What is a Depth Finder 101 (2024 Complete Guide)

One of the most important pieces of fishing gear to have if you fish from a boat or on ice is a depth finder.

This device measures the water depth beneath you, and whether there are any fish worth catching.

Why use a depth finder?

The day I got a depth/fish finder, my fishing life changed.

I could suddenly see areas where fish hold, what depth they preferred and I could target places with underwater structures like drop-offs or sunken trees that fish like to hang out around.

I could locate fish in seconds instead of guessing where they might be based on instinct.

There are many different types of fish finders out there, but they all serve a similar purpose. If you’re new to fishing or just want some information about this important piece of equipment, read on!

What is a depth finder?

Fish Finder and Fishing Gear on a Boat
Fish Finder and Fishing Gear on a Boat

Depth finders are sonar (sound navigation) devices used on boats or sleds or when ice fishing that measures the water depth beneath you, shows you the bottom contours and structure, and whether there are any fish down there.

Like ice fishing gloves I’ve reviewed here and other fishing tools, these depth finders have a specific use and purpose.

Using these tools, we anglers can determine what depth we are fishing in, find areas that hold fish, and begin to see a picture of the underwater contours of our favorite fishing areas.

Vital Information

They can also provide information about potential hazards or obstructions that may lie under the surface such as rocks, logs, reefs, or other obstacles.

I have avoided some serious disasters where I would have beached my boat were it not for seeing a rapid depth change on my depth sounder.

Is a depth finder and a fish finder the same thing?

A depth finder and a fish finder are the exact same thing. Each device works in exactly the same way (sound navigation) and the only reason they have different names is for marketing purposes more than anything else.

Both a fish finder and a depth finder (depending on the model you have/use) will show you the water depth, if there are any fish beneath you, what depth the fish are at, and give you an image of the bottom contours too.

How does a depth finder work?

A depth finder comes in two parts, a transducer, and a display.

The first of the two parts, the transducer, sits in the water and is connected to the display which is mounted on the boat usually near the steering wheel.

Sound Waves

The transducer works as a transmitter of sound waves (ultrasonic or radio) and sends out a pulse of sound waves towards the bottom of the water.

Once the sound waves hit any underwater objects (fish, weeds, the bottom) they reflect off them and the reflected waves echo back to the transducer and are then sent to the second of the two parts, the display, for processing.


The time it takes for the reflected waves to return back to the transducer determines how deep an object is underwater.

The display takes all this sonar information and creates it into a sonar image for us to read on the screen, so we anglers can know what’s down there and whether the body of water is worth fishing in.

Depth/fish finders can also come with GPS navigation systems built-in.

Having a navigation system combined with a fish finder is incredibly useful as you’ll be able to mark a particular location that you have caught fish in and looks fishy so you can find the site again quickly and easily.

What is a depth finder used for?

Boat on a Lake
Boat on a Lake

There are a number of different ways depth finders can be used. The most common purpose for us anglers is for finding fish.

Locating Fishing Spots

It’s great for figuring out where you should fish and learning what types of terrains, structures, and depths fish like to hang out around the most.

Learning the patterns of where fish like to be and connecting them to seasons and conditions took my fishing from ok to semi-pro.

I won’t fish on a boat without this device!


Depth finders can also be used to assist with everything underwater from navigating dangerous shallow waters (such as rivers), to finding sunken ships, planes, and cars so that they can be safely removed from waterways.

How does a depth finder help for fishing?

A fishfinder is incredibly helpful when it comes to finding fish (hence the name). Fish love to hang out around areas with depth changes like drop-offs or around structures like underwater mountains in the sea or fallen trees in lakes.

Fish also change the depths they hang out in based on weather conditions, the body of water, and where their primary source of food is.

Access to helpful information

If you’re using a fishfinder you are given all this sonar information displayed as an image on your screen. Fish can appear as fish finder fish arches or as fish icons in the display as well.

You can see how deep it is, if there is a structure on the bottom, are fish holding there, what depth the fish are currently at based on weather conditions, where the baitfish are, and other information too.

All this info leads to finding a lot more fish and over time you’ll be able to see patterns and determine where the fish are and why.


Plus if you have GPS on the fishfinder, you can start making a map of where the fish like to hang out and find your favorite fishing spots with ease.

Efficient fishing

Fishing without a fishfinder is like fishing blind. Dare I say like ice fiishing without the best ice fishing boots.

You are just guessing based on instinct and surface conditions and you have no idea of the detail beneath the surface.

