Aquarium Gravel & Sand Calculator, Bags and Pounds
You know your tank in gallons. The bag is priced in pounds. And the number that joins them is the one nobody writes down: the footprint. Picture a 20 gallon high next to a 20 gallon long. Same water, floors 25% apart. So this aquarium gravel and sand calculator starts from the base of the fish tank instead.
Your result
What to buy
2 bags of aquarium gravel at 20 lb, 33 lb total, for 24 x 12 in at 2 in deep
That is a tight fit with only 7 lb spare. Rinsing washes a little away every time, so a spare bag is cheap insurance.
- Slope it back to front
- The figure above is a flat bed at 2 in. Most people run it about an inch shallower at the front and an inch deeper at the back, which looks better and hides equipment. The total stays roughly the same either way.
- How this is worked out
- 24" × 12" × 2" = 576 cu in ÷ 1,728 = 0.33 cu ft × 100 lb = 33 lb
What is aquarium substrate?
It is whatever covers the bottom of the tank. Gravel, sand, or fired clay granules sold as aqua soil. All three are inert, graded to a size that will not trap waste, and washed so they do not cloud the water.
But the bed is doing more work than it looks. Most of the bacteria that process fish waste live on it, not in the filter. A few thousand grains beat anything a cartridge offers for surface area. That is why a tank stripped of its substrate crashes, and why you rinse an established bed in tank water rather than tap.
Which is also why depth matters in both directions. Too shallow and there is not enough surface for the colony or grip for roots. Too deep and the bottom layer runs out of oxygen, turns black and starts producing gas you do not want released into the water.
Calculate substrate for your tank
Length × width × depth, all in inches, then divide by 1,728 to reach cubic feet and multiply by what the substrate weighs. Take a 20 gallon high. It is 24 by 12 inches, so a two inch bed is 576 cubic inches. That is 0.33 cubic feet, or 33 lb of gravel.
Notice what is missing. Tank height never enters the sum. A tall tank and a shallow one with the same base take exactly the same substrate, because you are covering a floor rather than filling a box.
Measure inside the glass if you are not using a preset. Rimmed tanks lose roughly half an inch each way to the plastic trim, and on a small tank that is enough to matter.
Substrate depth by tank type, and what each one is for
Two inches is the answer for most tanks, and if you stop reading here you will be fine. The other depths exist for specific reasons rather than preference.
| Depth | Use | Why | 20 gallon high, gravel |
|---|---|---|---|
| 1" | minimum | Enough to cover the glass. Too shallow for roots or for burrowing fish | 17 lb |
| 2" | standard | The usual community tank depth, and what most people should pick | 33 lb |
| 3" | planted | Root feeders like swords and crypts need this much to spread | 50 lb |
| 4" | deep bed | Deep sand bed for a marine tank, where the lower layer does the filtering | 67 lb |
Read down that last column and the cost of changing your mind is obvious. Going from two inches to four doubles the order, on a tank where the difference is barely visible from the front.
Bottom dwellers are the one case that overrides the rest. Corydoras, kuhli loaches and anything that sifts want sand rather than gravel, at a depth they can actually dig into. An inch of sharp gravel is a worse home for them than three inches of soft sand.
Tank size pulls the other way, though. A deep bed in a small tank eats the space the fish were going to swim in. Two inches in a 20 gallon high is already a sixth of the height, gone before any water goes in. So nano tanks tend to run shallower than the chart suggests, and they should.
Check it with a ruler held against the outside of the glass rather than guessing from above. Substrate always looks deeper looking down through water than it turns out to be from the side, and that illusion is what leaves people a bag short.
Gravel vs sand quantity, and why sand weighs more
Sand is heavier for the same space. Not by much, but enough to change a bag count. Gravel runs about 100 lb a cubic foot and sand about 105, because the fine grains sit closer together and leave less air between them.
So a tank switching from gravel to sand needs roughly 5% more by weight for an identical bed. Aqua soil goes the other way, and further. Fired clay is mostly hollow. At 55 lb a cubic foot, one bag covers close to twice the ground.
| Substrate | lb / cu ft | Weight needed | 20 lb bags | Grain |
|---|---|---|---|---|
| Aquarium gravel | 100 | 33 lb | 2 | 3 to 5 mm |
| Aquarium sand | 105 | 35 lb | 2 | 0.5 to 2 mm |
| Play sand | 105 | 35 lb | 2 | Under 0.5 mm |
| Aqua soil | 55 | 18 lb | 1 | 2 to 4 mm |
The bag column is what you actually buy, and it hides the detail. Gravel and sand land on the same count here even though the weights differ, because bags only come whole. That stops being true on a bigger tank, where the gap is wide enough to add a bag.
Common tank sizes chart, with the substrate each one takes
Every standard tank at a two inch bed, in both materials. Find yours and the answer is the row.
| Tank | Base | Floor | Gravel | Sand | Bags |
|---|---|---|---|---|---|
| 10 gallon | 20 × 10" | 200 sq in | 23 lb | 24 lb | 2 |
| 20 gallon high | 24 × 12" | 288 sq in | 33 lb | 35 lb | 2 |
| 20 gallon long | 30 × 12" | 360 sq in | 42 lb | 44 lb | 3 |
| 29 gallon | 30 × 12" | 360 sq in | 42 lb | 44 lb | 3 |
| 40 gallon breeder | 36 × 18" | 648 sq in | 75 lb | 79 lb | 4 |
| 55 gallon | 48 × 13" | 624 sq in | 72 lb | 76 lb | 4 |
| 75 gallon | 48 × 18" | 864 sq in | 100 lb | 105 lb | 5 |
| 125 gallon | 72 × 18" | 1,296 sq in | 150 lb | 158 lb | 8 |
The two twenties are the row pair worth staring at. Same water, same price on the shelf, and the long one needs 25% more substrate because its floor is bigger. Buy off the gallon number and you come up short.
