How much does your catch weigh?

Fish Weight Calculator Tool: How To Weigh A Fish Without Scales

Calculate the weight of your best catch even if you don’t have any scales. Just enter the girth and length of the fish in the fields below

Fish weight

- - lb

Ounces

- - oz

Kilos

- - kg
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How To Weigh A Fish Without Using Scales

If you caught a fish and don’t have any scales or weight calculator on hand, you can estimate the weight of your catch from a few simple measurements. 

Please be aware that technique isn’t 100% accurate, but it will give you a ballpark estimate for the weight of your catch that you might not otherwise be able to get. 

I see many people asking questions about which numbers are correct to use for it to be more accurate and I think this is mainly down to a lack of understanding about how it really works.

In-Depth View

Below I’ve written a step by step guide about how the formula for the fish weight calculator above is created. 

I hope by reading it you can gain a better understanding of what the numbers in the formulas mean and what makes some measurements work better than others.

Let’s get started!

How to Calculate Fish Weight

Here’s how the fish weight estimation formula works:

Step 1: Shape Approximation

To calculate the fish weight, first, we are going to take a rough approximation of the shape of the fish as though it was made of blocks. 

Let’s cut away all the extremely lightweight parts of the fish like the fins, tail and snout to be left with a simplified fish shape.

Simplified 2D Fish Model

Simplified 2D Fish Model

Step 2: Straightening The Lines

Next, for this part, let’s make things even simpler by replacing the rounded curves of the fish with straight lines that roughly match its shape. 

We are going to start our lines at both ends of the fish and have them meet at the widest part of the fish where we will measure the girth.

Simplified Straight Line Fish 2D Model

Simplified Straight Line Fish 2D Model

Step 3: Making it 3D

Let’s make this fish three-dimensional by saying it is the same height and width when cut in half. 

By that I mean that the cross-section of the fish would be a square, so its height and width are the same. 

If we know the girth of the fish is 20 inches, then the four even sides of our blocky fish should equal our girth.

So to calculate the length of each side of our blocky fish we can simply divide the girth by 4 to get the length of one side. Now we are left with our three-dimensional blocky fish.

3D Fish Model

3D Fish Model

Step 4: Cutting It In Half

Now we want to try to calculate the volume (or three-dimensional area) of the blocky fish. 

The easiest way of doing that is to cut the blocky fish in half at the widest point. 

This leaves us with two wedge-shaped blocks; one that roughly matches the head of the fish and one that roughly matches the body.

Split 3D Blocky Fish Model

Split 3D Blocky Fish Model

Step 5: Starting With The Head

Now we can use some high-school math to calculate the volume of each block. First, let’s start with the head:

3D Fish Head Model

3D Fish Head Model

Step 6: Calculating The Volume Of A Wedge

To calculate the volume of a wedge shape we can use the formula.

To calculate the volume of a wedge shape we can use the formula

½ x (WEDGE LENGTH) x (Wedge height) ^ 2

Step 7: Calculating The Volume Of The Head

Putting in our values for the head we get:

½ x (HEAD LENGTH) x (GIRTH/4) ^ 2

Which can be simplified to:

½ x (HEAD LENGTH) x (GIRTH) ^ 2/16
3D Fish Tail Model

3D Fish Tail Model

Step 8: Calculating The Volume For The Tail

Doing the same for our tail wedge we get:

½ x (TAIL LENGTH) x (GIRTH) ^ 2 / 16

Step 9: Calculating The Total Volume

The total volume of our blocky fish must be the volume of the head wedge added to the volume of the tail wedge:

(½ x (HEAD LENGTH) x (GIRTHt) ^ 2 / 16 ) + (½ x (TAIL LENGTH) x (GIRTH) ^ 2 / 16)

And we can simplify this to:

½ x (HEAD LENGTH + TAIL LENGTH) x (GIRTH) ^ 2 / 16

Step 10: Simplifying The Volume Of a Fish

Since we know that (HEAD LENGTH) + (TAIL LENGTH) = FISH LENGTH we can simplify this further to:

½ x (FISH LENGTH) x (GIRTH) ^ 2 / 16

And finally one more simplification gives us:

(FISH LENGTH) x (GIRTH)^2)/32

So now we can calculate a pretty good guess for the volume of our fish from our simple blocky fish. 

To calculate the fish weight all we need now is a good approximation of the density of our fish. 

We can do this by estimating how many 1 inch chunks of fish we need to make 1lb of fish.

From averaging lots of actual fish, a reasonable starting estimate to use is that 25 1-inch actual chunks of fish weigh about 1 lb.

Step 11: Calculating Weight From Volume?

So we now want to figure out the weight by the following:

FISH WEIGHT = FISH VOLUME * 1/25

Step 12: The Final Formula For Weight

Substituting in our existing formula to be used for FISH VOLUME gives us:

FISH WEIGHT = (FISH LENGTH) x (GIRTH)^2)/32 * 1/25

Which can be simplified to:

FISH WEIGHT = (FISH LENGTH) x (GIRTH)^2)/(32 * 25)

Or….

FISH WEIGHT = (FISH LENGTH) x (GIRTH)^2)/800

TADAAA! Now we have our famous formula to estimate the weight of a fish, based on its length and its girth.

But the measuring doesn’t end quite yet… There is another part of the fish weight calculator to be discussed. 

If you look at various species of fish you will notice that their shape and the weight of their meat is different. 

It means when measuring other types of fish, different values should be used to get to the estimated value.

Making Adjustments

For example, let’s compare this Tuna with this Bass. 

If we overlay our idealized blocky fish over the actual fish we can see that while for the Bass it’s a reasonably good approximation. 

It doesn’t look like an accurate shape for the Tuna. We can’t stick with one shape. It will affect the estimate if we don’t make proper adjustments.

Tuna Fish With Model Outline Overlay

Tuna Fish With Model Outline Overlay
Bass Fish With Model Outline Overlay

Bass Fish With Model Outline Overlay
This is where tweaking our magic number helps. We just calculated our magic number as 800, this magic number is usually called the weight shape factor. 
 
We can actually tweak this number based on the fish species we are approximating to give a better estimate for that fish. 

Here’s a list of the fish/shape factors we use for a bunch of common species. 
 

When measuring fish, you can even use these weight shape factors to get an estimate for other fish that have similar shapes to the ones in the list below:

  • Trout – 900
  • Steelhead – 900
  • Mackerel – 900
  • Snook – 900
  • Redfish – 900
  • Bonefish – 900
  • Salmon – 800

  • Bass – 800
  • Walleye – 3500
  • Pike – 2700
  • Gar – 2700
  • Muskie – 2700
  • Sunfish – 1200
  • Bluegill – 1200

When you calculate, you can even come up with your own magic shape/weight factors by measuring and weighing your catch with scales and tweaking the numbers in the formula until it spits out the correct results. 

That’s one feature that the fish weight calculator menu above doesn’t have. 

Once you know your way around these values, using different measurements and getting the right estimate will be way easier.

Wrapping up

So there you have it, the process behind the fish weight calculator, We’ve successfully created our fish weight formula from our two simple measurements. 

We’ve also got a better idea of what the magic number is and how adjusting it affects the measurements. 

Try playing around with the formulas above and get a feel for how it works. 

Grab some measurements of your next catch and compare the fish weight calculator output to the actual weight. 

To make it more accurate, you can try to work out your own magic number that better matches the type of fish you caught. 

You can even make your own fish weight calculator that is suited to your needs.

 
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