Believe me, I know. I fished for years without one and then the day I started using one literally changed my life.

I could suddenly see everything under my boat and knew why I had never hit any fish in some areas and why I always caught fish in others.

Are all depth finders created equal?

Fish Finders on Display
Fish Finders on Display

Not every depth/fishfinder is the same, they come in a variety of shapes and sizes and there can be a huge difference between one and another.


There are 3 main types of fishfinders: portable, ice, and boat/mounted. Portable options have the lowest sonar power ratings, while ice and mounted options can penetrate and reflect to serious depths if you buy a unit with enough sonar power.


Mounted fishfinders are the best, and I’d recommend getting one with both charts and GPS as you will get the best picture of where the fish are, might be, and you can navigate to them quickly and safely.

How can you determine the depth of the water without a depth finder?

While depth finders are a great way to get accurate information about the terrain below you, they’re not the only way to determine the depth.

Many anglers opt for another option instead of buying an expensive unit – using your fishing line and then measuring how long it is once you hit the bottom by winding it back through a depth finder gauge! This will give you an estimate of the water’s depth that can be used until you have access to better technology.

You can also use the same technique to feel where the bottom of the water bed is and take multiple measurements over time to map out the terrain’s topography.


There are also some helpful chart/nav apps available which use the GPS from your cell phones or tablets and show your location on a depth chart, such as Navionics. It is not as detailed as a sonar image created by a fish finder but it does provide a good alternative for anglers.

What depth should I fish at in a lake?

View of Bait Underwater
View of Bait Underwater

There isn’t a definite answer to this question since it depends on the type of fish species you’re looking for and the lake you’re fishing in.

While fishing deep water can be more effective when targeting larger predators, there are also plenty of different types of gamefish who are native to shallower waters as well so don’t limit yourself!

If in doubt, use your depth finder device to get an accurate picture displayed on screen. Visualizing always helps.

Where can I find a fish depth finder?

There are many places you can purchase fish finders including sporting goods stores, Amazon and even local bait & tackle shops.

However, there’s no replacement for personal recommendations from other experienced anglers who have been able to test out different products at this stage of the game.


Your best bet is to ask around – chances are someone else has an idea of what might work well based on their own experiences, or check out our in-depth reviews.

How to setup a depth finder for fishing?

How to setup a depth/fish finder depends on the model you have bought.

No matter what type you purchase, it will come with a transducer that needs to be in the water (either floating, dropped through an fishing hole, or mounted to the hull of your boat) and be connected to a screen to show you the sonar image.

Once connected, you might require (depending on the device) a marine battery to power it – this is certainly the case for mounted/boat fish finders.

If you’re looking for the best marine batteries, click here.

How to mount a depth finder to a boat?

Fishfinder Mounted at the Back of a Boat
Fishfinder Mounted at the Back of a Boat

Mounting a depth finder onto your boat is typically as simple as clamping the transducer in place or suction cupping it to the hull or side of your fishing vessel.

This is the most important aspect to ensure you can get accurate readings. Once your transducer has been mounted to your hull, it’s time for some testing in order to make sure everything is hooked up correctly and calibrated to locate fish and objects properly.

Test your depth finder

You’ll want to do this before heading out on a day of fishing so that you have plenty of time if something isn’t quite right or needs adjustment. The first thing you need to do is turn on your system and put it into shallow water mode (usually -20 feet).

You should see a reading showing exactly what depth the boat sits at as well as any bottom features which lay under the boat such as fish nests, old tires, etc… If there are no readings when putting in shallow water mode then likely you may not.

If you’re getting incorrect readings, here’s our guide on how to tell if a transducer is bad. Be sure to check it out, as it can save you a boatload of cash just by knowing the root of the problem and fixing it yourself.

How to use a depth finder ice fishing weight?

Measuring Depth With a Sounding Weight

A depth finder ice fishing weight is designed so that you can have your bait sitting just off the bottom without using any kind of technology.

These are great so you can actually spend some time in your shelter instead of in the cold.

All you need to do is:

  • Attach the depth finder ice fishing weight to your hook and drop it down to the bottom.
  • Then adjust your float on the surface so that it begins to sit 1-2 feet below the water.
  • Now wind in the depth finder ice fishing weight, remove it, and send your bait out.

How to read a depth finder for fishing?

Depth finders can often seem confusing at first…

There’s a lot of information on the display and it’s not always intuitive to understand for new users. But with a bit of guidance, you’ll get the hang of it in no time.

Power up the unit

Before you start fishing, turn the unit on and place it in water.

