About the Aquarium Substrate Calculator
Planted aquariums and aquascapes are usually built on specialised substrates: nutrient-rich aquasoils, sometimes over a thin base layer of powder, root tabs or nutrient substrate, all sloped from a shallow front to a deep back. These products are sold by volume in litres rather than by weight, because their density varies and what matters is how much space they fill.
This aquarium substrate calculator works out the total litres for a sloped planted tank, separates the base layer from the cap layer, and tells you how many bags of each to buy. It works in centimetres or inches, but always reports litres, matching how the products are sold.
How to Use the Aquarium Substrate Calculator
Choose centimetres or inches.
Enter the tank's inside length and width (front to back).
Enter the total depth at the front and at the back, including any base layer.
Enter the base layer depth, or 0 if you are using a single substrate.
Enter the bag sizes in litres for the cap and base products.
The Formulas
total volume = length × width × (front depth + back depth) ÷ 2
base volume = length × width × base depth
cap volume = total − base
bags = volume ÷ bag size, rounded up
1 litre = 1,000 cm³ = 61.02 in³Step-by-Step Example
A 90 × 45 cm tank sloped from 4 cm at the front to 8 cm at the back, with a 2 cm base layer.
total = 90 × 45 × (4 + 8) ÷ 2 = 24,300 cm³ = 24.3 L
base = 90 × 45 × 2 = 8,100 cm³ = 8.1 L → 3 bags of 3 L
cap = 24.3 − 8.1 = 16.2 L → 2 bags of 9 L
the cap slopes from 2 cm at the front to 6 cm at the backWhy Planted Tanks Use Layers
Aquasoils are made from baked, granulated soil. They release nutrients, lower pH and hardness slightly and provide a light, porous bed that fine roots penetrate easily. A base layer underneath, often a nutrient powder or mineral substrate, feeds heavy root-feeding plants for longer. The cap keeps the base from clouding the water and gives the visible surface its colour and texture. Many successful tanks use aquasoil alone; layers are a choice rather than a rule.
Planning the Slope
A slope from front to back is one of the simplest ways to add depth to an aquascape. Common depths are 3 to 5 cm at the front, rising to 8 to 10 cm or more at the back. Steeper slopes and hills need support, since substrates slump over time; rocks, wood or mesh bags of lava rock are often used to build up the back without using as much soil. Keep the base layer thinner than the front depth so the cap covers it everywhere, otherwise base material shows against the glass.
Buying the Right Amount
Bags come in common sizes such as 3 L, 9 L and 10 L. The calculator rounds up to whole bags, but it is worth adding 10 to 15 percent to the calculated volume: aquasoil settles and compacts after flooding, granules break down over months, and landscapes with hills use more than a smooth slope. Leftover soil keeps well in a sealed bag for later topping up.
Using Inches
If your tank is measured in inches, switch the units. The calculator converts cubic inches to litres, 1 litre being 61.02 cubic inches, so you can compare directly with bag sizes. A 36 × 18 inch tank sloped from 1.5 to 3 inches holds about 23.9 litres, close to the metric example, because the two tanks are nearly the same size.
Setting Up the Substrate
Lay any base layer first and keep it a little back from the front glass so it stays hidden. Add the cap gently, building up the slope from front to back, then place rocks and wood before filling. Fill slowly over a plate or plastic bag to avoid washing the soil into craters. Many aquasoils release ammonia for the first few weeks, so frequent large water changes during that time protect plants and any livestock added later.
Substrate Life Span
Aquasoils gradually break down into finer particles and lose their nutrient load over one to three years, depending on the product and how often the bed is disturbed. Root tabs pushed into the substrate can extend its useful life. When a tank is rescaped after a long run, it is often easiest to replace the soil completely rather than top it up, and this calculator gives the volume to buy.
Substrate and Water Chemistry
Active aquasoils buffer the water toward soft and slightly acidic conditions, which suits many plants, shrimp and fish such as tetras and rasboras. Inert substrates such as sand and gravel leave the water chemistry as it is. If you keep livebearers or African cichlids that need hard water, an inert substrate with root tabs is usually a better choice than aquasoil.
Understanding Your Result
The headline gives the total volume of substrate in litres.
The cap layer line gives the cap volume and the number of bags.
The base layer line gives the base volume and the number of bags, or says there is none.
The slope line shows how thick the cap is at the front and back above the base.
When Should You Use This Calculator?
Use it when buying aquasoil and base layers for a new planted tank.
Use it to plan the slope of an aquascape before buying.
Use it to compare the cost of different depths and bag sizes.
Common Mistakes
Using outside dimensions. Measure inside the glass.
Forgetting the slope. Use the average of the front and back depths.
Making the base layer too thick. It must stay below the cap at the front.
Buying exactly the calculated amount. Allow for settling and hills.