment, or high-pressure lines, follow the equipment maker’s procedure or use a qualified technician.

Step-by-Step

Identify the manometer type. A U-tube has two vertical limbs containing liquid; pressure is read from the height difference between the columns. An inclined manometer uses a sloped scale, which can make small pressure differences easier to see. A digital model shows a value on a screen. Check the label or manual rather than assuming all models work the same way.
Check what pressure it reports. A gauge manometer compares pressure with local atmospheric pressure. A differential manometer compares pressure at two ports. An absolute-pressure instrument uses a vacuum reference. On a U-tube, one limb may be open to the atmosphere while the other connects to the measured system; both limbs may instead connect to separate points. Confirm the setup before interpreting the result.
Confirm the units and range. Look for units such as inches of water column (in. H₂O), millimeters of water, millimeters of mercury (mmHg), pascals (Pa), or kilopascals (kPa). Do not treat these units as interchangeable without conversion. Make sure the expected reading is within the instrument’s range; overpressure can damage it or cause fluid to spill.
Set the instrument up correctly. Put a liquid-column manometer upright on a stable, level surface. Check that the fluid is at the expected starting level, the tubing is secure, and there are no visible leaks, bubbles, or kinks. For a digital instrument, check the battery, zero setting, and hose connections as its instructions specify.
Zero or reference it. For a U-tube open to air on both sides, the fluid levels should be equal when there is no pressure difference. A digital model may require a zero operation with its ports exposed to the same reference pressure. Follow the manufacturer’s method; do not adjust a liquid scale or move fluid to force a zero.
Connect the pressure source safely. Attach the correct port or ports using suitable tubing and fittings. For a differential reading, connect the high-pressure side to the high port and the low-pressure side to the low port if they are labeled. Keep connections snug, but do not overtighten. Open valves or apply pressure gradually, following the system’s procedure.
Allow the reading to settle. Wait until the liquid columns or digital display stop changing. Pulsing pressure, vibration, temperature changes, or a leak can prevent a steady reading. Do not put your face near an open tube or loosen a connection while the system is pressurized.
Read the liquid levels at eye height. For a U-tube, note the level in each limb and calculate the vertical difference between them. Read the same part of the curved surface, or meniscus, in both limbs; for many clear liquids, this is the bottom of the curve, but follow the instrument’s marking or manual. Avoid viewing from above or below, which creates parallax error.
Apply the scale correctly. If a scale directly shows the difference in height, record that value once. If the scale gives each column’s position separately, subtract the lower position from the higher one. Do not add the two readings just because both limbs have numbers. On an inclined instrument, read along its marked scale and use any required correction specified by its maker.
Record the value and direction. Write down the numerical result, units, and which port or side was at higher pressure. A larger rise on one side means that side has lower pressure, because the fluid is pushed down on the higher-pressure side and up on the other. Digital instruments may show a signed value; preserve the sign and note the port connections.
Depressurize and disconnect. Close the source using its proper procedure, safely vent pressure if the setup requires it, and then remove the tubing. Avoid spills, clean up any liquid according to its safety information, and store the instrument upright or as its instructions direct.

Common Mistakes

Reading one side only: A U-tube reading usually depends on the difference between the two levels, not the height of one column alone.
Adding instead of subtracting: If two scale positions are shown, determine the vertical separation. Add only if the instrument’s instructions explicitly define the scale that way.
Mixing up pressure direction: The column rises on the lower-pressure side. A higher fluid level does not mean higher pressure on that side.
Ignoring the reference: Gauge, differential, and absolute pressure are different quantities. A gauge value is not an absolute pressure value.
Using the wrong units: “Inches” alone is incomplete; inches of water and inches of mercury represent different pressure scales.
Reading at an angle: Looking down at a scale can shift the apparent level. Put your eyes level with the meniscus or display.
Assuming every liquid behaves the same: The fluid’s density affects how column height relates to pressure. Use the instrument’s specified fluid and scale.

Tips

Write down the two column positions before calculating, so the subtraction can be checked.
Record the units, pressure reference, and connection direction alongside the number.
Use a scale with graduations fine enough for the pressure difference you need to measure.
For a digital instrument, verify that the selected unit and range match the measurement.
Repeat a stable measurement when accuracy matters; a second consistent reading can reveal a connection or reading error.
Keep liquid-column instruments vertical and avoid disturbing them while observing the meniscus.

Troubleshooting

The columns do not move: Check that the pressure source is on, valves are in the correct position, tubing is connected to the intended ports, and no line is blocked or kinked. Do not exceed the manometer’s rated range while investigating.

The reading keeps changing: Look for loose fittings, leaks, pulsing pressure, vibration, or a source that has not stabilized. Secure the instrument on a steady surface and wait again. If the system itself fluctuates, record that fact rather than reporting a false steady value.

The fluid levels are unequal at zero pressure: Make sure both sides are at the same reference pressure, the device is vertical, and the scale is being viewed at eye level. Check for fluid loss, contamination, or a damaged instrument. Use the maker’s zeroing procedure; do not compensate by guessing.

The value seems implausible or has the wrong sign: Confirm the units, scale direction, and high/low port connections. Check whether the instrument reports gauge or differential pressure and whether a displayed minus sign is meaningful. Re-zero and repeat only after removing pressure safely.

Liquid enters the tubing or spills: Stop the measurement and depressurize the setup according to its procedure. Do not continue if the fluid is unknown, contaminated, or incompatible with the system. Consult the manual for cleanup and service instructions.

Frequently Asked Questions

What does a manometer measure?

It measures pressure, often the difference between two pressures. Some designs compare a pressure with atmospheric pressure, while others display absolute pressure.

How do I read a U-tube manometer?

Read the fluid level in each limb at eye height, then find the vertical difference between them. Use the specified meniscus point and the scale’s units.

Which side has higher pressure?

The pressure is higher on the side where the fluid is pushed lower. The fluid rises on the lower-pressure side.

Do I add or subtract the two levels?

Usually, you subtract the lower level from the higher one to get the height difference. If the scale is already marked in differential pressure, read it directly as instructed.

Why does a manometer reading need units?

The number has no clear pressure meaning without units, such as pascals or inches of water column. Different units represent different pressure amounts.

Can I use any manometer for any system?

No. The pressure range, fluid, fittings, and materials must suit the system. Check the instrument and system instructions, and use a qualified technician for hazardous or specialized equipment.

Further reading: National Institute of Standards and Technology. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.