Many depth finders have a screen that’s not illuminated until they are placed underwater (if yours does this then you need to do this step before placing your equipment into the water).

Once turned on, there should be an easy-to-read screen with basic information ranging from temperature to speed displayed at all times.

If nothing is shown after turning it on then check if the battery is installed correctly or needs charging.

Inspect the display

Simrad Fishfinder
Simrad Fishfinder

Now we’ve got our device powered up let’s take a look at what each part of the display means:

Top Section

The top left corner usually has your current boat location plotted by GPS technology.

You may also see other icons like waypoints or fish marks to help locate fish in this corner as well.

The top right often has the depth of your location displayed, you can change between meters and feet depending on preference (I would recommend using metric). If it’s a lake fishing trip then that single number is all you need to look at but if there are both shallow and deep sections of water where you’re casting your rod then we’ll need to pay attention to the bottom screen too for more sonar information.

Middle Section

The middle section will have an icon representing the structure below your boat – I’ve drawn one here which shows a reef/rocky area with stones and mixed levels of relief under our boat since it’s located near some islands. This type of object or structure can be very productive so knowing what kind it is helps us decide whether to hang out in one spot or to move around.

Bottom Section

The bottom screen is usually a split-screen of either two circles (left and right) or an A and B marker which can be switched between by pressing the Mode button on your unit.

This section tells you both the depth at that specific location as well as how far reflected away from you it is, perfect for targeting fish exactly where they are holding so we know when to cast and set our bait before those valuable bites happen!

A depth finder offers a huge amount of information about what’s going on under your boat while you’re out on the water – but only if it’s used properly.

How does a depth finder gauge work on a fishing pole?

A depth finder gauge is a simple little device that clips onto your rod.

Once you have cast your line out or let it sink to the bottom, simply thread your line through the depth finder gauge and wind it in.

The depth finder gauge will then measure how much line you wind in, in feet or meters, and let you know what the current water depth beneath you is.

What is fishing mode on a depth finder?

Humminbird Helix Fish ID Feature
Humminbird Helix Fish ID Feature

Fishing mode isn’t all on depth finders but if you have a model with it, using it can make life easier.

This mode basically shows you what is a fish and what isn’t on your sonar image by changing any sonar arcs they reflect which the device recognizes as fish, into fish-shaped symbols.

The fish symbols are also sized so you can see smaller/larger fish, and their depth can also be displayed and labeled on some models.

It’s a good mode to use when learning to read a depth sounder.

How to use a depth finder for ice fishing

Using a depth finder for winter fishing is simple. All you need to do is have your depth finder powered up (via a marine battery or rechargeable battery), have the screen and transducer connected, and have two fishing holes, one for fishing in and one for the transducer.

Then you just need to drop your transducer into the water through the fish hole and turn on your display.

You should start seeing an image of the bottom, bottom data, and any fish down there immediately. You’ll have a hit in no time.

How big is a depth finder and how much does it weigh?

A depth finder can be as small as a golf ball, or as big as a laptop with a great big transducer and body that needs to be connected to it, the difference of size and weight depends on the model you own.

Portable options usually come with a transducer the size of a tennis ball or larger which connects to your smartphone via wifi/Bluetooth and uses it as the display and weighs around 1-2 lbs. 

Mounted options range in size from 4-12 inch displays and the transducers can be the size of a security light or up to the size of the boat grill or the pot you cook a family stew in. These can weigh upwards of 30lbs.

How to adjust the gain on a fishing depth finder?

How to set up your Fish Finder. Adjusting your Sonar for PRO RESULTS!

Gain is an important adjustment that controls the amount of detail you see on your depth finder. The higher the gain, the more details it will display and a wider range of what’s underwater.

But, too much gain can cause a cluttered image, and reflect false echoes, which can result in losing track of whether you’re seeing fish or smaller objects like weeds or debris.

Specs and Conditions

But, it does also depend on how high quality your unit is as well as how deep the water is. Most units come with adjustability from 0 to 30 for gains ranging from 200 kHz up to 250 mHz (MegaHertz), more than enough to see large fish and smaller baitfish.

Wrapping Up

Thanks for reading my article, I hope you enjoyed it and now know the answer to the question “what is a depth finder?” and about how helpful a depth finder is to locate fish and understand what happens beneath the surface.

If you fish on lakes, in the sea, or through the winter, they are well worth the investment.

If you liked the article, please share it with your friends and don’t forget to check my other articles, such as this review of underwater fishing cameras. If you now think a depth finder would be useful they probably will too.

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