So you want to cover the arid bottom of your new aquarium with some nice pebbles, but you have no idea how much to get.

Luckily, the formula for how much gravel you would need for your aquarium isn’t complicated, and I can help.

All you need to make this work is a ruler and a basic calculator.

**A substrate about 2 inches deep will suffice even in planted aquariums, so that’s what I recommend if you’re new to this.** Also, the gallon capacity of your tank is irrelevant when calculating the gravel needed, but its dimensions are.

## How much gravel do you need for your new aquarium?

Whether you have a 10 or 20 gallon aquarium is irrelevant when calculating how much gravel you need for your new aquarium setup. **This is because even standard aquariums come in different shapes and vary in size.**

For example, there is a 20 gallon “long” tank and a 20 gallon “tall” tank.

Both hold 20 gallons of water but have completely different bottom footprints.

The bottom dimensions of your new aquarium are what really matters here.

To find out the proportions of an aquarium, you can either grab a ruler and measure them, or use my resource page on aquarium dimensions. I’ve listed the measurements of all the standard-shaped tanks and their capacities in gallons, as well as the most popular irregular-shaped tanks.

Find the tank you have or want to get and check its width and length.

From there, you need to follow two steps:

- Note the length and width of the aquarium.
- Multiply the dimensions to find the bottom footprint of the tank in square inches or centimeters and write down the answer.

So, with that out of the way, we can move on to formulas.

Generally speaking, there are three main ways to calculate how much gravel an aquarium needs:

**Using the weight of the gravel.**This is the most generalized method and it is used most of the time;**Using the volume of the gravel if stated on the bag it comes in.**This is the most accurate methodology for calculating the amount needed, but most brands don’t mention the volume;**By measuring the volume of the bag after manually flattening it on the ground.**This method is a bit more hands-on but can be somewhat inaccurate and you need to physically have the bag in front of you.

Let me explain in detail each calculation method and the formulas behind them while sharing their pros and cons:

### 1. Calculate the weight of the gravel

by thatsruff

Unfortunately, most brands of aquarium gravel list the weight of their product instead of its volume.

You would usually see how many pounds the bag weighs, but that’s about it.

Why this has become a popular practice among manufacturers is beyond me.

Also, the label would usually have some vague indications like:

*“Use 1 to 2 pounds of gravel per gallon.”*

For the reasons I listed above, such instructions can be quite misleading and, sometimes, just plain wrong.

If someone tells you to just use “X” pounds of gravel per gallon, they’ve either never done it or don’t know how density works.

This is because it is very likely that we initially focused on a particular depth of substrate.

Different aquascapes and configurations require different depths for our gravel beds.

**Author’s note:** A freshwater planted tank typically requires no less than 2 inches of gravel. Even if you only want artificial plants in your aquarium, a depth of 2 inches would work best to contain them. In addition, this depth is ideal for fish that like to burrow into the substrate such as certain species of small plecos or eels.

**Also, keep in mind that the formula I’m about to share works best with classic pea aquarium gravel and Dolomite gravel.** Each brand of gravel has varying weights relative to its volume, depending on the grain size and material.

That being said, here is the formula for calculating the gravel needed for an aquarium based on weight:

**(Length of tank in inches × Width of tank in inches) ÷ 15 = pounds of gravel per 1 inch average depth of substrate**

If you need to achieve the classic 2 inches of gravel depth, multiply the answer from the above formula by two.

Anyway, if you’re using the metric system, the formula looks like this:

(Length of tank in centimeters × Width of tank in centimeters) ÷ 211 = kg of gravel for 2.5 cm average depth of substrate

**Let me continue with a practical example:**

Suppose I have a standard 10 gallon aquarium with 16 inches in length and 8 inches in width.

I would like to cover the bottom of this tank with 2 inches of gravel.

I would multiply the length and the width, which gives me 16×8 = 128 square inches of bottom footprint.

I would then divide 128 by 15, as in the formula above, and I would get ~8.53 pounds of gravel as the answer.