The 55 gallon is the opposite surprise. It is a big tank, but only 13 inches front to back, so it takes less than a 20 plus a 20 would. Tall and narrow is cheap to floor.
Rimless and custom tanks are not in the chart, and they are the ones worth measuring rather than guessing. Switch the tool to the base measurement and use the inside of the glass. Trimmed tanks lose about half an inch each way to the plastic, which barely registers on a 75 and matters on a nano.
These figures are all at two inches because that is what most people build. For any other depth, scale straight from the row: three inches is the two inch number plus half again, four is double it. Depth is linear, so the arithmetic stays easy.
Planted tank substrate depth, and how to layer it
Three inches at the back, two at the front. Root feeders such as amazon swords and the larger crypts put down a real root system, and an inch of gravel simply does not hold them. They come loose every time you rescape.
Most planted tanks use two layers rather than one. Aqua soil or a nutrient base goes down first, an inch of it, then gravel or sand on top to cap it. The lower layer feeds the roots and the cap keeps it from clouding the water every time something disturbs it.
Work out each layer on its own here, changing the substrate type between runs, then add them. Let’s say a 20 gallon high, one inch of aqua soil under two inches of gravel. That is 9 lb plus 33 lb. So the soil is the cheap part by weight, even though it costs more per bag.
One habit worth borrowing from the planted crowd: slope the bed up toward the back. It reads as depth from the front. It hides heaters and intakes. And it gives you root depth where the tall plants go, without paying for it across the whole floor.
How much substrate per gallon
One pound a gallon is the rule everybody repeats. Most people assume it scales. It does not, because it ignores the shape of the tank entirely.
| Tank | Gravel at 2" | lb per gallon | Rule says |
|---|---|---|---|
| 20 gallon high | 33 lb | 1.67 | 20 lb |
| 20 gallon long | 42 lb | 2.08 | 20 lb |
| 55 gallon | 72 lb | 1.31 | 55 lb |
| 75 gallon | 100 lb | 1.33 | 75 lb |
The spread runs from about 1.31 to 2.08 lb a gallon across four common tanks, all at the same depth. That is not a rule, it is a coincidence that holds for one tank shape and fails on the rest.
If you want a rule that survives, use area instead: a two inch bed of gravel runs about 11.6 lb per hundred square inches of floor. That works on any tank, in any shape, including the odd ones.
The pound a gallon idea did come from somewhere. It lands close on the tall rectangular tanks that were standard decades ago, at the shallow beds people ran then. Tanks got wider and beds got deeper, and the rule never moved with them.
Where it still bites is at the till. Someone with a 20 gallon long follows the rule, buys 20 lb, and gets home with roughly 52% less than the tank needs. Not enough to notice in the bag, plenty to notice spread across the floor.
What aquarium substrate costs
Between $12 and $30 for a 20 lb bag of gravel or aquarium sand, with most sitting near $18. Aqua soil runs $30 to $45, which is the real reason people cap it rather than fill with it.
Play sand is the outlier and the reason it comes up so often. A 50 lb bag from a hardware store is $6 to $12, so flooring a 55 gallon costs under $20 instead of $70. That gap is worth the extra rinsing to a lot of people.
Buy one bag more than the sum says. Here’s the trap: substrate settles into the corners over the first month, and rinsing carries a little away every time. Come back for a single bag of colored gravel six months later and the dye lot no longer matches.
Aquarium substrate questions
Can you use play sand in an aquarium?
Yes, and plenty of people do. It is washed silica, the same mineral as aquarium sand, at a fraction of the price. The catch is grain size. Play sand is finer, so it packs down tighter, and a bed deeper than about two inches can go stale in the lower layer.
Is play sand safe for aquariums?
The sand itself is inert and safe. What you have to check is the bag: buy plain washed play sand with nothing added, never a product listing a mold inhibitor or a binder. Rinse until the water runs clear, then rinse again, because the dust is what causes trouble.
Can play sand be used in a fish tank?
It can, and it suits bottom feeders especially well because their barbels do not get scraped the way they can on sharp gravel. Add it to a drained tank, not a full one. Pouring sand through water is how you end up with a week of cloud.
How much sand for a 55 gallon aquarium?
About 76 lb at two inches deep, which is 4 bags. A 55 gallon has a 48 by 13 inch base. That narrow 13 inches front to back is why it takes less than people expect for a tank that size.
How much sand for a 75 gallon aquarium?
Around 105 lb at two inches, or 6 bags. Same 48 inch length as a 55, but 18 inches front to back instead of 13. That lifts the floor area by 38%, and the sand goes up with it.
How much gravel do I need for a 20 gallon tank?
A 20 gallon high takes about 33 lb at two inches. A 20 gallon long holds the same water but has 25% more floor, so it needs about 42 lb. Same gallons, different answer, which is the trap here.
How deep should aquarium gravel be?
Two inches for a normal community tank. One inch is enough to cover the glass but nothing will root in it. Go to three if you are growing plants that feed through their roots, and only go deeper than that for a marine deep sand bed.
How many pounds of substrate per gallon?
The old rule says one pound a gallon, and it under-buys almost every time. A 20 gallon high at two inches actually needs 1.7 lb a gallon. The rule ignores footprint, and footprint is the only thing that matters.