About

Substrate quantity miscalculations waste money or leave bare spots that destabilize plant roots and disrupt biological filtration. This calculator computes the required volume and mass for over 25 substrate types - from pool filter sand (ρ ≈ 1.56 kg/L) to lightweight expanded clay (ρ ≈ 0.45 kg/L) - using actual bulk densities rather than generic estimates. It handles rectangular, cylindrical, bow-front, and corner tank geometries, including sloped substrate beds where front and back depths differ. Results include total mass, volume in liters, and the number of commercial bags required at your chosen bag size.

Note: bulk density values assume dry, loosely packed substrate. Wet substrate compresses roughly 5 - 15% depending on grain geometry. For planted tanks, target a minimum depth of 5 cm to allow root anchoring. For Walstad-method setups with capped soil, calculate each layer independently. This tool approximates bow-front curvature as a parabolic bulge; actual volume may vary ±3% based on glass curvature radius.

Formulas

The substrate volume depends on tank geometry. For a standard rectangular tank:

V = L × W × D1000

where V is volume in liters, L is tank length in cm, W is tank width (front-to-back) in cm, and D is substrate depth in cm. For a sloped bed with different front and back depths:

V = L × W × (D_front + D_back)2000

For cylindrical tanks:

V = π × r² × D1000

where r is the tank radius in cm. For bow-front tanks, the additional curved volume is approximated:

V_bow = L × W × D1000 + 2 × L × B × D3000

where B is the bow bulge depth in cm (how far the curved glass extends beyond the straight width). The substrate mass is then:

M = V × ρ

where M is mass in kg and ρ is the bulk density of the chosen substrate in kg/L. Finally, the number of bags:

n = ⌈ Mm_bag ⌉

where m_bag is the mass per bag in kg and the ceiling function ensures you always round up to whole bags.

Reference Data

Substrate Type	Bulk Density (kg/L)	Grain Size (mm)	Best For
Pool Filter Sand	1.56	0.4 - 0.8	Budget planted tanks, Corydoras
Play Sand (Silica)	1.58	0.1 - 0.5	Aesthetic, budget setups
Black Diamond Blasting Sand	1.68	0.5 - 1.2	Dark aesthetic, inert substrate
Natural Aquarium Gravel	1.60	2 - 5	General freshwater, goldfish
Pea Gravel	1.65	5 - 10	Large cichlids, undergravel filters
Fluorite (Seachem)	1.00	1 - 7	Planted tanks, CEC nutrient exchange
Eco-Complete (CaribSea)	0.96	1 - 5	Planted tanks, live bacteria
ADA Aqua Soil Amazonia	0.85	1 - 3	High-tech planted, shrimp
Fluval Stratum	0.78	2 - 4	Shrimp tanks, planted
Tropica Aquarium Soil	0.82	1 - 3	Planted, pH-lowering
UNS Controsoil	0.80	1 - 3	Planted, shrimp breeding
Crushed Coral	1.45	2 - 10	African cichlids, pH buffering
Aragonite Sand	1.50	0.5 - 2	Marine, reef, pH buffering
Live Sand (Marine)	1.48	0.5 - 2	Saltwater, reef DSB
CaribSea Arag-Alive	1.52	0.5 - 1.7	Marine, bacteria-seeded
Laterite (Iron-rich clay)	1.10	1 - 5	Lower layer for planted tanks
Potting Soil (capped)	0.55	Varies	Walstad method, budget planted
Expanded Clay (LECA)	0.45	8 - 16	Aquaponics, biological media
Pumice	0.50	3 - 10	Lightweight planted, biological
Lava Rock Gravel	0.90	3 - 12	Biological filtration, hardscape
River Rock / Pebbles	1.70	10 - 30	Large fish, decorative
Glass Beads	1.55	3 - 8	Decorative, easy cleaning
Onyx Sand (Seachem)	1.40	0.5 - 2	Planted, mineral-rich
Mr. Aqua Aquarium Soil	0.76	1 - 3	Planted, shrimp
SafeTSorb (Oil Absorbent)	0.70	1 - 4	Budget planted, high CEC
Turface MVP	0.72	1 - 3	Budget planted, CEC

Frequently Asked Questions

Finer substrates (grain size below 2 mm) compact more tightly, providing better root anchorage at shallower depths. A minimum of 5 cm is recommended for fine sand. Coarser substrates like pea gravel (5-10 mm) require 7-8 cm minimum because roots slip through larger voids. For carpeting plants such as Hemianthus callitrichoides, use 3-5 cm of fine-grained active soil like ADA Amazonia (ρ ≈ 0.85 kg/L) to avoid root lift.

Mineral density (e.g., quartz at 2.65 kg/L) describes a solid, void-free particle. Bulk density accounts for the air gaps between grains when loosely packed in a bag or tank. Aquarium gravel has a bulk density around 1.60 kg/L because roughly 40% of the volume is interstitial space. Using mineral density would overestimate the required mass by approximately 65%.

Run the calculator twice. First, set the substrate to "Potting Soil (capped)" at ρ ≈ 0.55 kg/L with a depth of 2.5 cm for the bottom nutrient layer. Then run it again with your cap material (e.g., Pool Filter Sand at ρ ≈ 1.56 kg/L) at 2.5 cm depth. Sum both mass results. The cap layer must be at least equal in depth to the soil layer to prevent anaerobic gas breakthrough.

Yes. The calculator uses a trapezoidal cross-section, meaning the substrate surface slopes linearly from the front depth to the back depth across the tank width. This is the standard aquascaping technique for creating depth perspective. If you sculpt a concave or terraced slope, actual volume will differ. For terraced layouts, calculate each terrace as a separate rectangular section.

Active soils like ADA Amazonia and Fluval Stratum absorb water and swell slightly (3-5%), but then compact under their own weight by roughly 10-15% over the first 48 hours. Inert substrates like sand and gravel show minimal change (under 3%). This calculator uses dry bulk density values. For active soils, consider adding 10% extra to compensate for settling. The tool does not apply this correction automatically to maintain transparency.

Place a straightedge across the front opening of your bow-front tank, touching both front corners. Measure the maximum perpendicular distance from the straightedge to the curved glass at the center point. This is the bulge value B, typically 5-10 cm for standard bow-front aquariums (36-72 inch models). The calculator approximates the curved area as a parabolic segment, which is accurate within ±3% for most commercial bow-front tanks.

Crushed coral (ρ ≈ 1.45 kg/L) is calcium carbonate and will raise pH to 7.8-8.4 and increase KH by 2-5 dKH depending on CO₂ levels and water volume. This calculator provides mass and volume only. For chemistry impact, the rule of thumb is 1 kg of crushed coral per 40 liters raises KH by approximately 1 dKH at equilibrium. African cichlid setups typically use 2.5-5 cm depth of crushed coral as the sole substrate.