Substrate looks simple until you stand in the shop trying to guess how many bags you need. Buy too little and the tank floor is patchy, plants will not root, and a second trip is needed. Buy too much and heavy bags sit in the garage for years. This free substrate calculator turns your tank base and desired depth into litres, kilos, and bag counts for gravel, sand, or aquasoil.
Depth matters for more than looks. Most rooted plants need at least five centimetres to anchor, and deeper beds of seven or eight centimetres suit large plants and burrowing fish. Too shallow and plants float free at the first water change. Too deep without plants or burrowers, and the lower layers can go stagnant, so match depth to what actually lives in the tank.
Using the calculator is simple. Enter the tank length and width in centimetres, the substrate depth you want, and the substrate type. Press Calculate substrate and you get the volume in litres, the weight in kilos, and how many ten-kilo and twenty-five-kilo bags that means. Gravel, sand, and aquasoil weigh differently per litre, so the type choice changes the kilo result.
A worked example shows the maths. A tank base of one hundred by forty centimetres with five centimetres of gravel holds twenty litres of substrate. At one point six kilos per litre, that is thirty-two kilos, which means four ten-kilo bags or two twenty-five-kilo bags. The same volume in sand is slightly heavier, so the bag count can change even when the volume does not.
Slope the substrate if you want a natural look: deeper at the back, shallower at the front. Calculate for the average depth, then add one extra bag for the slope and for topping up over the first months, since substrate settles and gets vacuumed away slowly. Rinse gravel and sand well before adding, but do not rinse aquasoil, which is meant to stay intact.
Gravel suits most community tanks and is easy to vacuum, while sand suits bottom feeders like corydoras that sift it through their gills. Aquasoil feeds plants and softens water, which helps many plants but changes water chemistry, so it suits planted tanks rather than every community. Pick the type for the fish and plants first, and let the calculator handle the quantity.
Do not forget hardscape displacement in a rough sense: large rocks and driftwood take up floor space, so a heavily scaped tank needs a little less substrate than the bare-base calculation. The difference is usually one bag at most, so buying one bag extra still covers it. Keep the spare sealed and dry for future top-ups.
This calculator gives a planning quantity from your own measurements, not a shop quote. Measure the inside of the tank, not the outside, and recheck depth after planting, since planting pushes substrate around. It is free, it needs no sign-up, and it runs entirely in your browser.
Substrate cleaning is the chore that decides whether a deep bed stays healthy. A gravel vacuum at each water change lifts waste from the top layers without disturbing the whole bed, and that is enough for most tanks. Push the vacuum deep only in small sections each week, rotating around the tank, rather than churning the entire bed at once, since deep beds hold bacteria and even small pockets of gas that are better left undisturbed. Planted tanks need a lighter touch still: vacuum the open areas and leave the planted zones mostly alone, since plant roots hold the bed together and feed from it. If you ever smell rotten eggs when disturbing the bed, that is hydrogen sulphide from a stagnant patch, so vacuum that area gently over several weeks and consider thinner substrate or more burrowing snails there.