However, since I want an average of 2 inches of substrate depth, I would multiply 8.53 by 2, which equals ~17.07.

Therefore, I learned that my standard 10 gallon tank needs about 17 pounds of gravel.

Similarly, a standard sized 20 gallon “long” tank would require approximately 50 pounds of gravel for 2 inches of substrate depth.

But what if you want to obtain a sloping substrate?

by @tigahboy.h20 on Instagram

Let’s say I wanted my aquascape to have a slope that was 4 inches deep at one end, tapering to 2 inches at the other end.

To find the average gravel depth of this setup, I would add 4 and 2 and divide the result by the number of ends, which is 2.

This gives me (4 + 2) ÷ 2 = 3 inches of substrate on average.

By following the formula, we can calculate how many pounds of gravel would be needed for 2 inches of substrate depth in each of the popular standard aquariums:

- 17 pounds for a 5 gallon tank;
- 28 pounds for a 10 gallon tank;
- 50 pounds for a 20 gallon “Long” tank;
- 50 pounds for a 29 gallon tank;
- 88 pounds for a 40 gallon “Breeder” tank;
- 82 pounds for a 55 gallon tank;
- 119 pounds for a 75 gallon tank;
- 179 pounds for a 125 gallon tank.

**Author’s note:** Fortunately, there are more sophisticated aquarium gravel calculators. For example, Aqua-Calc takes into account the specific weight of quality gravel brands like CaribSea Eco-Complete. The limitations of this calculator, however, are that it can only calculate gravel in rectangular aquariums.

### 2. Calculation of gravel by volume

It is very rare to see an aquarium gravel mark listed in liters or cubic inches on the bag. Only aquatic soils used for planted tanks are systematically listed in litres.

It’s a shame because the liter is a measure of volume:

**1 liter equals a volume of 1 cubic decimetre, or 1000 cubic centimeters or 61.02 cubic inches.**

Doing the calculations by volume is the easiest and most accurate way to determine how much gravel an aquarium needs.

The formula for doing this is:

**(Length of tank × Width of tank × Desired average depth of substrate) = The volume of gravel in cubic inches or cubic cm**

You then divide the result by the amount of gravel each bag contains in cubic centimeters or liters and get the number of bags you would need.

**A practical example:**

Let’s say I have a 20 gallon “Long” tank that has a bottom footprint of (30.25″ × 12.5″) = 378.125 square inches.

I want to cover the bottom of this tank with 2 inches of gravel so I multiply 378.125 by 2 which gives me 756.25 cubic inches.

Let’s say a bag of gravel of my choice contains 550 cubic inches of substrate according to the label.

I then go on and divide 756.25 by 550 which tells me I need about 1.38 bags of gravel for my 20 gallon tank.

So basically two bags cover me.

The same methodology applies if you use the metric system and calculate in centimeters.

The only difference is that in the end you are converting liters to cubic cm (1 liter = 1000 cubic cm).

### 3. Measure the volume of the gravel on site using a ruler

by weedabix

Some may find this method impractical, but there are scenarios that justify it.

The method itself is basically arming yourself with a ruler and heading to the pet store.

You already know the bottom area of your tank and the desired gravel depth:

- You arrive at the store and choose a brand of hammer that appeals to you.
- You continue to carefully flatten the bag on the ground until it has the desired height (depth) of substrate.
- After that, you measure its length and width and calculate how many bags you would need to cover the bottom of your tank.

For example, you want 2 inches of gravel depth.

You level the unopened bag and even it out until it’s 2 inches high all over.

You measure the length and width of the bag and multiply them to get its area.

You then divide the area of the bottom of your aquarium by the area of the bag to estimate how many bags you would need.

This method is usually useful when you have no idea what kind of substrate you want and you have time to visit the pet store yourself.

Although there is room for small errors, it also works with aquarium sand and water soils.

## Conclusion

Don’t think too much and just choose the method that works best for you.

Determining how much gravel your new aquarium needs shouldn’t be complicated.

Leave me a comment below if you need more answers.

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