Buying substrate is one of those jobs where it is easy to be far out. Too little leaves a thin layer that does not reach the depth you want, and too much means carrying bags you did not need and displacing water you did. The sum is simple geometry, but it hides one trap: substrate is sold by weight and measured by volume, and the conversion depends on what the product is made of. This guide shows both steps, gives a table for common tank sizes and depths and explains how to allow for slopes, hardscape and the weight on your stand.
The two formulas
The Aquarium Substrate Volume Calculator uses:
volume (litres) = length × width × depth in cm ÷ 1,000 weight (kg) = litres × density (kg per litre)
The default density in the calculator is 1.5 kg per litre, which is a planning figure for a typical gravel or sand. Real products differ, and the bag often gives a density or the volume it covers. Use the bag’s figure when you have it.
A worked example: a tank 60 cm long and 30 cm wide with 5 cm of substrate. 60 × 30 × 5 = 9,000 cm³ = 9.0 litres. At 1.5 kg per litre that is 13.5 kg.
A table for common tanks and depths
Inside length and width, in centimetres, at three depths. Weight uses 1.5 kg per litre.
| Tank (L × W, cm) | Depth | Volume | Weight at 1.5 kg/L |
|---|---|---|---|
| 60 × 30 | 3 cm | 5.4 L | 8.1 kg |
| 60 × 30 | 5 cm | 9.0 L | 13.5 kg |
| 60 × 30 | 7 cm | 12.6 L | 18.9 kg |
| 90 × 40 | 3 cm | 10.8 L | 16.2 kg |
| 90 × 40 | 5 cm | 18.0 L | 27.0 kg |
| 90 × 40 | 7 cm | 25.2 L | 37.8 kg |
| 120 × 50 | 3 cm | 18.0 L | 27.0 kg |
| 120 × 50 | 5 cm | 30.0 L | 45.0 kg |
| 120 × 50 | 7 cm | 42.0 L | 63.0 kg |
The numbers climb quickly. A 120 cm tank with 7 cm of substrate needs 63 kg at this density, which is several large bags and is part of the weight your stand must carry.
Measure inside the glass
Use the inside length and width, not the outside, because the glass takes up space. Our guide to how to calculate aquarium volume shows how much difference that can make: for a 12 mm glass tank, a few centimetres on each side. For substrate, an error of 2 cm in each direction in a 90 × 40 tank changes the volume by about 10%.
Sloped and uneven substrate
Many keepers slope the substrate from the back to the front for looks and for plants. The average depth is what matters for volume. If the back is 8 cm and the front 3 cm, the average depth of a straight slope is (8 + 3) ÷ 2 = 5.5 cm. For a 90 × 40 tank, that is 90 × 40 × 5.5 ÷ 1,000 = 19.8 litres, 29.7 kg at 1.5 kg per litre. If you add mounds or terraces, estimate in sections, or buy a bag more than you calculate and keep what is left.
Bags to buy
Divide the weight by the bag size and round up.
| Weight needed | 2 kg bags | 5 kg bags | 10 kg bags |
|---|---|---|---|
| 13.5 kg | 7 | 3 | 2 |
| 27.0 kg | 14 | 6 | 3 |
| 45.0 kg | 23 | 9 | 5 |
Check the bag for the volume it covers, since some are sold by litre. Buy a little extra: fine sand packs differently from gravel, and you will lose some when rinsing. Most retailers let you return unopened bags; ask.
Wet and dry weight
Substrate weighs more wet than dry, because water fills the gaps between grains. The bag’s weight is the dry weight; once in the tank, the substrate and the water in it weigh more, though the water in the gaps is part of the tank’s water, not extra. For stand-loading calculations, our Aquarium Weight Calculator takes the substrate into account, and the guide to how much a fish tank weighs walks through the total.
Substrate and water volume
Substrate and decor displace water. If a tank holds 180 litres to the water line without substrate, 5 cm of substrate in a 100 × 40 tank takes up 100 × 40 × 5 ÷ 1,000 = 20 litres of space, though some of that volume is the gaps between grains, which fill with water. In practice the net loss of water volume is a fraction of the substrate’s bulk volume, depending on how it packs. For dosing treatments and calculating water changes, treat the real water volume as lower than the gross figure and measure it by filling if you need accuracy. The Aquarium Water Change Calculator works from the volume you enter.
Choosing depth
The right depth depends on what you keep and plant, and no single number suits every tank. Plants with roots, burrowing fish and different substrate types have different needs, and a specialist shop, a club or the maker’s guidance is the place to ask. The calculator tells you how much of a given depth you will need; it does not tell you which depth is right.
| If you want… | Ask about… |
|---|---|
| Rooted plants | Depth and type of substrate for the species |
| Burrowing fish | Fine, smooth substrate and sufficient depth |
| A bare or thin layer | The effect on cleaning and appearance |
| A deep bed | Weight, water volume and maintenance |
Planning costs
Substrate is sold by weight, and prices vary widely with type. Multiply the weight by the price per kilogram to estimate the cost. For example, 27 kg at 1.60 per kg is 43.20. A larger tank costs much more, and the cost of the cheapest substrate may be small compared with the stand, filter and lighting. Add substrate to your total set-up budget.
