Aquarium substrate calculator: liters for a flat or sloped bed

Published October 15, 2026Not yet reviewed by the editor3 sources

Short answer

Liters = length × width × average depth (all in cm) ÷ 1000, which is the formula Tropica gives for its Aquarium Soil: 90 × 60 cm at 8 cm needs 43.2 L [1]. For a slope, the average depth is (front depth + back depth) ÷ 2.

Substrate volume

L
kg/L
Only if the maker publishes it.

Substrate needed9L2.38 US gal, 549 in³
front 3 cmback 7 cm
Average depth5 cm (1.97 in)
Bags of 9 L1 (1 exactly)

A level bed has the same depth front and back. Mounds, rock and wood displace substrate, so treat this as the amount for a plain slope.

The formula

A substrate bed is a block, or a wedge when it slopes. Its volume in liters is the inside footprint times the average depth, all in centimeters, divided by 1000 because a liter is 1000 cm³ [2]. Tropica publishes exactly this calculation for its Aquarium Soil, with the example of a 90 × 60 cm tank at 8 cm needing 43.2 L, and suggests making the layer thicker toward the back [1].

For a straight slope from front to back the average depth is the mean of the two ends, so a bed that is 3 cm at the front and 7 cm at the back uses the same substrate as a flat 5 cm bed. The side view in the calculator redraws the slope as you type.

Worked example

A 60 × 30 cm tank with 3 cm of soil at the front rising to 7 cm at the back:

  • Average depth (3 + 7) ÷ 2 = 5 cm
  • Volume 60 × 30 × 5 ÷ 1000 = 9.0 L
  • With 3 L bags: exactly 3 bags; with 9 L bags: 1 bag.

The calculator rounds bags up only when the shortfall is real, so 9.0 L of a 3 L product is three bags, not four.

Converting units

If you measure in inches, the calculator converts with 1 in = 2.54 cm and reports US gallons with 1 gal = 3.785412 L [3]. For products sold by weight, enter the bulk density only if the manufacturer publishes it; densities of soils, sands and gravels differ widely, and a guessed figure would make the weight meaningless.

What changes the real amount

  • Hardscape. Rocks and wood pushed into the bed displace substrate; mounds and terraces add to it. The calculator handles a plain slope only.
  • Settling. Granular soils settle and compact over time, and fine powder layers fill gaps between grains.
  • Under-layers. If you put a nutrient layer or powder under the main substrate, calculate each layer separately with its own depth.

Aqua soils change water chemistry

Some substrates are not chemically inert. Tropica states that its Aquarium Soil lowers pH and slightly affects water chemistry, and recommends changing 25–50 % of the water at least twice a week for the first four weeks after setup [1]. That matters for the CO2 from KH and pH calculator, which assumes carbonate is the only buffer in the water.

Questions

Why slope the substrate?

Tropica's own instructions suggest making the layer thicker toward the rear wall for visual effect [1]. A slope also gives deeper substrate where larger, root-feeding plants usually go.

My substrate is sold by weight. How do I convert?

You need the product's bulk density in kg per liter. If the manufacturer publishes it, enter it and the calculator gives the weight; if not, the bag's stated volume is the reliable figure.

Other calculators

Background reading

Sources

  1. Aquarium Soil. Tropica Aquarium Plants. Quantity formula length × width × layer ÷ 1000; thicker layer toward the rear wall; 2–3 mm grains; lowers pH; 25–50 % water changes at least twice a week for the first 4 weeks. Accessed October 12, 2026.
  2. SI units: volume. National Institute of Standards and Technology, Office of Weights and Measures. The liter is one cubic decimeter (1000 cm³). Accessed October 12, 2026.
  3. NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically. National Institute of Standards and Technology (SP 811). inch 2.54 cm; US gallon 3.785 412 L; pound 0.453 592 4 kg. Accessed October 12, 2026.