You can measure the depth of a well using a weighted tape, an electronic water level indicator, or a sonic meter. The simplest approach for most homeowners is lowering a weighted line until it hits bottom, then reading the length. Any method you choose should be accurate to at least 0.1 foot, per EPA measurement standards. Before you pick a technique, it helps to understand what you’re actually measuring, because “well depth” can mean more than one thing.
Total Depth, Static Water Level, and Pumping Level
Three measurements matter when it comes to a well. Total well depth is the distance from the top of the casing (or ground level) to the very bottom of the well. Static water level is how far down the water surface sits when the pump hasn’t run for a while and the water has settled to its natural resting point. Pumping level is how far the water drops while the pump is actively running. Each tells you something different about your well’s condition.
Total depth tells you the physical size of the well. Static water level tells you how much water is sitting above the pump. If you track these over time, you can spot problems early: a rising static water level after heavy rains is normal, but a steadily dropping one over months or years can signal that the aquifer is declining or that the well is silting in. The pumping level, compared to the static level, reveals how quickly your well recovers. A large gap between the two suggests the well may be struggling to keep up with demand.
Your driller’s log, which should have been filed when the well was installed, records the original depth and water levels. Comparing current measurements to those original numbers is one of the best ways to monitor your well’s health over time.
The Weighted Tape Method
This is the most straightforward technique and the one the U.S. Geological Survey uses as a standard field method. You need a graduated steel tape (essentially a long measuring tape marked in feet and tenths of feet) with a weight attached to the end. For a DIY version, a fishing weight tied to a long, marked line works in a pinch, though it won’t be as precise as a proper steel tape.
Here’s how the USGS procedure works:
- Measure the weight. Record the distance from the zero mark on the tape to the bottom of the weight. In the USGS example, this was 1.20 feet. You’ll add this to your final reading.
- Lower the tape. Feed it into the well until the weight hits bottom and the tape goes slack.
- Find the true bottom. Pull the tape back up slowly until you feel a slight jerk as the weight shifts from lying flat to standing upright. That jerk tells you the weight is resting on the bottom but the tape is taut and giving you an accurate reading.
- Read the tape at the top of the casing. Note where the tape meets your reference point (called the measuring point).
- Repeat for consistency. Pull the weight up a foot or two so it hangs freely, then lower it again. Do this until you get two readings that match.
- Calculate total depth. Add the tape reading at the top to the length of the weight. If your casing sticks up above ground level, subtract that height to get the depth below ground. In the USGS example: 84.30 feet (tape reading) plus 1.20 feet (weight length) minus 3.40 feet (casing height above ground) equals 82.10 feet of total well depth.
This method also gives you the static water level as a bonus. When you pull the tape out, the portion that was submerged will be wet. The boundary between wet and dry tape marks where the water surface is. Some technicians chalk the lower portion of the tape before lowering it so the waterline shows up more clearly.
Electronic Water Level Indicators
An electronic water level indicator is a probe on the end of a spool of marked wire. When the probe touches water, it completes an electrical circuit and triggers a light, a buzzer, or both. You then read the depth off the markings on the wire at the top of the casing.
These instruments are the standard tool for measuring static water level because they’re fast and precise. The EPA requires that measurements taken with electronic indicators be recorded to the nearest 0.01 foot. They won’t tell you the total well depth on their own (the probe detects water, not the bottom), but you can sometimes feel the probe hit bottom after passing through the water column, giving you a rough total depth reading as well.
Electronic indicators come in various cable lengths, typically ranging from 100 to over 1,000 feet. For most residential wells, a 300-foot model is more than sufficient.
Sonic Water Level Meters
A sonic meter sits at the top of the well and fires a sound pulse down the casing. It measures how long the pulse takes to bounce off the water surface and return, then calculates the distance using the speed of sound. You never lower anything into the well, which eliminates any risk of contamination.
The main advantage is convenience. The main limitation is physics. The speed of sound changes with temperature, so these devices include a temperature sensor to compensate. Even so, the error is relatively small: roughly 1% for every 10°F of temperature difference between the sensor and the air in the well. A bigger factor is well diameter. In wells wider than 8 inches, the sound pulse weakens as it spreads out, making readings unreliable. Manufacturers recommend inserting a narrow drop tube from the top of the well down past the water surface to guide the sound pulse in larger-diameter wells.
Sonic meters measure only the distance to the water surface, not total well depth. They’re useful for ongoing monitoring but won’t tell you how deep the well goes.
Pressure Transducers
If you already have a pressure gauge on your well system (many submersible pump setups do), you can calculate water level from pressure readings. The principle is simple: water exerts pressure based on how tall the column is above the sensor. Every 1 psi of pressure equals about 2.31 feet of water above the gauge.
To find the static water level, you need three numbers: the total well depth, the pressure reading when the pump is off, and the depth where the pressure sensor sits. For example, in a 300-foot well with a sensor at 250 feet that reads 26 psi when the pump is off, the math works out to: 250 feet minus (26 psi × 2.31 feet per psi) minus any casing height correction. In this case, that gives a static water level of about 187 feet below ground.
Pressure transducers are the standard tool for continuous, automated monitoring. They sit in the well permanently and log data over time, which is useful for tracking seasonal changes or the effects of nearby pumping. For a one-time depth check, they’re overkill. But if your well system already has a pressure gauge, you can use it to estimate your water level without opening the well cap.
Safety When Opening the Well
Any method that requires lowering something into the well means removing or partially opening the well cap. This creates two risks: contamination of the well and personal safety hazards.
Gases can accumulate inside well pits and casings. The CDC specifically warns against entering a well pit because of this buildup. When you remove a well cap, work in an open, ventilated area and don’t lean directly over the opening to breathe in whatever comes out. If your well has an electrical connection at the cap, turn off the power before you start.
Contamination is the other concern. Anything you lower into the well, whether it’s a tape, probe, or wire, can introduce bacteria from one well to another. The USGS protocol calls for disinfecting and rinsing any equipment that was submerged below the water surface. For homeowners measuring their own single well, the risk is lower, but it’s still good practice to clean your measuring line with a diluted bleach solution before and after. Make sure the well cap goes back on securely when you’re done. A loose or damaged cap is one of the most common ways surface water and insects get into a well.
Choosing the Right Method
For most homeowners who want a one-time measurement, the weighted tape method is the most practical. A long tape measure or marked rope with a weight costs very little, and it gives you both total depth and a rough water level in a single trip. The accuracy won’t match a calibrated steel tape, but for home purposes, getting within a foot or two of the true depth is usually all you need.
If you want to monitor water levels over time without opening the well each time, a pressure gauge on your existing plumbing (if your system has one) lets you calculate approximate water levels from inside your house. For precise, repeated water level measurements, an electronic indicator is the professional standard. Sonic meters offer the advantage of no-contact measurement but are limited to water level only and work best in wells under 8 inches in diameter.
For wells deeper than a few hundred feet, cable stretch becomes a factor with weighted tapes, and signal strength can weaken with sonic meters. Deep wells that deviate from vertical (which is common at greater depths) also introduce error, since the cable or tape follows the actual path of the borehole rather than a straight vertical line. In these cases, professional measurement with calibrated, stretch-corrected instruments gives the most reliable results.