Common mistakes
- Using the outside dimensions.
- Forgetting the slope.
- Assuming every product has the same density.
- Not buying a margin for rinsing and waste.
- Ignoring the weight on the stand.
- Treating gross volume as water volume.
- Choosing depth without advice.
Questions keepers ask
Can I mix two substrates? Many keepers do, for example a nutrient layer under a cap of sand or gravel. Work out the volume of each layer separately with its own depth.
How do I clean substrate before use? Follow the maker’s instructions; many products need rinsing in a bucket until the water runs clear. Rinse in small batches, and do not use soap.
What if my tank has an irregular base? Use the average depth over the base, or work out the volume in sections.
How do I estimate substrate for a cylinder tank? Use π × radius² × depth for the volume in cubic centimetres, divided by 1,000 for litres.
Does the weight of substrate matter for my stand? Yes, along with the water and tank. Add it to the weight estimate; see how much a fish tank weighs.
Can I reduce the amount I need? A thinner layer needs less, but your plants and fish may need a minimum depth. Ask a specialist.
A planning sheet
| Item | Your figure |
|---|---|
| Inside length (cm) | |
| Inside width (cm) | |
| Average depth (cm) | |
| Volume (litres) = L × W × D ÷ 1,000 | |
| Density from the bag (kg per litre) | |
| Weight (kg) | |
| Bag size (kg) | |
| Bags to buy (rounded up, plus a margin) | |
| Price per kg and cost |
Write the figures down before you shop, so that you can compare products on the same basis. If a product states its coverage per bag in litres, compare it directly with the volume you calculated.
A note on safety and sourcing
Choose substrate sold for aquarium use, and rinse it as the maker says. Avoid collecting sand or gravel from outdoors for a tank, since it may contain contaminants or materials that change the water. If you are unsure whether a product suits your fish or plants, ask the shop.
Quick summary
- Volume (litres) = length × width × depth (cm) ÷ 1,000.
- Weight (kg) = litres × density; the calculator’s default is 1.5 kg per litre, so check the bag.
- A 60 × 30 cm tank at 5 cm needs 9 litres, about 13.5 kg.
- Measure inside the glass, average the depth of a slope and buy a small margin.
- Substrate adds weight and displaces water, so allow for both.
- Use the Aquarium Substrate Volume Calculator.
Frequently asked questions
How do I calculate how much substrate I need?
Multiply length × width × depth in centimetres and divide by 1,000 to get litres. Multiply by the density to get kilograms.
What depth of substrate should I use?
It depends on what you keep and grow. Ask a specialist shop or club; this guide shows the arithmetic for any depth.
How heavy is aquarium gravel or sand?
It varies by material and whether it is wet. The calculator's default density is 1.5 kg per litre; check your product's label.
How many bags do I need?
Divide the total weight by the bag weight and round up, adding a small margin for sloping.
Does substrate reduce the water volume?
Yes. Substrate and decor displace water, so real water volume is lower than the tank's gross volume.
Should I slope the substrate?
Some keepers slope it higher at the back. Average the depth, or work out the front and back separately.
Sources
- Pure geometry: volume = length × width × depth; 1 litre = 1,000 cubic centimetres
- Substrate density depends on the product; the calculator default of 1.5 kg per litre is a planning figure (check the bag)
Related calculators
- Aquarium Substrate Volume CalculatorWork out how many litres of gravel or sand you need for a layer of a chosen depth.
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Water Weight CalculatorWork out how much the water in a fish tank weighs so you can check your stand and floor.
Keep reading
- How to Calculate Aquarium Volume (Litres/Gallons)How to calculate aquarium volume from length, width and height in cm or inches: the formulas, US and UK gallon conversions and real water volume.
- How Much Does a Fish Tank Weigh? Full Tank WeightHow to estimate the weight of a filled fish tank: water at 1 kg per litre or 8.35 lb per US gallon, plus glass, substrate, rock and stand, with examples.
- Fish Tank Size Chart: Gallons, Litres and WeightA fish tank size chart for common US gallon sizes with typical dimensions, volume in gallons and litres and water weight, plus how to read nominal sizes.
- US Gallons vs UK Gallons for AquariumsA US gallon is 3.785 litres and a UK gallon 4.546, a 20% gap. Convert tank sizes both ways, see why 55 gallons can mean 208 or 250 litres, and fix water sums.